GEOLOGY LIBRARY 2001.313082 DISS 1811 F•SI GIOL ..AR'~ THE UN.-r...1-t'::il..i OF; . TEXAS AT AUSTIN fir:-l.. .l . 1987 GEOLOGY LIBRAR'( y OFTEXASAT AUSTIN THE UNIVERSIT AL LIBRARIES T~~~~NER ~ . --: MAR 1 3 199QET;; £8 19 1998 /")1:'0'' our To 1Ls ""' • ocr os7998 ')' ·~ ,. , n.Docr os 199a GEoL ~~ 1~f'fRED DEC 0 7 1998 GEOL~~ R: TD M 1 Lrr To lf.S AR 7 8 1999 GEOL ~~2~1v, 25 1999 ~CE DEPOSITIONAL SYSTEMS IN THE LOWER CRETACEOUS MORRO DO CHAVES AND COQUEIRO SECO FORMATIONS, AND THEIR RELATIONSHIP TO PETROLEUM ACCUMULATIONS, MIDDLE RIFT SEQUENCE, SERGIPE-ALAGOAS BASIN, BRAZIL APPROVED BY SUPERVISORY COMMITTEE: Dedicated to my wife and daughter Celia and Paula DEPOSITIONAL SYSTEMS IN THE LOWER CRETACEOUS MORRO DO CHAVES AND COQUEIRO SECO FORMATIONS, AND THEIR RELATIONSHIP TO PETROLEUM ACCUMULATIONS, MIDDLE RIFT SEQUENCE, SERGIPE-ALAGOAS BASIN, BRAZIL by ANTONIO MANUEL FERREIRA DE FIGUEIREDO, Ph.D. DISSERTATION Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY THE UNIVERSITY OF TEXAS AT AUSTIN August, 1981 ACKNOWLEDGEMENTS I express my appreciation to L. F. Brown, Jr., chairman, and to w. L. Fisher, R. o. Kehle, M. Dorfman, and F. c. Ponte, other committe members. Gratitude is also extended to PETROBRAS colleagues z. Matos and G. Fernandes who helped with valuable discussions during the research and who helped to establish the seismic framework of sequence II in the Sergipe-Alagoas Basin. Graduate student s. Ghazi helped with thin section interpretation and preparation of photomicrographs. I am grateful to PETROBRAS {Petroleo Brasileiro S/A) for providing financial assistance and releasing subsurface data and seismic data analyzed in the qisser­tation program, and to the Bureau of Economic Geology {W. L. Fisher and L. F. Brown, Jr.) for providing sup­port needed to execute the research. iv DEPOSITIONAL SYSTEMS IN THE LOWER CRETACEOUS MORRO DO CHAVES AND COQUEIRO SECO FORMATIONS, AND THEIR RELATIONSHIP TO PETROLEUM ACCUMULATIONS, MIDDLE RIFT SEQUENCE, SERGIPE-ALAGOAS BASIN, BRAZIL Publication No. ~~~­ Antonio Manuel Ferreira de Figueiredo, Ph.D. The University of Texas at Austin Supervising Professor: Leonard F. Brown, Jr. In the Sergipe-Alagoas Basin, along the north­east coast of Brazil, the lacustrine, middle rift sequence is composed of the Lower Cretaceous Morro do Chaves and Coqueiro Seco Formations. Subsurface analy­sis permitted recognition and mapping of four principal types of depositional systems that infilled the basin with more than 3,000 meters of elastic-carbonate sediments: Morro do Chaves carbonate platform, Coqueiro Seco fluvial-deltaic, Coqueiro Seco fan delta, and Coqueiro Seco slope systems. The generally poor quality of seismic profiles in this rift sequence precludes con- v ventional seismic stratigraphic approaches. Morro do Chaves lacustrine carbonate platform sediments were deposited on shallow positive areas flanking the principal point sources (rivers), and are composed of massively bedded, high energy limestones. Contemporaneous with shallow-water sedimentation, deep­water euxinic and bituminous lacustrine shales were deposited under starved basin conditions. Sublacustrine canyon excavation attested to the presence of a destruc­tional slope episode. Coqueiro Seco fluvial-deltaic, fan delta, and slope sediments are principally terrigenous. Fluvial­deltaic and fan delta facies display high sand/shale ratios and blocky to massive E-log patterns; slope facies display serrate to digitate E-log patterns and are less sandy. Delta plain channel-fill facies and coarse­grained meanderbelt fluvial facies are dominant in fluvial-deltaic systems, and proximal to medial conglom­erates and coarse conglomeratic sandstones are dominant facies in fan delta systems. Slope facies are composed of sublacustrine fans composed of fine-to medium­grained sandstones enveloped by thick, subbituminous shales, and thin, marly, lacustrine limestones. vi Coqueiro Seco elastic systems prograded across the basin and buried Morro do Chaves carbonate platforms in response to tectonic pulses related to rift develop­ment. Cyclic sedimentation occurred in the highly unstable Alagoas Sub-basin where fluvial-deltaic and slope systems are dominant, but fan delta and slope N systems in the less complex Rio Sao Francisco Sub-basin do not exhibit cyclicity. Coqueiro Seco fluvial-deltaic, fan delta and slope sedimentation terminated because of continued basin subsidence and diminishing sediment supply as source areas were leveled. Consequently, the basin became the site of lacustrine shale deposition repre­sented by the Ponta Verde Formation in the Alagoas Sub-basin. The rift sequence is truncated by a pre-Aptian N unconformity in the Rio Sao Francisco Sub-basin. Evaluation of petroleum occurrences in relation­ship to defined depositional systems permitted recogni­tion of several types of plays characterized by unique structural and stratigraphic relationships exhibited by reservoirs, source beds and structure. The Coqueiro Seco slope play, formed by updip pinchout of turbidite fans, is judged the most promising in the sequence. vii TABLE OF CONTENTS Introduction. • • • • • • 1 Pre-Rift Cratonic Continental Phase 4 Rift Phase. • . • • • • 4 Sequence I . 5 Sequence II 7 Sequence III 8 Post-Rift Marine Phase •• 9 Methods and Procedures •• 10 Previous Studies. 20 Regional Tectonic Setting • 28 Definition of Sub-basins. 43 Alagoas Sub-basin . . • . 43 Coruripe Sub-basin. • • • • • 44 Rio Sao Francisco Sub-basin 44 Mosqueiro Sub-basin • • 45 Stratigraphy••• 46 General Considerations and Basic Concepts • 46 Morro do Chaves Carbonate Platform System • 60 Coqueiro Seco Systems, Alagoas Sub-basin 77 Coqueiro Seco Slope System . 89 Sinimbu Low. • 96 Lagoa Manguaba and Paripueira Lows. • . . • . • . • • 103 Core Analysis. . • • . • • 106 Analysis of Operationa_l Units. 120 Coqueiro Seco Fluvial-Deltaic and Fan Delta Systems. • • • • • . • • • • 125 viii Rio Jequi~ Fan Delta System . • 127 Coqueiro Seco Fluvial-Deltaic System. . • • • • • . • . • 128 Sinimbu Low 138 Lagoa Manguaba and Paripu­ eira Lows • • • . • • • 140 Core Analysis 143 IV Coqueiro Seco Systems, Rio Sao Francisco Sub-basin. . . . . . . . . . . . . . . 148 Coqueiro Seco Slope and Basin systems. 150 Coqueiro Seco Fan Delta System . 153 Seismic Stratigraphic Evaluation. • 159 General Considerations. • . • ••• 159 Alagoas Sub-basin • • • • • . • • • . 161 Rio Sao Francisco Sub-basin • . 171 Ancient Stratigraphic Analogs 176 West African Basins • • 176 Green River and Uinta Basins, u. s.. . 181 Ridge Basin, U.S ••• 184 Holocene Depositional Analogs 188 Great African Rift System . 188 Omo Delta, Lake Rudolf 190 Lake Kivu and Lake Tanganyika. • 194 Alpine Lakes, Europe •• 195 Lake Brienz, Switzerland 195 Lake Geneve, Switzerland 198 Hydrocarbon Occurrences and Analysis. • 201 General Considerations. • 201 Morro do Chaves Carbonate Plays 204 Coqueiro Seco Slope Plays • • • 208 Coqueiro Seco Fluvial-Deltaic and Fan Delta ix Plays. 214 Conclusions 220 Appendix A -Stratigraphic Cross Sections 229 Bibliography. 264 Vita. 276 x ILLUSTRATIONS Figures 1. Location map, Sergipe-Alagoas Basin, Brazil • 2 2. Schematic stratigraphic column, Sergipe-Alagoas Basin, Brazil • . . • . . • • • 3 3. Schematic stratigraphic column of Rift phase, Rio Sao Francisco, Coruripe, and Alagoas sub-basins, Sergipe-Alagoas Basin, Brazil • • • • • 6 4. Location map, wells utilized in the study, and location of strike and dip strati­graphic cross section, Sergipe-Alagoas Basin, Brazil • • • . • • . . 11 5. Location map, seismic profile network utilized in the study, and time-isopach map, Morro do Chaves, Coqueiro Seco and Ponta Verde Formations, Sergipe-Alagoas Basin, Brazil . • • • . . • 14 6. Simplified Bouguer map, Sergipe-Alagoas Basin, showing major gravimetric lows 16 7. Isopach map, Morro do Chaves and Coqueiro Seco Formations, Sergipe-Alagoas Basin, Brazil . . . 18 8. Representative type-section, Morro do Chaves Formation (Well 1-CS-l-AL} 22 9. Representative type-section, Coqueiro Seco Formation (Well l-BC-1-AL} . 24 10. Structural framework map, Sergipe-Alagoas Basin, Brazil 29 xi Figure 11. Principal fracture systems, Sergipe­Alagoas Basin, Brazil . . . . . • . 31 12. Generalized stratigraphic cross section A-A', strike section, Alagoas Sub-basin rift sediments, Sergipe-Alagoas Basin, Brazil . . . . . . . . . . . . . . . . . . 33 13. Generalized stratigraphic cross section B-B', dip section, Alagoas Sub-basin rift sediments, Sergipe-Alagoas Basin, Brazil • 34 14. Generalized stratigraphic cross section c-c•, dip section, Alagoas Sub-basin rift sediments, Sergipe-Alagoas Basin, Brazil • 35 15. Generalized stratigraphic cross section A-A', strike section, Rio sa'o Francisco Sub-basin rift sediments, Sergipe-Alagoas Basin, Brazil • • • • . • . • • • • • • . 36 16. Generalized stratigraphic cross section B-B', dip section, Rio Sao Francisco Sub-basin rift sediments, Sergipe-Alagoas Basin, Brazil 37 17. Schematic types of lake models: (1) Open or drainage lake model, and (2) Closed lake model • • • . • 48 18. Schematic diagram of basic types of inflows (hyperpycnal, homopycnal, and hypopycnal) and resultant type of deltas . 51 19. Types of inflow in thermally stratified lakes (oligotrophic lakes), showing underflows, interflows, and overflows 53 20. Map of net elastic facies, Coqueiro Seco and Morro do Chaves Formations, Sergipe-Alagoas Basin, Brazil • . . • . • • . • 55 xii Figure 21. Map of net coarse elastic facies, Coqueiro Seco and Morro do Chaves Formations, Sergipe-Alagoas Basin, Brazil • • . • • • 56 22. Map of percentage of coarse elastic facies, Coqueiro Seco and Morro do Chaves Formations, Sergipe-Alagoas Basin, Brazil . • • • • • • 57 23. Map of net limestone facies, Coqueiro Seco and Morro do Chaves Formations, Sergipe-Alagoas Basin, Brazil • • . 58 24. Map of net massive limestone facies, Morro do Chaves Formation, Sergipe-Alagoas Basin, Brazil • • • . • • • • • . • • • • 61 25. Morro do Chaves carbonate platform system photomicrographs (A to H) • 63 26. Map of net coarse elastic facies, map of , • N Morro do Chaves Formation, Rio Sao Francisco Basin, Sergipe-Alagoas Basin, Brazil • • . . • 72 27. Typical electric log pattern, Morro do Chaves carbonate platform system (Well l-RJ-1-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil . . . • • • • . • . 73 28. Typical electric log pattern, Morro do Chaves carbonate platform system (Well l-SES-47) Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil • • • • 74 29. Typical electric log pattern, Coqueiro Seco fan delta and slope systems (Well l-SES-23) Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil • • • • 78 xiii Figure 30. Map of net non-massive limestone facies, Coqueiro Seco Formation, Sergipe-Alagoas Basin, Brazil • • • . • . • . • 80 31. Typical electric log patterns, Coqueiro Seco Formation, Sergipe-Alagoas Basin, Brazil • • . • • • • • • . . . . . . . . . 82 32. Schematic representation of various Mid-Continent fluvial-deltaic sandstones and respective E-log patterns • • • • • . 84 33. Isopach map, serrate and digitate E-log pattern, Coqueiro Seco Formation, Sergipe-Alagoas Basin, Brazil • • • • 86 34. Isopach map, blocky and serrate E-log patterns, Coqueiro Seco Formation, Sergipe-Alagoas Basin, Brazil • • • • 87 35. Schematic relationship between operational units {A to H) and depositional systems, Coqueiro Seco and Morro do Chaves Formations, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • • • • • 90 36. Schematic models of constructive (1) and destructive (2) shelf-slope systems, associated with sustained sediment supply and with sediment-starved slope and canyon excavation, respectively • • • • • 91 37. Schematic block diagram of facies rela­tionships during deposition of slope operational units B-1 and B-2, Coqueiro Seco Slope System, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • • • • • • 94 38. Map of net coarse elastic facies, serrate and digitate E-log patterns, Sergipe-Alagoas Basin, Brazil • • • • • . . • . • 95 xiv Figure 39. Typical electric log pattern, Coqueiro Seco slope system, Jequia area (Well 1-JC-l-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil . • • . • . • 97 40. Schematic model of turbidite fan (l)~ and hypothetical vertical facies sequence in a prograding slope system (2). (From Walker, 1978) • . • • • . • • • • • • 99 41. Typical electric log pattern, operational unit B-1, Coqueiro Seco slope system, Jequia area (Well 1-JA-l-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • 101 42. Map of net shale facies, operational unit B-1, Coqueiro Seco slope system, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • 102 43. Typical electric log pattern, Coqueiro Seco slope system, Boca do Caixa area (Well l-BC-1-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil . • . • 104 44. Typical electric log pattern, operational unit B-2, Coqueiro Seco slope systems, Coqueiro Seco area (Well l-CS-1-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • • • • • • • • • • . • • • 105 45. Representative core of carbonate facies, operational unit B-2, Coqueiro Seco slope system, Coqueiro Seco area (Well l-CS-1-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil 107 46. Coqueiro Seco slope system (B-2 operational unit) photomicrographs (A to F). • • • . . 108 47. Map of net coarse elastic facies, operational unit B-2, Coqueiro Seco slope system, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • . . . • . . . • . • 114 xv Figure 48. Map of net limestone facies, operational unit B-2, Coqueiro Seco slope system, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • • • . . • . • . • 115 49. Representative core, Coqueiro Seco slope system, Jequi' area (Well 3-JA-4-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil . . . . . . . . . . . . . . . . . . 117 50. Representative core, Coqueiro Seco slope system, Ponta Verde area (Well l-PV-1-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil . • • . 118 51. Representative core, Coqueiro Seco slope system, Frances area (Well 1-FS-l-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • • • 119 52. Schematic block diagram of facies rela­tionships during deposition of operational units C and D, Coqueiro Seco slope system, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • • • • • • • . • . • • • • • • 121 53. Map of net coarse elastic facies, operational unit C, Coqueiro Seco slope system, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil . • • • • • • . . • . . • • 123 54. Map of net coarse elastic facies, operational unit D, Coqueiro Seco delta and slope systems, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • . • • • • 124 55. Map of net coarse elastic facies, based on blocky and massive E-log patterns, Sergipe-Alagoas Basin, Brazil . • • • • • 126 xvi Figure 56. Map of net coarse elastic facies, operational unit E, Coqueiro Seco fluvial­deltaic, fan delta, and slope systems, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil . • . . . . • . • • • • . . . . 130 57. Map of net coarse elastic facies, operational unit F, Coqueiro Seco fluvial­deltaic, fan delta and slope systems, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • • • • • • • • • • • • . . • . 131 58. Map of net coarse elastic facies, operational unit G, Coqueiro Seco fluvial­deltaic system, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • • • • 133 59. Map of net coarse elastic facies, operational unit H, Coqueiro Seco fluvial-deltaic system, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil . 135 60. Isopach map of Ponta Verde Formation, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • . • • • • . . • . • . • . 137 61. Typical electric log pattern, Coqueiro Seco fluvial-deltaic system, Jequia area (Well 1-JC-l-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil . • • • 139 62. Typical electric log pattern, Coqueiro Seco fluvial-deltaic system, Lagoa Manguaba area (Well 2-LMST-1-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin • . . 141 63. Typical electric log pattern, Coqueiro Seco fluvial-deltaic system, Boca do Caixa area (Well l-BC-1-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil . . . • • . 142 xvii Figure 64. Representative core, Coqueiro Seco fluvial-deltaic system, Jequia area (Well l-JA-1-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • • • • 144 65. Representative core, Coqueiro Seco fluvial-deltaic system, Tabuleiro dos Martins area (Well 3-TM-2-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • 145 66. Representative cores, Coqueiro Seco fluvial-deltaic system, Coqueiro Seco area (Well 1-CS-l-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil 146 67. Representative cores, Coqueiro Seco fluvial-deltaic system, and Ponta Verde deep lacustrine system, Satuba area (Well l-SA-1-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil 147 68. Schematic block diagram of facies relationships during deposition of Coqueiro Seco fluvial-deltaic, fan delta and slope systems, operational units E, F, G, and H, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • • • • • . • • 149 69. Typical electric log pattern, Coqueiro Seco slope/basin systems, Pia9abu9u area (Well l-PIA-1-AL), Rio Sao Francisco Sub-basin Sergipe-Alagoas Basin, Brazil 152 70. Typical electric log pattern, Coqueiro Seco fan delta system, Serrao area (Well l-S0-1-SE), Rio Scfo Francisco Sub-basin Sergipe-Alagoas Basin, Brazil 155 71. Typical electric log pattern, Coqueiro Seco fan delta system, Praia de Santana area (Well l-PS-1-SE), Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil • 156 xviii Figure 72. Comparative stratigraphic column and depositional systems operative during the Morro do Chaves, Coqueiro Seco, and Ponta Verde Formations deposition in the Rio Sao Francisco and Alagoas Sub-basins, Sergipe-Alagoas Basin, Brazil • • • • • • 158 73. Dip seismic profile composed of profiles 42-RL-7, 42-RL-4, and 28-RL-336, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • 163 74. Dip seismic profile 27-RL-412, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • 165 75. Schematic conceptual models of slope types associated with types of sedimenta­tion control: I--Uplap {tectonic control}; II--Offlap {sediment control}; III--Onlap {sediment starved}; and IV--Carbonate shelf-slope • • . • • . 166 76. Dip seismic profile 42-RL-22, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • • • • • • • • • • . • • • 167 77. Strike seismic profile composed of profiles 42-RL-169, 42-RL-157, 42-RL-156, 42-RL-153, and 42-RL-151, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • • • 170 78. Dip seismic profile composed of profiles 42-RL-100, 7-RL-406, 37-RL-42, and 28-RL-176, Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil • • • . • • 173 79. Dip seismic profile composed of profiles 37-RL-117 and 48-RL-668, Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil 175 80. Location map, coastal basins East Brazilian and West African Coast • 177 81. Northeast-southwest and east-west cross sections of Gabon Basin, Africa • . • • . 179 xix Figure 82. Generalized geologic map, Uinta and Green River Basins, Utah and Wyoming, U.S.A••• 182 83. Schematic diagram illustrating deep-water lacustrine sedimentation model, Ridge Basin, California • • • . • • • 185 84. Location and distribution map of the Great African Rift System 189 85. Regional location map and structural setting of Lake Rudolf (l)~ and location map of Rudolf Basin and Lake Rudolf (2) 191 86. Geomorphologic units of the Omo delta Lake Rudolf . • • • • • • • • • • • • 193 87. General stratigraphy of Lake Kivu bottom sediments • . • • • • . • • • • • • 196 88. Schematic block diagram of Lake Geneve (Lac Leman) • • • . • • • 199 89. Location map, oil and gas fields and non­commercial shows, Morro do Chaves and Coqueiro Seco Formations, Sergipe-Alagoas Basin, Brazil • • • • • • • • • 205 90. Location map, oil and gas fields and non­ commercial shows, Coqueiro Seco slope system, Sergipe-Alagoas Basin, Brazil 209 91. Representative cores and typical section, Coqueiro Seco slope and delta systems, Cidade de S~o Miguel dos Campos Field, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • • • . • • . • . • • • . 210 92. Structural map on top of gas-bearing sandstone, Cidade de Sao Miguel dos Campos Field • • • • . • • . • 212 xx Figure 93. Location map, oil and gas fields and non­commercial shows, Coqueiro Seco fluvial­deltaic and fan delta systems, Sergipe-Alagoas Basin, Brazil 216 94. Structural cross section, Coqueiro Seco Field, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil. • ••..• 217 95. Generalized depositional model consisting of facies tract and facies distribution (plan view) of Morro do Chaves and Coqueiro Seco Formations, Sergipe-Alagoas Basin, Brazil • • • • . • • • . • • • • • 222 96. Paleogeographic map, Morro do Chaves and Coqueiro Seco Formations, Sergipe-Alagoas Basin, Brazil . • • • • • . • . • . . • 224 Plates I. Stratigraphic cross section 1-1', strike section, Coqueiro Seco, Morro do Chaves and Ponta Verde Formations, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil . 231 II. Stratigraphic cross section 2-2', strike section, Coqueiro Seco, Morro do Chaves and Ponta Verde Formations, Alagoas Sub-basin, Brazil • • • • • . • . 233 III. Stratigraphic cross section 3-3', strike section, Coqueiro Seco, Morro do Chaves, and Ponta Verde Formations, Alagoas Sub-basin, Brazil • . • . • • . • 235 IV. Stratigraphic cross section A-A', dip section, Coqueiro Seco and Morro do Chaves Formations, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil . • • • 237 xxi Plates v. Stratigraphic cross section B-B', dip section, Coqueiro Seco and Morro do Chaves Formations, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • • • • 239 VI. Stratigraphic cross section C-C', dip section, Coqueir.o Seco and Morro do Chaves Formations, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil . • • • . 241 VII. Stratigraphic cross section D-D', dip section, Coqueiro Seco, Morro do Chaves, and Ponta Verde Formations, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • 243 VIII. Stratigraphic cross section E-E', dip section, Coqueiro Seco, Morro do Chaves, and Ponta Verde Formations, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil • 245 IX. Stratigraphic cross section F-F', dip section, Coqueiro Seco, Morro do Chaves, and Ponta Verde Formations, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil . 247 X. Stratigraphic cross section 1-1', strike section, Morro do Chaves Formation , Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil • • . • • • • • 249 XI. Stratigraphic cross section 2-2', strike section, Coqueiro Seco and Morro do Chaves Formations, Rio Sa~o Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil . 251 XII. Stratigraphic cross section 3-3', strike section, Coqueiro Seco and Morro do Chaves Formations, Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil . 253 xxii Plates XIII. Stratigraphic cross section 4-4', strike section, Coqueiro Seco and Morro do Chaves Formation, Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil • 255 XIV. Stratigraphic cross section A-A', dip section, Coqueiro Seco and Morro do Chaves Formations, Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil • • • . 257 xv. Stratigraphic cross section B-B', dip section, Coqueiro Seco and Morro do Chaves Formations, Rio S~o Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil • • • • 259 XVI. Stratigraphic cross section C-C', dip section, Coqueiro Seco and Morro do Chaves Formations, Rio SaNo Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil 261 XVII. Stratigraphic cross section D-D', dip section, Coqueiro Seco and Morro do Chaves N Formations, Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil • . • • • • 263 xxiii INTRODUCTION Sergipe-Alagoas Basin, Brazil is located in Sergipe and Alagoas States along the northeast coast of Brazil between latitude 9° and 11°30 1 s and longitude 37° and 35°30' w. The onshore and offshore basin covers 26,000 km2 (Fig. 1). This investigation deals with the Morro do Chaves and Coqueiro Seco Formations. It is intended to define the depositional systems that operated during the middle of rift evolution and resulted in deposition of a thick elastic-carbonate stratigraphic section con­tained between the Penedo and Muribeca Formations (Fig. 2). Additionally, the petroleum potential of this section was evaluated by establishing the several types of plays that can be expected from the structural and stratigraphic style exhibited by the Morro do Chaves and Coqueiro Seco Formations in each sub-basin, as well as the nature of probable reservoirs and source rocks. Sergipe-Alagoas Basin contains a maximum of about 10,000 meters of sedimentary strata. The sedimentary 1 Figure~l~.:---r:~;-;~::--~~~­ area of map, Se Basin, Brazil, ~·p;=A~:;;::::-:::-::-~~~ I study. showi~g Location rgipe-Alagoas column was deposited during three tectonic phases (Fig. 2}; the pre-rift, rift, and post-rift phases, equivalent to continental, lake, gulf and sea sequences of Asmus and Porto (1980). Pre-Rift Cratonic Continental Phase Upper Jurassic strata are composed of bolson and playa lake sediments which were buried by alluvial fan deposits (Bananeiras, Candeeiro,and Serraria Formations, respectively). These strata overlie remnant Paleozoic sediments (Batinga and Aracare Formations}. Upper Jurassic pre-rift strata up to 400 meters thick are distributed throughout the basin, and are also known as the continental sequence. Coarse sandstones deposited by alluvial fan systems constitute the best petroleum reservoirs in the basin. Rift Phase Lower Cretaceous rift strata are composed of very complexly related carbonate and elastic sediments that were deposited in lacustrine environments during initial breakup of Pangea and the separation of the South American and African continents. The rift strata can be divided into three sequences equivalent to lake and gulf sequences of Asmus and Porto (1980), deposited in response to the tectonic and sedimentary evolution of the rift basin (Fig. 3). In this dissertation, a sedimentary sequence is considered a stratigraphic unit composed of a succession of genetically related facies or depositional systems, deposited by a continuous and single tectonic episode, normally bounded by unconformities. Examples of major depositional sequences are illustrated by Galloway and Brown (1973) in West Texas and Vail et al. (1977) in offshore Northwest Africa. Since the sedimentary sequence is bounded by unconformities, it constitutes a time-stratigraphic unit. In the same way, a Holocene depositional system, according to Fisher and Brown (1979) is an assemblage of related facies, environments and associated processes; an ancient depositional system is therefore, a stra­tigraphic analogue or three-dimensional assemblage of sedimentary f acies linked genetically by inferred sedi­mentary environments and depositional processes. SEQUENCE I This basal Lower Cretaceous sequence is composed of (1) lacustrine starved basin, slope and turbidite fan systems of the Barra de Itiuba Formation; (2) lacustrine SEO. JI[ JI I fluvial-deltaic system of the Penedo Formation in the Alagoas, Coruripe, and Rio Sao Francisco Sub-basins; and (3) contemporaneous lacustrine fan delta system of the Rio Pitanga Formation (Fernandes et al., 1981). A elastic and carbonate platform system (The Morro do Chaves Formation of official PETROBRAS usage) on the Sergipe Platform and in the adjacent Mosqueiro Sub-basin is actually a platform system which is equivalent to the Rio Pitanga, Penedo, and Barra de Itiuba Formations, and is informally called the "Caioba Platform System" (Fernandes et al., 1981). Sequence I is 3,000 meters thick in depocenters. Sandstones of the Barra de Itiuba slope system are oil reservoirs in several oil fields in the Sergipe-Alagoas Basin. SEQUENCE II Sequence II is composed of (1) Morro do Chaves carbonate platform system, and (2) Coqueiro Seco fluvial­deltaic, fan delta, and slope systems in the Alagoas, N Coruripe and Rio Sao Francisco Sub-basins. Only one well (l-SES-7) possibly penetrated this sequence in the Mosqueiro Sub-basin. In the Alagoas Sub-basin deposi­ tion of this sequence was terminated by deposition of a lacustrine shale called the Ponta Verde Formation; in N the Rio Sao Francisco Sub-basin the top of sequence II was eroded and is truncated by a pre-Aptian unconfor­mity. Sequence II is 3,000 meters thick in depocenters and contains the least productive petroleum reservoirs of the Sergipe-Alagoas Basin. The goal of this disser­ tation is the analysis of rift sequence II. SEQUENCE III Sequence III comprises the Muribeca Formation, equivalent to gulf sequence of Asmus and Porto (1980) and occurs in all sub-basins and on most of the platform areas. Muribeca Formation consists of (1) pre-evaporite, lacustrine, fan delta and platform systems of the Maceio Member, (2) platform carbonate and evaporite systems of the proposed Paripueira Member, (3) post-evaporite lacustrine fan delta systems of the proposed Po~ao Member, (4) elastic and carbonate platform system of the Tabuleiro dos Martins Member in Alagoas Sub-basin, and (5) an unamed elastic and carbonate system in the Coruripe Sub-basin. On the Sergipe Platform and in the adjacent Mosqueiro Sub-basin this sequence comprises (6) pre-evaporite valley-fill and fan delta systems of the Carmopolis Member, which overlies a pre-Aptian unconfor­mity, (8) platform evaporite and carbonate system of the Ibura Member, and (9) a fine-grained elastic and car­ bonate platform system of the Oiteirinhos Member. This is the most widespread rift sequence in the basin. The / Carmopolis Member of the Muribeca Formation contains most of the petroleum discovered to date in the Sergipe-Alagoas Basin. Post-Rift Marine Phase Middle Cretaceous and Tertiary strata are corn-posed of (1) fan delta, carbonate shelf, and calcareous slope systems of the Riachuelo Formation, (2) carbonate platform and calcareous slope systems of the Cotinguiba Formation, (3) an onlap and offlap slope system of the Calumbi Member of the Pia~abupu Formations, and (4) con­temporaneous fan delta and carbonate platform systems of the Marituba Member of the Pia9abu9u Formation. Each phase was deposited in response to tectonic evolution of the basin ranging from initial rift-valley to a proto-oceanic gulf and ultimately into an open ocean. These phases are-represented by strata deposited in continental, rift lacustrine, and open marine passive margin environments. METHODS AND PROCEDURES The stratigraphic sequence analyzed in this investigation comprises the Coqueiro Seco and Morro do Chaves Formations of Sergipe-Alagoas Basin, which con­stitute rift sequence II (Fernandes et al., 1981). These formations were penetrated completely or partially by about 150 wells. Approximately 95 wells have pene­trated the entire sequence. Some of these wells are cut ,.,, by faults. The study used 47 wells from the Rio Sao Francisco Sub-basin and 102 wells from the Alagoas Sub-basin. No wells penetrated this sequence in the Coruripe Sub-basin, and only one well was drilled into strata that are questionably Coqueiro Seco in the Mosqueiro Sub-basin (Fig. 4). The first step in this investigation involved tabulation of all available lithologic data for the 150 wells. To obtain these data, borehole geophysical logs, sample descriptions, and other stratigraphic data of all wells penetrating the sequence were consulted. Besides composite logs (1:1000 scale), bore-hole geophysical logs of each well (1:1200 scale), including combined IES (electrical-induction) and BHC (bore-hole compensated 10 / -WELL C<>Hf"OLLED CROSS lECT/ON / -SCHEMATIC CROSS SECTION l"VVV'-EROSIONAL LIMIT /FAULT NR-NOT REACHED E -ERODED Figure 4. Location map, wells utilized in the study, and location of strike and dip stratigraphic cross sec­tion, Sergipe-Alagoas Basin, Brazil. sonic) logs were used. Other logs, such as FDC (forma­tion density) and SNP (side-wall neutron porosity) or CNL (compensated neutron log) were normally available for wells where hydrocarbons have been detected, espe­cially in gas-bearing sandstones. All well data were furnished by Petroleo Brasileiro s. A. (PETROBRAS). Accuracy of lithologic data from well logs pre­sented a problem because the lithic composition of wells had been interpreted by different log technicians using different criteria to define the electrical characteris­tics of various rock types. In order to solve this problem, the lithology of 150 wells was reinterpreted to ensure uniform interpretation throughout the basin. Around 126,000 meters of hole was analysed and interpreted to produce a table of net sand, conglom­erate, shale, and limestone isolith values; thicknesses of Morro do Chaves, Coqueiro Seco and Ponta Verde Formations; percentages of each lithic type; and elastic/carbonate ratios. A base map was prepared of Sergipe-Alagoas Basin, and the location of 150 wells utilized in the study were plotted. Location of strike and dip stra­tigraphic cross sections prepared during the study are shown (Fig. 4) and are exhibited in Appendix A. Using the well-log data base, isopach maps of principal basinwide lithogenetic units and individual minor operational units were prepared in an attempt to define the geometry of depositional systems, the princi­pal source areas (input) of sediments, and the basin­fill geometry for rift sequence II. Exclusive use of well data would not permit proper delineation of deposi­tional systems, basin geometry, and sediment input, because of the limited density of wells in most areas as well as the incomplete penetration of the Morro do Chaves and Coqueiro Seco Formations by most wells in the central part of the basin. To solve this problem of limited well data, seismic data were used to correlate sequence II throughout the basin. A network of seismic profiles (Fig. 5) covering most of the onshore and off­shore area of Alagoas Sub-basin, and offshore area of Coruripe and Rio S~o Francisco Sub-basins was used to prepare a time-isopach map of the Coqueiro Seco, Morro do Chaves and Ponta Verde Formations. All seismic lines were furnished by PETROBRAS. The limited quality of the seismic profiles, especially those in onshore areas, preclude normal seismic stratigraphic interpretation, but normally they were adequate to delineate and map the general geometry of sequence II. Use of seismic data, ;.______ map, seismic profile network uti­lized in the study, and time-isopach map, Morro do Chaves, Coqueiro Seco and Ponta Verde Formations, Sergipe-Alagoas Basin, Brazil. tied to available wells and gravity data (Fig. 6}, per­mitted realistic mapping throughout the structurally complex basin where faults and unconformities had to be recognized and accounted for before the geometry of the sequence could be mapped. The final result (Fig. 5} was a time-isopach map (later converted to depth} of sequence II, which is far more reliable than maps based only on well data. The seismic mapping was in cooperation with PETROBRAS geophysicist G. Fernandes. All wells drilled near the seismic profiles were used to verify and control sequence thickness needed to convert time values to depth. A Bouguer gravity map (Fig. 6) provided by Petroleo Brasileiro S/A was used to support and verify the time-isopach map prepared of the sequence. In the onshore area the gravity map outlined the gross geometry of the basin and sub-basins. In offshore area the Bouger map displays a regular increase seaward in grav­vity values, masking possible anomalies; this is probably caused by the proximity of oceanic crust. Compilation of seismic, gravity and well data permitted construction of an isopach map of the combined Coqueiro Seco and Morro do Chaves Formations. The geom­etry and limits of sequence II are shown in Figure 7. It was then possible to construct lithologic maps; e.g., ified Bouguer map, Sergipe-Alagoas Basin, Figure 6. Simpl vimetric lows. showing major gra net elastic, net coarse elastic, net limestone and vari­ous lithic percentage maps using the isopach map of sequence II as a guide of thickness trends throughout the basin. To understand basin evolution during deposition of sequence II, dip and strike stratigraphic cross sec- N tion for Alagoas and Rio Sao Francisco Sub-basins were constructed using the top of rift sequence II as datum. The absence of wells in the Coruripe and Mosqueiro Sub-basins precluded well log cross sections for these areas. Analysis of all lithic data, as well as analysis and distribution of SP and GR log shapes provided the basis to define depositional systems and component sedi­mentary facies. Well log correlation permitted sub­division of the Coqueiro Seco and Morro do Chaves Formations into operational units (A to H) in the Alagoas Sub-basin. Net sand maps and other lithic maps were prepared of each operational unit to permit more detailed analysis of the sequence. Electric logs and lithology were used to characterize each depositional system: carbonate platform facies, slope, and delta and fan delta facies. Further discrimination of delta and slope systems were made using various electric log pat­terns such as blocky, serrate, and digitate shapes. Figure 7. Isopach map, Morro do Chaves and Coqueiro Seco Formations, Ser9ipe-Alagoas Basin, Brazil, showing the geometry and limits of rift sequence II and princi­pal depocenter areas. Once a tectonic and sedimentary framework of the basin was constructed, and the various depositional systems that infilled the sub-basins had been mapped, paleogeographic and diagrammatic models were developed. These conceptual models illustrate the inferred evolu­tion of sedimentation in each sub-basin and the effect that tectonic events imposed on sedimentary patterns. This integrated picture of Sergipe-Alagoas Basin during the mid-rift phase of development was based on all available data, using state-of-the art concepts of sedi­mentary processes, environments and facies. Finally, oil occurrence and types of traps affecting the Coqueiro Seco and Morro do Chaves Forma­tions were identified and mapped. Analysis of the structural and stratigraphic nature of Coqueiro Seco and Morro do Chaves oil fields and significant oil and gas shows defined a series of plays for each depositional system and for each structural and stratigraphic setting. Based on the principal factors controlling petroleum occurrences, these plays assist in evaluating untapped petroleum potential of sequence II. Knowledge of the distribution of depositional systems and their predic­table three-dimensional facies fabric provided a tool to predict and map favorable areas for further exploration. PREVIOUS STUDIES Since 1939 the Sergipe-Alagoas Basin has been studied, explored and drilled for petroleum, ini­tially by the CNP (National Council of Petroleum) and some small private companies, and after 1954, exclusi­ / vely by Petroleo Brasileiro s. A. (PETROBRAS), a govern­ment company. After several wildcats and a few oil and gas discoveries, the first comprehensive geologic regional study was done in 1960, based on surface and subsurface geology, gravimetry and seismic data. In 1964 Ritcher prepared a study of surface • / w geology along the Jequ1a and Sao Miguel Rivers in the Alagoas Sub-basin; he described outcrops of Morro do Chaves and Coqueiro Seco Formations. Besides Ritcher's study several other surface studies (Perrela, 1963; Ponte et al. 1964; Ponte et al. 1966; Ponte et al. 1967), were carried out to complete the geological map of the Sergipe-Alagoas Basin. A compilation based on these studies was published by DNPM and PETROBRAS (Ponte, 1975). Discovery of several oil fields in the Sergipe-Alagoas Basin in the mid-sixties accelerated 20 petroleum exploration throughout the basin. Consequent­ly, studies were undertaken to evaluate the petroleum potential of all sedimentary sequences. At the end of the sixties, a team of PETROBRAS geologists (Schaller, 1969) critically reviewed the lithostratigraphy, biostratigraphy, and chronostratigraphy of the basin, using the current rules of International and American Code of Stratigraphic Nomenclature. At that time a new stratigraphic column was defined which is still offi­cially accepted. The Morro do Chaves Formation was defined by Schaller (1969) as a sequence of coquinoid limestones and dolomites containing elastic intercalations, overlying the arenaceous Penedo Formation and underlying the elastic Coqueiro Seco Formation (Fig. 8). The Morro do Chaves Formation is characterized by carbonate beds which are normally chalky and light colored~ brown and green shale beds intercalated with fine-to medium­grained, calcareous sandstones are common. Thickness of the Morro do Chaves Formation ranges from a few meters to more than 1500 meters. The upper and lower contacts with the Coqueiro Seco and Penedo Formations are grada­tional. RES. PO COQUE!RO SECO 2 547 ( -2 452) M. CHAVES Interca.la.tions of LTMESTONE, white, chalky; COQUINA, ~rey to cream; SHALE, grey; and SANDSTONE fine-to mediuw­gra.ined, rarely coarse-gr8in ed, calcareous. PENEDO Figure 8. Representative type-section, Morro do Chaves Formation (Well l-CS-1-AL), as defined by Schaller, 1969. The Coqueiro Seco Formation was defined by Schaller (1969) as a section of sandstones and bitumi­nous shales overlying the Morro do Chaves Formation (limestones) and underlying green shales of the Ponta Verde Formation. The Coqueiro Seco Formation is com­posed of thick sections of alternating sandstone and shale beds. Sandstones are immature, fine-to medium-grained, and may contain coarse sand and conglomerate beds, kaolinite, abundant feldspar grains, and shale. Shales are normally brown to greenish gray, sub-bituminous, commonly laminated and may contain very thin dolomitic limestone beds. The Coqueiro Seco Formation grades downward into the Morro do Chaves Formation, and upward into the Ponta Verde Formation (Fig. 9). The Morro do Chaves and Coqueiro Seco Formations are Early Cretaceous (Wealden?) in age, and have been I chronologically classified in the Brazilian Jequia and Alagoas Stages. Absence of marine fossils in these for­mations precludes good correlation with international chronostratigraphy. A series of stratigraphic studies was carried out to evaluate the petroleum potential of each rift sequence; Morro do Chaves and Coqueiro Seco Formations were studied by Ojeda (1969). The study addressed two PV cs "4..'JGUABA SHALE, brown and dark grey, subbituminous, intercala.ted with SAND­ ROrEIO STONE, medium-to coarse-grained, mica­ceous and clayey. FRANC is SANDSTONE, medium-to coarse-grained,quartzos and i'eldspa.tic, inter­calated with SHALE, brown, subbituminous. ARAM SIPE SAhDSTONE, medium-to fine-grained, kaolin­ i tic, with intercala­ tions of SHl\LE, green­ ish grey and dark brown, and LIMY.:STONE, white and pinky, cryptocrystalline (Calcilutite) to chalky. .::core 2547 '-24521 _R_R_O_O_O__:..:VE_S;:...;..=..:"'-­ CH~A~ MC Figure 9. Representative type-section, Formation (Well 1-BC-l-AL), as Coqueiro Seco defined by Schaller, 1969. N isolated onshore areas in the Rio Sao Francisco and Alagoas Sub-basins. Among several conclusions, Ojeda ~ believed that Barra de Sao Miguel (well l-BSM-1-Al), Lagoa Jequi~ and s:o Miguel dos Campos (well l-SMC-3-AL) areas exhibit conditions that justified an exploratory program, but almost all other areas were considered geologically uninteresting or sufficiently tested. Also in 1969, Sa published a study of Coqueiro Seco Field, the first oil and gas field discovered in the Coqueiro Seco Formation. Oil and gas is produced from Coqueiro Seco reservoirs (sandstones) directly beneath the Penta Verde Formation. Present production from Coqueiro Seco Field is 20 mld, and the cummulative production in June, 1980, was 55.240 m3. The Coqueiro Seco Field is situated on the upthrown block of the Tabuleiro dos Martins Fault, and is a structural (faulted anticline) trap. Sa; concluded that the oil occurrence was areally limited but that gas zones might be extended. Beltrami and Della Favera (1977) divided Sergipe-Alagoas Basin into four sub-basins similar to the present investigation. The section between the Penedo Formation and the top of the rift strata was divided into four depositional cycles; the first and second cycles correspond to rift sequence II. The cycles are based on concepts of Karagodin (1975), and are composed of coarsening or fining-upward sequences reflecting lithology and bed thickness variations as indicated on electric logs. Cycles I and II of Beltrami and Della Favera, equivalent to Morro do Chaves, Coqueiro Seco and Ponta Verde Formations, were interpreted to represent a large sedimentary lobe with source points located in the Lagoa Manguaba (well 2-LMST-1-AL) and Japaratinga (well 2-JRST-1-AL) areas along the onshore margin of the N Alagoas Sub-basin, and in the Serrao (well l-S0-1-SE) ~ . area along the onshore margin of Rio Sao Francisco Sub-basin. These depositional cycles were inferred to represent regressive-transgressive sedimentation, and the cyclic origin is believed by Beltrami and Della Favera, to be mainly of tectonic origin. Piccard and High (1968) suggested that cyclicity in lacustrine environments resulted from climatic variations controlling lake levels. Nevertheless, Beltrami and Della Favera believed that seasonal climatic variations, as well as lateral variation of environments, were not important factors and generated only second-order cycles; they considered the tectonic factor to be res­ ponsible for cyclic sedimentation in Sergipe-Alagoas Basin. The most recent study of the Morro do Chaves and Coqueiro Seco Formations by Fernandes et al., (1981) involved a basin analysis of the entire rift phase of Sergipe-Alagoas Basin, and an analysis of oil and gas plays. REGIONAL TECTONIC SETTING The structural framework of the Sergipe-Alagoas Basin, which was established during the rift phase, is a series of regional tensional faults forming half-grabens called sub-basins separated by elevated horst blocks defining stable platforms (Fig. 10). Post-rift tectonic movements were of minor importance and consisted of small readjustments along older faults. The principal post-rift movement was subsidence and seaward tilting of the sub-basins. These readjustments may have caused rearrangement of oil accumulations initially generated and trapped during the rift phase. Rifting tensional forces in the crust probably developed as the result of a thermal anomaly in the ~thenosphere (Hsu, 1956; Sleep, 1971, 19731; Falvey, 1971), created an initial bulge in the crust, and resulted in formation of normal faults oriented generally north-south. Examples include the Coruripe, Piranhas, Ponta dos Mangues, Rio Vaza Barris, and Tabuleiros dos Martins Faults. The age of these faults is approximately Late Jurassic to Early Cretaceous. They bound Alagoas, Coruripe, Rio sto Francisco and 28 t!AAl1 Mosqueiro sub-basins and the principal platforms, such as the Sergipe and Palmeira Alta. Many minor grabens and horsts exist throughout the sub-basins. Once the South American and African continents began drifting in the Early to Late Cretaceous a new stress pattern was developed. Rotation occurred because the spreading rate at the south was faster than in the north. These differential spreading rates created a complex system of both compressional and tensional stress (Perrela, 1963; Ojeda and Fugita, 1974; Fig. 11) which resulted in a new set of faults that cross-cut the older system. These east-west trending faults, such as the Atalaia, Penedo, and Japaratuba Faults, are oriented almost 90° to the earlier rift faults. These younger faults coincide in a general sense with the pre­sent landward limit of Coqueiro Seco and Morro do Chaves deposition. This was the final tectonic event in the basin. It created a series of horst blocks such as the Penedo, Japoata ~ and Muribeca horsts, showing that there was principally vertical fault displacement associated with this event. Termination of continental breakup in the northernmost area of rift (Pernabuco-Paraiba area) marked the beginning of continuous tilting presumably 36° 09° ~o I I co -compression cz -shear tr -traction B 10° 10° --Basement Liniments Rift -Drift iSQ 0 8 -·-Rift -Drift C Drift 11° D··············· Dritt · 0 25 50Km Figure 11. Principal fracture systems, Sergipe-Alagoas Basin, Brazil, associated with rift and drift phases. (From Ojeda and Fugita, 1972). as a direct response to cooling of the continental plate and the newly generated oceanic crust. Rift structure and its control of the geometry of the sub-basins and the definition of the main deposi­tional systems that infilled these sub-basins are shown on diagrammatic sections across the Rio Sao Francisco and Alagoas Sub-basins (Figs. 12 to 16). Dip-oriented sections demonstrate very clearly a half-graben struc­tural style; strike sections parallel the structural grain of faulted basins. All faults detected in the Sergipe-Alagoas Basin are nearly vertical normal faults, displaying displacements up to 5,000 meters; e.g. Coruripe, Atalaia and Tabuleiro dos Martins Faults (Fig. 10). The landward limit of the Sergipe-Alagoas Basin is marked by a series of faults, except along the Palmeira Alta Platform which forms a large domal area separating the Coruripe and Alagoas Sub-basins from the Rio Sao Francisco Sub-basin (Fig. 10). The fault system along the western margin is oriented approximately N 45° E. Data derived from several wells drilled very closed to the northeast faulted margin of the basin indicate that the present faulted margin was not its margin A ~t--r-~--~~~~......--~~~--r___j~IB£LJ't!f,---T~~~~~~j'f:h::i;;_ Figure 12. Generalized stratigraphic cross section A-A' dip section, Alagoas Sub-basin rift sediments, Sergipe-Alagoas Basin, Brazil, showing depositional systems. c I c 'TAB DOS MARTINS FAULT D FAN DELTA DDELTA OsLOPE 1~ ... 1BASIN D PLATFORM (:2.J EVA PO RI TES D FAN! BOLSON PALEOZOIC ALAGOAS SUB-BASIN GENERALIZED STRIKE CROSS -SECTION C -CI Figure 14. Generalized stratigraphic cross section c-c•, strike section, Alagoas Sub-basin rift sediments, Sergipe-Alagoas Basin, Brazil, showing depo­sitional systems. + km ~(I FAN DELTA [ ]DELTA D PLATFORM I;J PALEOZOIC ~ L:'._j E VAPORITES D VALLEY-FILL RIO SAO FRANCISCO SUB-BASIN GENERALIZED DIP CROSS -SECTION CORUR/P E ? FJ\ULT D SLOPE D FAN! BOLSON Figure 15. Generalized stratigraphic cross section A-A', dip section, Rio Sao Francisco Sub-basin rift sediments, Sergipe-Alagoas Basin, Brazil, showing depositional systems. during deposition of sequence II. During deposition of pre-rift and rift strata (Bananeiras, Serraria, Barra de Itiuba, Penedo, Coqueiro Seco and Morro do Chaves Formations) the landward limit of the basin was inland from its present position. In the northernmost part of the Sergipe-Alagoas Basin, where the rift system ter­minates, the present basinal limit probably corresponds to the boundary during rifting. During deposition of the uppermost rift sequence (Muribeca Formation) the present faulted limits of the basin were finally established (Aptian-Albian). Synsedimentary faults (growth faults or faults associated with salt tectonics) occur in the basin, but they are not directly associated with the genesis of the rift basin structure. Small contemporaneous faults displace the Barra de Itiuba slope system but of the Coqueiro Seco-Morro do Chaves sequence, this type of fault was not detected in the slope system. Salt tec­tonism occurred within Sergipe-Alagoas Basin, but salt­controlled deformation mainly affected passive-margin strata and had no impact on the Coqueiro Seco and Morro do Chaves Formations. The complexity of the structural activity increased during the rift phase. The initial rift tee­tonic episode in Sergipe-Alagoas Basin involved subsi­dence of a broad continental basin in which bolson, playa-lake, and alluvial fan systems were operative (Bananeiras and Serraria Formations). The second epi­sode involved the relatively simple tectonic style of half-grabens that were infilled by lacustrinedeltaic, 1slope and basin sediments (Barra de Itiuba and Penedo Formations). Where tectonic activity was more intense, such as in Japaratuba and Mosqueiro Lows (Fig. 16), alluvial fans and fan deltas deposited coarse elastics associated distally with shelf limestone facies (Rio Pitanga Formation and equivalent "Caioba" carbonate and elastic platform). A third episode of rifting further faulted and defined the sub-basins and increased the subsidence rate in the main depocenters. At that time very sandy slope and deltaic systems occupied a marginal carbonate plat­form (Coqueiro Seco and Morro do Chaves Formations) in all sub-basins except perhaps the Mosqueiro sub-basin. Platform areas were stable areas (e.g., Sergipe and Palmeira Alta) where only limited deposition, princip­ally carbonates, occurred: subsequent erosion created an extensive unconformity (pre-Aptian unconformity) (Figs. 15, 16). The Alagoas Sub-basin was segmented into several subsidiary lows (Sininbu, Logoa Manguaba, Paripueira) which resulted in complex sedimentary facies patterns (Figs. 12, 13, 14). A fourth and major tectonic episode followed deposition of lacustrine shales of the Penta Verde Formation. This final tectonic episode of the rift phase created major faults, rejuvenated other pre­existing faults, formed large grabens and essentially defined the limits of the rift sequence. During this episode alluvial fans and fan deltas were responsible for depositing the Muribeca Formation over most of the basin, including the Sergipe Platform and probably other platform areas that were subsequently eroded. A great volume of coarse siliciclastic sediments were deposited in wedges paralleling the principal faults. Evaporites and carbonates were deposited during tectonically inac­tive periods when the input of elastics was temporarily terminated. Over the pre-Aptian unconformity on the Sergipe Platform, valley-fill conglomerates and sand­stones were deposited and later covered by an extensive, shallow evaporitic and carbonate facies, producing the best petroleum play in the basin (Figs. 12, 13, 14, 15, 16). Tectonic events progressively shifted eastward indicating the end of the rift phase and initiation of passive-margin tectonic conditions (Fernandes et al., 1981). During the passive-margin phase, tilting and accentuated seaward subsidence permitted progressive, extensive marine inundation of the basin with conco­mitant development of fan delta, carbonate platform and slope systems. Deposition during this time is represented by Riachuelo fan-delta, carbonate platform and slope systems, the Cotinguiba carbonate platform and calcareous slope systems, the Calumbi onlap and offlap slope systems, and the Marituba fan-delta and the Mosqueiro carbonate platform systems. These deposits were influenced by the buried tectonic rift framework which controlled the thickness and facies distribution of these later marine and nearshore sediments. Consequently, platforms, shelf edges, submarine canyons and slope and basin depocenters conform with the struc­tural elements of the rift. (Fernandes et al., 1981). In summary, the fundamental framework of Sergipe-Alagoas basin was established during the rift phase. Rift tectonism controlled the nature and distri­bution of all lithofacies deposited during the rift phase and determined the spacial distributions of petro­leum reservoirs, source beds and traps. DEFINITION OF SUB-BASINS As shown in Figures 10 and 12 to 16, the Sergipe­Alagoas Basin was divided into several sub-basins. Each sub-basin exhibited its own unique structural evolution and depositional systems during deposition of the Morro do Chaves and Coqueiro Seco Formations. Alagoas Sub-basin This sub-basin is the largest sub-basin and is situated entirely in Alagoas State. It is defined by gravity, seismic and well data and composed of several isolated lows or more negative areas where elastic depo­sition dominated and adjacent stable platforms where car­bonate deposition occurred. The Alagoas Sub-basin is bounded on the south, west and southwest by faults (Coruripe, Tabuleiro dos Martins, and other bordering faults), and by stratigraphic pinchout or erosion on the north and northeast. Offshore (east) the basin termi­ nates against the eroded Maceio Horst (Fig. 10, 14). Principal lows defined in the Alagoas Sub-basin are Sininbu, Lagoa Manguaba and Paripueira, all of which persisted during Morro do Chaves and Coqueiro Seco 43 deposition (Figs. 12, 13, 14). The present limits of the Alagoas Sub-basin, as demonstrated by the various lithic maps, are not the original depositional limits of the sub-basin, which appear to have been much larger at the time of deposition. Coruripe Sub-basin This sub-basin in Alagoas State is limited at the southeast end of the Alagoas Sub-basin by the Coruripe and Tabuleiro dos Martins Fault (Fig. 10). It was not an isolated sub-basin during Morro do Chaves and Coqueiro Seco deposition but was a southward exten­sion of the Alagoas Sub-basin that paralleled the Sininbu Low. No wells have been drilled in the sub-basin and seismic data quality is poor. The Coruripe Sub-basin apparently became an independent, well-defined sub-basin as a result of Aptian tectonics asso­ciated with Muribeca deposition. ~ Rio Sao Francisco Sub-basin Located in both Sergipe and Alagoas States, this is. the second largest sub-basin in Sergipe-Alagoas Basin. It is bounded by faults (Piranhas, Japaratuba N and large platform areas (Sergipe, Japoata, and Palmeira Alta) on the south, southwest and west (Fig. 10). Erosional limits terminate the subbasin on the north and northeast: offshore (east) the basin was uplifted and eroded during the Aptian tectonic episode (Figs. 15, 16,). An offshore extension of the Palmeira Alta Plat­ ~ form separates the Rio Sao Francisco Sub-basin from the Coruripe and Alagoas Sub-basins (Fig. 10). Evidently, present limits of the sub-basin do not represent the original margins, which were established during the Aptian tectonism when the Penedo and Japoata ~ Horsts were formed. Mosqueiro Sub-basin This southeast sub-basin within Sergipe-Alagoas Basin is defined by gravity and seismic data, as well as by borehole information (Fig. 10). The well l-SES-7 drilled near the southwest margin of the sub-basin penetrated a thick coarse elastic section paleontologi­cally dated as time-equivalent to the Coqueiro Seco ' N Formation (Jequia Stage) and Penedo Formation (Sao ~ Sebastiao Stage). No other wells drilled in the sub-basin reached the pre-Aptian section because of the very thick Muribeca section present in the sub-basin. Since the Mosqueiro Sub-basin was active during the deposition of rift sequence I and III, it is concluded that it could have been active also during deposition of Sequence II. STRATIGRAPHY General Considerations and Basic Concepts Initial rift development in Sergipe-Alagoas Basin produced several major half-graben sub-basins separated by stable platforms. After deposition of the Penedo fluvial-deltaic sediments throughout the basin, renewed tectonic activity initiated the deposition of Morro do Chaves and Coqueiro Seco Formations (Sequence II) within an extensive lake, which occupied each sub-basin. Lacustrine sedimentation in rift-valleys is still poorly understood. According to Kinsman (1975) rates of subsidence of rift-valley floors may exceed O.SKm/MY and source areas are normally small in area. Nevertheless, rates of denudation are very high and sediment supply may keep up with rates of subsidence, permitting devel­opment of fluviatile facies throughout an entire basin. However, if subsidence rates are high relative to sedi­ment supply, the lakes may become starved basins. Such extensive and deep rift-valley lakes may have floors that lie below sea level as in the present Lake Tanganyika. This environmental setting may explain the lower Coqueiro Seco section, which is composed of dark 46 bituminous shales and turbidites which were deposited in the relatively deep Coqueiro Seco-Morro do Chaves lake. Deltaic systems normally operated at one end of the lake, such as the Omo Delta in modern Lake Rudolf (Butzer, 1971). In the Morro do Chaves and Coqueiro Seco lake the net coarse elastic and percentage coarse elastic maps clearly show that sediments entered from several point sources (rivers) along the western margin of the lake. These point sources coincided with prin­cipal structural lows and bathymetric deeps in each sub­basin, and are surrounded by more positive, stable areas where Morro do Chaves carbonate platforms developed. Two types of lakes may occur (Fisher and Brown, 1979): (1) Open or drainage lakes in which water and sediment are contributed to the system but sediment supply may vary from abundant to limited (or starved), and (2) Closed lakes in which input is restricted to rainfall and output to evaporation or seepage. Closed lakes can develop unique depositional systems because of thermal stratification, where an upper, less dense epi­limnion and a lower, denser hypolimnion are separated by a thermocline. Circulation and oxygenation occur above the thermocline while stagnation and a highly reducing environment occurs below (Fig. 17). Normally in temper­ Topsets LAKE MODEL (Open) ~~eset~ ~-.Bottom sets GILBERT DELTA ~~ Blanket-like, Qroded beds MARGINAL FACIES• Shorefoce, (1) Beach, Gilbert deltas, Burrowed muds (2) OPEN LA>ufoco Open or Figure 17. Schematic types of lake models: (1) (From drainage lake model, and (2) Closed lake model. Fisher and Brown, 1979). ate climates this zonation is destroyed annually by tur­novers, but in tropical climates the stratification could exist for very long periods of time. As will be documented in this report, the Morro do Chaves and Coqueiro Seco lake or series of lakes in the Sergipe-Alagoas Basin probably were open. Periods of sediment starvation in open lakes are recognized by the occurrence of black, fissile, bituminous shales in the slope-basin systems. Another important aspect of lacustrine environ­ments is the homopycnal type of flux that is developed by discharge of river water into fresh or nearly fresh lake water. Homopycnal deltaic deposition is typified by rapid mixing because of approximately equal river and lake water densities, resulting in types of deltas called "Gilbert deltas" because of their recognition in U.S. Pleistocene lakes by G. K. Gilbert (1885). As stated by Bates (1953), homopycnal inflow occurs where sediment-laden fluid enters a basin filled with fluid of comparable density. Mixing takes place readily in three dimensions and the flow pattern is that of an axial pat­tern, forming the classical type of top-, fore-, and bottom-set deposits, as shown by Gilbert. Gilbert pre­sented the classic description of this process following a study of the bedding of symmetrical arcuate deltas (fan deltas) deposited in lakes by mountain torrents. Homopycnal inflow can best take place where a river flows into a well-mixed lake (meromictic lake) having a water temperature about the same as that of the river. Under such conditions three-dimensional mixing causes immediate deposition to take place at each stream mouth, blocking the outflow no matter how many distributaries develop in succession (Fig. 18). This results in devel­opment of flat topset fluvial beds that truncate or abruptly grade into foreset beds resting at the angle of repose beyond the distributary mouths. Likewise there is limited deposition in the deeper parts of the basin of fine-grained sediment comprising thin bottomset suspension beds. Over a period of time under low lacus­trine wave energy the shifting distributary outlets will build an arcuate, symmetrical delta lobe. If waters are very turbid due to the presence of large amounts of suspended silt and clay, the denser, turbid fluid can move directly down the lake floor by density flows (hyperpycnal flow) to produce turbidite deposits on the floor of the lake basin. Turbidites were first recognized in lacustrine systems (Forel, 1887) 2-GILBERT-TYPE DELTA (Homopycnal In.flow) 3-MARINE LITTORAL DELTA (Hypopycnal Inflow) (hyperpycnal, homopycnal, and hypopycnal) and resultant type of deltas. (From Bates, 1953). and studies by Houbolt and Jonker (1968} in Lake Geneva and Gould (1960} in Lake Mead document the process. Gould concluded that a river carrying an appreciable amount of suspended sediments and entering a quiet body of water at the same temperature and salinity will pro­duce a turbid, dense water mass which will plunge beneath the lake water and travel along the floor by gravity, forming a turbidity current called an underflow by Bell (1942}. However, in nature the temperature and salinity of a lake are rarely uniform from top to bot­tom, creating thermally or chemically stratified lakes. Under these conditions, another type of flow can be expected when river water sinks beneath the lake surface and flows along one of the lower water layers of higher density. This type of flow is called interflow. Also interflows occur when a muddy river is less dense than the water at the surface of the lake. These types of flows are shown in Figure 19. Underflows or turbidity currents are the domi­nant type of current that transports sediments within Lake Mead (Gould, 1960}. Water discharged by the Colorado River from October to April is colder and more saline than the deep water in the eastern part of the lake, causing the muddy river waters to sink rapidly shore terroct oos1n plain delta orto overflows (surface currents) w beneath the clear lake water and travel along the sub­merged Colorado River Channel, after the coarsest frac­tion of bed-load sediments has been desposited in the Colorado delta. Houbolt and Jonker (1968) studied the sedimentation of Lake Geneva (Lac Leman) using cores and seismic profiles and recognized the existence of a large sublacustrine channel in front of the Rhone River mouth that extended to a depth of 280 meters, cutting the prodelta and slope. This channel terminates in the central plain of the lake where a sandy turbidite fan is developed. The Rhone River is depositing a high sand delta separated from the sublacustrine fan by an exten­sive shaly slope or by-pass area. The depositional history of the lacustrine Morro do Chaves-Coqueiro Seco sequence is demonstrated by several lithic maps (net elastic, net coarse elastic, percentage of coarse elastic and net limestone) shown by Figures 20 to 23. The maps of elastic and coarse elastic facies clearly indicate that sediments entered the basin at several source points along its landward margin. Seismic and well data (Fig. 7) demonstrate that the Morro do Chaves-Coqueiro Seco sequence thins east­ward (offshore direction) toward the spreading center. Central parts of the lake were infilled by a thick sand­ and Morro do Chaves F c acies, Coqueiro Seco Brazil, showing princ~pr=~t!ons, Sergipe-Alagoas Basin, ource areas and depocenters. SOURCE AREAJI 9 <30 .. CJ .J0-4~­ mm .. ~o l'VV'V"-EltOSJON.U Llltll1 /FAULT-, Nf-N01 REACHED E -ERODED • -WELL / /"'""" BASIN PERCENTAGE CLASTIC -5% I j /°''" 1...... depocenters. shale facies exhibiting a high sand-shale ration (Figs. 20, 21, 22} which was deposited by density currents generated by slumping associated with the progradation of coarse-to fine-grained fluvial-deltaic systems into the lake. Carbonate platforms existed in the inter­deltaic areas as shown by net limestone values (Fig. 23). Basinal limestones were deposited during periods of low terrigenous sediment influx. Four main depositional systems were recognized in all sub-basins studied: (1) Morro do Chaves car­bonate platform systems; (2) Coqueiro Seco fluvial­deltaic systems; (3) Coqueiro Seco fan-delta systems; and (4) Coqueiro Seco slope and basin systems. In Alagoas Sub-basin the sequence was divided further into several operational units or cycles separated by well-defined and extensive shale units. The operational units are designated A to H: A is a basal carbonate unit (also called massive limestone) of the Morro do Chaves Formation; B, C, and D units are principally slope deposits and some minor deltaic facies; and E, F, G, and H are mainly fluvial-deltaic facies; and E, F, G, and H are mainly fluvial-deltaic strata (Plates I to XVII--See Appendix A). Each operational unit will be ~ discussed in this report. In the Rio Sao Francisco Sub­ basin, these cyclic operational units were not observed, and the entire section is composed of three superposed depositional systems: Morro do Chaves carbonate plat­ form, and Coqueiro Seco fan delta and slope systems. Stratigraphic cross sections illustrate the relationship . tV of these systems and operational units in both Rio Sao Francisco and Alagoas Sub-basins (Plates I to XVII). Morro do Chaves Carbonate Platform System The Morro do Chaves Formation (Fig. 8), as de­ fined by Schaller (1969), was redefined in this study to include only massive carbonate facies associated with platform environments in Sergipe-Alagoas Basin. Non- massive carbonate facies intercalated with elastic rocks are considered to be components of the Coqueiro Seco Formation (Fig.23). The Morro do Chaves carbonate platform system (Fig. 24) is composed of massive-bedded lacustrine limestones and shaly limestones, dolomitized locally in the Rio S~o Francisco Sub-basin. Petrographic thin sec­ tions of cores from the Alagoas and Rio sa"'o Francisco Sub-basins show that the massive-bedded limestone is composed of pelecypod grainstones or packstones (pelecypod biosparite of Folk, 1962) intercalated with some mudstones and pelecypod wackestones (micrite to J,fASSIVE LIMESTONE DEPOCENTER /\./V\J' -EROS/ONA L UMIT /FAUl..T-F Hit-HOT REACHED f -EROOED do Chaves Formation, Sergipe-Alagoas Basin, Brazil, showing areal distribution of Morro do Chaves carbonate platform system in Alagoas and Rio Sao Francisco Sub-basins. fossiliferous micrite and sparse biomicrite) rarely dolomitized and indicative of a shallow, high enery environment exhibiting a very high organic productivity (Figs. 25, A to H). In Alagoas Sub-basin the massive limestone f acies rarely contains inter-bedded coarse elastics or ~ even shale beds, but in Rio Sao Francisco Sub-basin pro­minently intercalated coarse elastic facies indicate intermittent progradation of fan-deltas over a shallow carbonate platform (Plate X). Distribution of coarse elastic fan-delta facies is shown in Figure 26. In deep waters beyond the shelf edge, the Morro do Chaves car­bonate shelf facies grades to marls and very calcareous slope shale beds, and disappears toward the center of the lacustrine basin. These tongues of deep-water car­bonate facies represent both density flow deposits and suspension deposits in the starved deep basin environ­ment. Typical electrical log sections in the Alagoas "' and Rio Sao Francisco Sub-basins are shown in Figures 27 and 28. The logs display a characteristic massive-bedded aspect more than 400 meters thick. In cores this limestone is olive-gray to white, dense, commonly recrystallized, and contains large shells and fragments Figure 25-A. Well l-PIA-21-AL, core #7, 562.45 meters. Morro do Chaves Carbonate Platform System. Pelecypod biosparite showing typical large fragments of pelecypod shells (pelecypod grainstone). Figure 25-B. Well l-PTA-3-SE, core #9, 598.7 meters. Morro do Chaves Carbonate Platform System. Oyster biosparite showing detail of unbroken oyster shell; microspar totally obliterate original porosity (oyster grainstone). A B Figure 25-C. Well l-PTA-3-SE, core #9, 596.7 meters. Morro do Chaves Carbonate Platform System. Pelecypod biosparite showing large pelecypod fragments in recrystallized mass of microsparite and micrite. Figure 25-D. Well l-PTA-3-SE, core #9, 596.7 meters. Morro do Chaves Carbonate Platform System. Pelecypod biomicrosparite. In the center of the photomicrograph there are possible algal plates disseminated in a microsparite illite clay mass. D Figure 25-E. Well l-PTA-21-AL, core #7, 560.7 meters. Morro da Chaves Carbonate Platform System. Microsparite formed by recrystalliza­tion of mudstone; dead oil-filled fracture on the right side of photomicrograph. Figure 25-F. Well l-PIA-21-AL, core #7, 559.0 meters. Morro da Chaves Carbonate Platform System. Silty microsparite formed by recrystallized microspar; a few silty quartz grains and black organic matter. F Figure 25-G. Well l-PIA-21-AL, core #5, 473.50 meters. Morro do Chaves Carbonate Platform System. Fine-grained rhombs of dolomite originated by replacement of micritic limestone; note euhedral outline of crystals. Figure 25-H. Well l-PIA-21-Al, core #7, 562.45 meters. Morro do Chaves Carbonate Platform System. Dolomitized pelecypod biosparite showing typical pelecypod shell fragments in totally recrystallized sparry calcite and microspar matrix; some rhombs of dolomite. H of pelecypods and ostracods; in some localities it is v typically chalky and soft•. In the Rio Sao Francisco. Sub-basin the uppermost section of the carbonate platform is commonly dolomitized, grading to coarse to finely crys­tralline dolomite (Fig. 25 G and H). Dolomitization seems to be associated with post-depositional events, especially with the erosion of the pre-Aptian unconfor­mity when highly saline, and Mg-rich solutions derived from Muribeca evaporite deposits percolated into the carbonate platform sediments beneath the unconformity. The base of Morro do Chaves carbonate sediments is gradational with the coarse elastic deposits of underlying Penedo fluvial-deltaic sediments. A map of net coarse elastic facies of the Penedo Formation shows that principal Morro do Chaves platform areas typically coincide with areas of highest subjacent Penedo coarse elastic values. Apparently Morro do Chaves carbonates developed on shoals supported by slowly subsiding Penedo deltas. The top of the Morro do Chaves Formation is IV erosional throughout most of the Rio Sao Francisco Sub- basin. Normally it grades upward into deltaic and fan­ deltaic deposits of the Coqueiro Seco Formation in the ,., Alagoas and part of the Rio Sao Francisco Sub-basins. As shown by Picard and Williamson (1974) the ~ FAN DELTA DEPOCENTER ~SOURCE AREAS /V\A/"-EROS/ONA L LIMIT /FAULT-F NR-NOT REACHED E -ERODED • -WELL PALMEIRA ALTA PLATFORM / / fO FRANCISCO SUB BASIN .. SERGIPE -ALAGOAS BASIN RIOS. FRANCISCO SUB BA­ NR NR. SIN -M. Chaves Formation ? NR• C. I. -25m oj:rl L 11okm ?Jr ~=N=E=T=C=O=A=R=S=E=C=L=A=S=T=l=C=:::::=.l 5 llOmi Figure 26. Map of net coarse elastic facies, Morro do Chaves Formation, Rio Sa~o Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil, showing source areas and fan delta depocenter. ALAGOAS SUB -BASIN TYPICAL SECTION OF MORRO DO CHAVES PLATFORM SYSTEM RIO JEQUIA AREA LITHOLOGY INTERPRETATION I CONGLOMERATE, quartzose and quartz­itic, highly feldspat1c, with sandy matrix, slightly calcareous. ~ :z::~ u:,~ (/) 0 >­ ~(/) ci SHALE ,light gray, m1caceous, cal­ 0 careous, some brown, carbonaceous. I (.)I LIMESTONE, coarse-gra i.ned, bio­ elastic (calcarenitel, cream to •...ih1te, massive, hard . LIMESTONE ,as above, so1ne white, ~ ) chalky, soft, cryptocrystall1ne, (calcilutite) . ~ ~ (/) J :::> ~ a: SHALE, gray , micaceous, calcar­0 eous as above . > , LL f--s dNSTST(lilS [:J0.3001110• 10'•10'11 1•11 12'>41111111• 10"-10'11{W)j j)C).\OQfl jlJ I 10'-10'111• 11 1.CNOITLON&L ~=&OATOI ~ ~LTili..._-....._--....""-­ Figure 32. Schematic representation of various Mid-Continent fluvial-deltaic sandstones and respective E-log patterns. (From Brown, 1979). decreases with increasing grain size, it is concluded that the GR log also may be used to infer grain size profiles in elastic strata. Stratigraphic cross sections of the sub-basins show (Plates I to XVII) that several log patterns display a mappable distribution within the sub-basins. Furthermore, it is possible to calibrate the "E-log facies" with samples, cores, and map patterns to permit basinwide mapping and interpretation of principal facies components of the elastic depositional systems. Isopach maps of digitate and serrate, blocky and massive E-log ·patterns define the distribution of these facies throughout the sub-basins. Serrate and digitate E-log facies (Fig. 33) occur in the principal depocen­ters or lows of the sub-basins and thin toward the mar­gins and platform areas of the sub-basins. Isopach con­tours of the serrate and digitate E-log facies suggest that it was sourced from points along the basin margin that coincide with fluvial-deltaic depocenters. Blocky and massive E-log facies (Fig. 34) display a widespread, uniform distribution, but the depocenters coincide essentially with serrate and digitate E-log depocenters. Distribution and stratigraphic relationships of these E­log facies, complemented by core and sample data, and ~ PfllNCIP~~E~~POCENTER ~SOURCE AREAS /\.IV\,/" -EROS/ONA l uw1r /FAUH-F HR-NOT lfEACHEO E -EROOEO • -WEU various lithic maps (to be discussed later), led to the conclusion that these facies represent slope (serrate and digitate) and fluvial-delta or fan-delta (blocky and massive) lithofacies. The coincidence of depocenters for facies displaying these E-log pattterns indicates that the sediments in sublacustrine fans were derived mainly from prograding Coqueiro Seco delta or fan-delta systems. More than 3,000 meters of sediments were depos­ited in the Paripueira depocenter in the Alagoas Sub-basin, and almost 3,000 meters were deposited in most of the other depocenters. The Coqueiro Seco slope system contains 500 meters of net sand in the Alagoas Sub-basin IV and 200 meters in the Rio Sao Francisco Sub-basin. Net sand in Coqueiro Seco fluvial-deltaic and fan-deltaic systems is more than 1,000 meters thick in Alagoas Sub- I\/ basin and about 500 meters thick in the Rio Sao Francisco Sub-basin. Cyclic sedimentary units in the Coqueiro Seco Formation, especially in the Alagoas Sub-basin, and the complex boundary between serrate and digitate facies and blocky and massive facies has permitted delineation of seven operational units from the top of the Penedo Formation to the base of the Ponta Verde Formation (Fig. 35). These operational units are separated by wide­spread shale units. (Plates I to IX). COQUEIRO SECO SLOPE SYSTEM Some modern slope systems are sites of active deposition with a sustained sediment supply derived from shallow-water depositional systems, principally delta and fan-delta systems, but most modern slopes are sediment-starved as a result of Holocene rise in sea level. These two basic types of slope systems, defined as constructional and destructional, are shown in Figure 36. A constructional slope is one that exhibits evi­dence of net progradation due to a sustained supply of sediment introduced to the shelf edge by delta, fan­delta, barrier-bar and strandplain and also, in some cases, shallow-water carbonate systems. Sediments are introduced to the slope system via gravity flows (grain flow, turbidity flow, fluidized flow, debris flow). If sediment supply is greater than basin subsidence, the slope system will prograde (offlap), but if the sub­sidence rate is greater than sediment supply, the slope deposits will superpose (onlap-fill or uplap). The mid­and upper-slope in a constructional system is prin­cipally a by-pass zone composed of fine-grained suspen­sion sediments which separate the coarse-grained, shal­ ALAGOAS SUB-BASIN SCHEMATIC RELATIONSHIP BETWEEN OPERATIONAL UNITS ( A TO H) AND DEPOSITIONAL SYSTEMS \0 Shelf -----­ ::'YI; .:::::::::~"~ ::::::::: :\ :::: ij : low marine or lacustrine systems at the shelf edge from coarse-grained submarine or sublacustrine fans at the base of the slope. Consequently, mid-and upper slope shale or fine-grained limestones lie stratigraphically between facies deposited on the shelf and facies depo­ sited at the base of the slope. A destructional slope is a sediment-starved slope that has no available sediment supply. Sediment starvation may occur because of a relative rise in sea level (basin subsidence or absolute sea level rise) which inundates the shelf and essentially eliminates shallow-marine or lacustrine sedimentation until deltas or coastline environments can prograde across the sub­merged shelf to its shelf-edge and again supply sediment to the slope. Without deltas or other shallow-water sources of sediments, the shelf edge and slope will be eroded by submarine or sublacustrine processes, causing extensive formation and development of canyons. In this way, submarine, or sublacustrine fans can be formed at the base of the slope at the mouths of the canyons, supplied with sediment eroded from the platform (shelf) and mid-and upper-slope. Submarine or sublacustrine erosion will cause the shelf-edge and slope to retreat, resulting in a progressive landward shift in the posi­ tion of submarine or sublacustrine fans (submarine onlap). Most authors believe that phases of construc­tion and slope and shelf destruction are cyclic. The cyclicity has been explained by global sea level changes (Vail et al., 1977) and others have suggested that basin subsidence is important (Pitman, 1978; Brown and Fisher, 1980). These basic concepts can be applied to explain the depositional environments responsible for facies in the basal part of the Coqueiro Seco Formation, but any absolute changes in lake level would probably be of cli­matic origin. Data from good quality seismic lines indicate a basic uplap or onlap-fill style of sedimen­tation, without significant evidence of progradation (offlap). This points to a tectonic control of the style of slope sedimentation and indicates an equilibrium between subsidence rate and sediment supply. This uplap constructional slope was deposited in depo­ centers between peripheral platform areas and was sourced by the advancing Coqueiro Seco deltas and fan­ deltas (Fig. 33). A very generalized model (Fig. 37) illustrates the initial depositional setting for the Coqueiro Seco slope system. Net sand distribution within the slope system documents this model (Fig. 38). Delta (not preserved) ~ SLOPE AR~~:OCENrElt SOURCE AREAS A/\/\/ -EROS/ONA l UMH /FAUlT -F NR-NOT REACHED E -ERODED • -WELL / Two different types of slope systems exist in Alagoas Sub-basin: one in the Sininbu Low and the other in the Lagoa Manguaba and probably Paripueira Lows. Each slope system has its own point source of sediments (Figs. 33, 38). The stratigraphic and E-log character of the systems are illustrated on cross sections (Plates I to IX). Approximately 300 m of slope sand occurs in the Sinimbu Low: about 500 m occurs in the Lagoa Manguaba Low. Sinimbu Low--In the Sinimbu Low, typified by the well l-JC-1-AL (Jequi~ da Costa tl), the Coqueiro Seco slope deposits are characterized generally by very well­defined, overall fining-and thinning-upward sequences. However, it is possible to recognize discrete coarsening-upward sequences within each individual sand unit (Fig. 39). Slope sandstones are mainly fine-but sometimes coarse-grained, very calcareous and indurated, intercalated with dark-brown to black, bituminous, calcareous, platy to blocky shale becoming fissile at the base. In the Sininbu area, (Plates II, III, IV and V) the slope section was divided into three operational units or cycles (B-1, C, and D). Each cycle is separated by relatively thick, dark, bituminous shale beds that drape the lobes of the sublacustrine fan com­ AL AG OAS SUB-BASIN TYPICAL SECTION OF COQUEIRO SECO SLOPE SYSTEM JEQUIA AREA SP LITHOLOGY INTERPRETATION FLUV AL­DELT IC FLUVIAL -DELTAIC SYSTEM SYS EM SANDSTONE •coarse-grained to conglomerati poorly sorted, highly feldspatic and kao Channel-fill deposits. linic, slightly calcareous. ~-UPPER SLOPE SHALE.dark brown to black, Sl.Lty, blocky carbonaceous and bl.tuml.nous ( burn). Suspens.ton shales with thin ll.me­stone beds of calcareous slope. SLOPE SYSTEM Operational Units B-1, C, and D SANDSTONE.very fl.ne-to fl.ne-grained, quartzose, calcareous. Cycles of thinnl.ng-upward sequen­ D ces formed by fine-grained sand­stones enveloped into dark brown shales, representing small, stack 1200,;;­ ed turb1dite fans deposlted by SHALE.dark brown, carbonaceous, also turbidity currents in a construct gray, with varved appearance . ive slope episode . Operatl.onal unit B-l. covering the Morro do Chaves carbonate platform, is made of very bitumi­ SANDSTONE fine-grained as above . nous black shale with a few very fi.ne-grained sandstone beds, pro­bably represents deposit.ion 1n closed lacustrine conditions. Cyclicity of section is probably control led by tectonic pulses, as well as by climatic changes . Each operatl.onal unl.t represents SANDSTONE coarse to rarely con­ short-Lived period of coarse glomeratic, quartzose, kaolinic, cal- elastic deposition (construct­ional phase)corresponding with tectonic episode; Long-term sus­pensl.on shale and carbonate IMESTONE.cream to white, chalky (calcl.­ deposl.tion correspond; to destruc tiona l epl.sodes . SANDSTONE. very fine-grained, Sl.lty, quartzose, ca Lcareous. S!{ALE brown and gray as above . to coarse-grain.TFORM SYSTEM carboPate faC°i"'eS (slooe) . Low ener FLUVIA.L _ DELTAIC SYSTEM Coarse-grained meanderbelt oeposits Figure 41. Typical electric log pattern, operational unit B-1, Coqueiro Seco slope system, Jequia area (Well 1-JA-l-AL), Alagoas Sub -basin, Sergipe-Alagoas Bas in, Brazil. ,/ ~ MAJt/MUAi THICKNESS ~ AREAS /'VV'\/-EROSIONA-L LJMIT rf' /FAULT-F HR-NOT ftEACHED E -ERODED •-WELL / / SERGIPE -ALAGOAS BASIN ALAGOAS sue BASIN NET SHALE Operational Unit 8 -1 C.1. -25m 1 , j , 1IO•m 5 llOmi Figure 42. Map of net shale facies, operational unit B-1, Coqueiro Seco slope system, Alagoas Subbasin, Sergipe-Alagoas Basin, Brazil, showing its characteristic distribution around the carbonate platform areas. basin conditions, because a thermocline was established. Lagoa Manguaba and Paripueira Lows--In the Lagoa Manguaba and Paripueira Lows, slope deposits contain much more coarse elastic sediment than the Sinimbu Low area and the electric log response to the sandier facies differs from logs in the Sinimbu Low (Plates I, II, III, VI, VII, VIII and IX). The log patterns (Fig. 43) exhibited by the slope facies are well-defined, serrate deflections and some digitate to blocky patterns near the top of the section. The amount of shale in the slope system in Lagoa Manguaba and Paripueira Lows is much less than in the Jequia area. The by-pass zone that typically separates the slope system from the overlying fluvial-deltaic system is not well-defined in the Lagoa Manguaba and Paripueira Lows. Also, there is no thick, black basal shale in these lows~ the basal unit is operational unit B-2. Operational unit B-2 (Fig. 44) is composed of about 70% elastic and 30% car­bonate sediment. The elastic component is very fine-to fine-grained highly calcareous sandstone, and black to dark-gray, laminated shale. The carbonate component is made of chalky mudstones, wackestones and some packsto­nes containing a high percentage (30-40%) of terrigenous grains, commonly quartz, disseminated in the micrite TYPICAL SECTION BY -PASS SHALE OPER UNIT D ~I >-• UJI 2000;n lJJ Cl.. OPER UNIT 0 ...J c (/) 0 (..) lJJ (/) 0 a: Li:i :::i 0 0 2400,;;­(..) OPER UNIT B-2 ALAGOAS SUB-BASIN OF COOUEIRO SECO SLOPE -SYSTEM BOCA DA CAIXA AREA LITHOLOGY INTERPRETATION SANDSTONE.white, medl.um-to coarse-grained h.Lghly feldspatJ.c, well sorted , calcare;:ms FLUVIAL -DELTA IC SYSTFM Delta p lain distributary channels SHALE. dark brown, laminated and subbitumi -t-----­ nous , and also greenish gray, platy, fis ­ PRODELTA -UPPER SLOPE sile. Suspension shale with distal del­ ta front sands (turbidites) SANDSTONE , l l.ght cream, coarse-grai.ned , SLOPE SYSTEM r are l y congl.omeratic, quartzose , some arkos1c , friable, intercalated •llth dark Operatior.al Units C and o brown bitum1nous shale . Rhy.thlnic sedimentation comp.:>sed of fine -to coarse-grained sand­ SHALE , dark brown. subb1tum1nous and green­ stones a nd black to brown sub-bi­ ish gC"ay, platy as above . tuminous shales representi.ng tur­ bidite fans deposited bv turbidi­ tic curr ents du r ing a c;nstruct.ive slope episode . Coarse sediments are provided by advancing, pr,,gra­ dational fluvial -deltaic systems . Subsidence rate ar.d sediment sup­ ply rate proba bly are s1mllar, re­ sulting 1n stacking of the slope fac1es . SANDSTONE , wh1te . fine -to med1um-gra1ned , occas1onally coarse to congtomerat1c, feldspati.c , ca lcareous . friable . LIMESTONE . white to cream, very fine-grain­ Ope rational Un.it B-2 ed {calc1lut1te), partially recrystalll.zed and chalky . Intercalau.ons of fine -to medium­ grained s a ndstor:es ; b rown , subbitu­ m1nous shales : and chalky limesto­ ne . The terrigenous elastic sedi­ SANDSTONE , greenish-gray, f 1ne­to medium­ ments represent tu r bid.ite fans co­ gra 1ned , h1gly calcareous, tight . vered by pelagic limestones formed during per i ods of low elastic in­ put to the basinal part of the lake . SHALE.brown, subb1tum1nous and gray . silty. LIMESTONE , (calcilutite) chalky. as above . PLUVIAL -DELTAIC SYSTEM SANDSTONE , white, coarse-grained, highly Coarse-grained meanderbelt depos­ feldspatic and kaol.ini t ic, micaceous , cal­ its . careous . Figure 43. Typical electric log pattern, Coqueiro slope system, Boca do Caixa area (Well l-BC-1-AL) Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil. Seco ALAGOAS SUB-BASIN TYPICAL SECTION OF OPERATIONAL UNIT B-2 COOUEIRO SECO SLOPE SYSTEM LITHOLOGY 1 CS-1-AL CHALK ,wh1te, soft, micaceous. SANDSTONE, liqht gray, fine-qra1n ed, mi-caceous, calca reous. CHALK, as above, also coou1no1d . grad ing to very recrystallized Limestone. SHALE, black and da rk gray, fl.s­s1le and laminated Intercalat1ons of CHALK. SAND­STONE and SHALE, as aoove. INTERPRETATION SLOPE SYSTEM Section composed of f1ne-ara1ried sar. ds tones and black to b~own, subb1tuminous shales represeiita­ tive of turbid ite fans: covered by suspension carbonates deposit­ed dun.r.g low elastic input peC'tods. Presel"'ce of i.mbricated coou1nas indicate contribution of displaced maten.al from the car::ionate pl a tforms . LIMESTONE;. coqu1no1d, yellow and f-----------------i br~wn, graaing to Marl. FLUVIAL-DELTAIC SYSTEM Coarse-ora1ned meanderbelt deposits SANDSTONE'. white, coarse -qra1ned, m1caceous ano calcareous. Figure 44. Typical electric log pattern, operational unit B-2, Coqueiro Seco slope systems, Coqueiro Seco area, (Well 1-CS-l-AL) , Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil. mass. In cores these carbonates exhibit a coquina of imbricated fragments of pelecypods in a carbonate mud and sand (Fig. 45). Petrographic thin sections of this carbonate facies confirm that it is composed of sandy wackestones to packstones containing 30-40% of very fine to medium quartz and feldspar grains and mica plates. Terrigenous grains are uniformly dispersed in the carbonate matrix and are associated with broken fragments of pelecypod shells, shale fragments and feldspars. When present, carbonate mudstones normally are partially dolomitized (Fig. 46 A to F). Operational unit B-2 probably represents depo­sition during an early stage of lake development. Ini­tial subsidence permitted deposition of an offshore facies composed of intercalated terrigenous and car­bonate sediments, probably equivalent to stratigraphic unit 1 of Koesoemadinata (1970) or transitional facies of Samborn and Goodwin (1965) in the Green River Formation, Uinta Basin. The relationship between slope sands and carbonates is well demonstrated by net sand and net maps of in operational unit B-2 (Figs. 47, 48). Core Analysis--As discussed by Walker (1980) classical turbidites exhibiting Bouma sequences comprise only a small volume of deep-water deposits. Massive ALAGOAS SUB-BASIN 10mv REPRESENTATIVE CORE , OPERATIONAL L.--1 UNIT B-2 SP Sllf\l, 1·:, massive. CC'•Ql!DJA massive, reworked. COQUH'iA made of imbricated she] 1 fragments of pelecypods in calcareous matrix SHAJ,E , massive and SIT.'l'S'l'ONr: parallel and convolute laminated. HIT'F.HJ'nWr1wrc1n -Displaced coqu inoid bed redeposited into relatively deep water lacustrine environment , a t center of the basin. COQUEIRO SEGO AREA -COO. SEGO SLOPE SYSTEM Figure 45. Representative core of carbonate facies, operational unit B-2, Coqueiro Seco slope system, Coqueiro Seco area (Well l-CS-1-AL), Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil. Figure 46-A. Well l-NF-1-AL, core #2, 1634.8 meters. Coqueiro Seco Slope System -Operational unit B-2. Arenaceous biosparite displaying very broken small fragments of pelecypods associated with about 30% terrigenous elastic grains, comprising abundant quartz and lesser feldspar and muscovite. The elastic fraction is poorly sorted. Quartz grains are subangular, fine-to very fine-grained, with common quartz overgrowths. Feldspar is partially illitized and corroded by calcite, but some is fresh, as demonstrated by the plageoclase near the center of the photomicrograph. Figure 46-B. Well l-NF-1-AL, core #2, 1634.1 meters. Coqueiro Seco Slope System -Operational unit B-2. Arenaceous biomicrosparite showing subangular, very fine-to fine-grained quartz grains dispersed in a mass of microsparite~ some broken, small pelecypod fragments. Organic matter is abundant, as well as residual oil. 1 09 Figure 46-C. Well l-CS-1-AL, core #10, 2669.45 meters. Coqueiro Seco Slope System -Operational unit B-2. Silty-sand microsparite showing detail of a coarse quartz grain, well rounded, with percussion marks (?), in a microsparite matrix. Figure 46-0. Well l-CS-1-AL, core #10, 2669.45 meters. Coqueiro Seco Slope System -Operation unit B-2. Silty­sand microsparite characterized by laminations of silty­sand microsparite and calcitic shale containing very fine sand grains disseminated in both lithologies. c D Figure 46-E. Well 1-BC-l-AL, core #8, 2290.0 meters. Coqueiro Seco Slope System-Operational unit B-2. Microsparite formed by recrystallization of mudstone, displaying typical pressure solution features with liberation of clay and very fine authigenic quartz. Figure 46-F. Well l-SMC-2-AL, core #1, 601.5 meters. Toe of Carbonate Platform System (Morro do Chaves Formation). Arenaceous biosparite, composed of very poorly sorted medium-grained sand pelecypod biosparite. Quartz grains are polycrystalline and exhibit large extinction angles (polarized light); derived from meta­morphic rocks. Potassium feldspar and low grade meta­morphic rock fragments are also present. 113 E F llIIIIl SLOPE DEPOCENTER r-SOURCE AREAS /\/\,/\/' -EROSIONA l LIMIT /FAULT-F HR-NOT REACHED E -ERODED • -WfU. / BASIN SUB BASIN Unit B-2 50m ' ,ro,m I 5 IOm1 MAXIMUM THICKNESS AREA -EROSIOHA l UMtr /FAULT-F NR-NOT REACHED E -ERODED • -WELL ./ SERGIPE -ALAGOAS BASIN ALAGOAS SUB BASIN NET LIMESTONE Opera/tonal Unit B-2 1 ; C. l. -25m! ; ,t011m !5 l,Om1 Figure 48. Map of net limestone facies, operational unit B-2, Coqueiro Seco slope system, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil, showing basinal con­centration of limestone deposition. sandstones, pebbly sandstones and several types of conglomerates are very common in this environment. According to Walker, the most distinctive structures characterizing deepwater sediments (in addition to the Bouma sequence) are sharp bases, parallel laminae, ripple laminae, convoluted bedding, load cast, flutes, and rip-up clasts associated with water escape struc­tures (e. g., dish, pipes). The Bouma sequence does not occur in massive arenaceous deep-water facies, and graded bedding is uncommon. Analysis of several cores from the Coqueiro Seco slope system in the Jequi~ area of the Sinimbu Low and Ponta Verde and Franpes areas of the Lagoa Manguaba, and Paripueira Lows shows (Figs. 49, SO, 51) that sediments exhibiting serrate log patterns are composed basically of several cycles of very fine to coarse-grained and rarely conglomeratic sandstones. These sandstones display massive to parallel laminae and contain many convolute structures, dewatering structures, and rip-up shale clasts enveloped by thin siltstone and shale beds exhibiting parallel-laminae, common slump structures, and rare bioturbation. In the Jequia area (Fig. 49) shales are more abundant and thicker than in other areas7 the shales are mainly black to brown, fissile, ALAGOAS SUB-BASIN REPRESENTATIVE CORE OF COQUE/RO SECO JEQU/A AREA 3 JA-4-AL I SP SN 81-PASS SHALE ,___ 1281 m , I • HC PRODUCT ION ~ I Core Description Della favera, 1976 . INTERPRETATION -Sublacustrine turbidite fans; deposited in relatively jeep-water;enveloped by slope shales . Turbidite fans are formed by channeled facies of supra fan and supra fan lobes , in a cyclic sequence . SLOPE SYSTEM SHALE.gray, fissile, horizontal l am1nated . SILTSTONE ,cross-laminated and horizontal Laminated, with slump structures. SHALE ,f1ss1le, b1oturba ted as above SANDSTONE .very fine-grained. mas ­ sive, ·..i1th rip-up clasts and ri.p­ ple-dr1ft, a few water-escape structures . SANDSTON~ c::mgl.o'.Tlerat1c to very f1ne-gra1ned, with finning-upward cycles . slump and water-escape structures, horizontal laminated in the very f1ne-gra1ned beds . SANDSTONE ,fine-grained, massive, with water-escape structures, and rip-up clasts. SHALE ,dark gray, fissile, parallel laminated . Missing core SANDSTONE, very fine-grained, para l ­lel laminated, weakly b1oturbated, associated with silty-shale beds . Convolute laminationsare present at the base of the bed. Missing core SHALE fissile, with ostracodes, pelecypod shells, fish fragments , and silty laminations with wavy cross laminations . slope Sub-basin, ALAGOAS SUB-BASIN 10mv L.___J REPRESENTATIVE CORE, OPERATIONALSP UNIT C 1-PV-1-AL SAlJilSTmn:, very fin e-c;rained, massive. ~;/\f'JJiS'rotrn ,fine-to very fine-c;rained , massive, with convolute lamination and water-escape structures. :;111\J,E and SIJ)l':J'rGNE, massive to parallel laminated. CC1NGJ,(;f1ll·:HA 'L'E, massive . SI1,1'S'rmn; , parallel to cross laminated. 3380m IN1'FHl'!l :·:'l'fl'PJC1T' -Channeled portion of suprafan lobes Description by: ~radin~ to inner fan facies. Slope Jlella 11avera, 1976 systern. PONTA VERDE AREA -COO. SEGO SLOPE SYSTEM Figure so. Representative core, Coqueiro Seco slope system, Ponta Verde area (Well l-PV-1-AL), Alagoas Sub­basin, Sergipe-Alagoas Basin, Brazil. 1-FS-1-AL Sp 1010m Description by : Della Vavera,1976 missing -----1016m FRAN<;ES AREA ALAGOAS SUB-BASIN REPRESENTATIVE CORE, OPERATIONAL UNIT D Cycles of Sf,IlDS1.l10t:1:, medi um to very fine­gra ined, massive to parallel laminated at top. ~:>!!l\T,I·' and SII.'J'S'l'Otll~ both wi tl1 slump :::;truc­tures, and convolute laminations Sl\irn~:·rrnJF, very fine-groined, massive. Cycles of Sl\f! l"i'.1~'01-:i': , as above. SAJ:T1S'l'OJ·1r, coarse to fine-c;rained, rnassive to cross l aminated. Cycles of StND~l'CM~.very fine-c;rained , mas­sj_ve, ~rading to paro.llel J arnin<•ted a t tor,, sometimes with inclined and cross lamina­tion. J; 'l' L'Hl'l::·"l''"l'Jr'ql -Cycles of suprafan lobe deposits , and some chrrnnel.s (mid­fan ) of a deep-1·iater l a custrine fan. COQ. SEGO SLOPE SYSTEM Figure 51. Representative core, Coqueiro Seco slope system, Franpes area (Well l-FS-1-AL), Alagoas Sub­basin, Sergipe-Alagoas Basin, Brazil. and sub-bituminous. In summary, slope systems of the Coqueiro Seco Formation are characterized by: (1) Distribution of serrate-digitate E-log facies throughout the central areas of the lake, dis­appearing toward the platform and point sources. (2) Less sand and finer-grained than overlying delta or fan-delta facies. (3) Presence of carbonate mudstone and wackestone containing terrigenous sediment in the central parts of the sub-basins. (4) Predominance of massive sandstones and other sedimentary structures produced by density currents. (5) Presence of basal black, bituminous shale throughout most of the sub-basin. (6) General evolution of the entire sub-basin from dominantly deep-water carbonate, basal black shale, turbidite, by-pass slope, to fan-delta and delta environments. A general model of the Coqueiro Seco slope system is shown in Figure 52. Analysis of Operational Units--Analysis of various maps of slope operational units (Figs. 53, 54) show that it is possible to define the sources of elastic and carbonate sediments. During deposition of operational units B-1 and B-2, deltas that supplied sediments to the lake were far landward from the present preserved margin of the basin. Consequently, the older operational units are composed exclusively of slope sediments; contemporaneous delta and fan-delta facies have been eroded from the original basin margin. Slope sandstone and carbonate facies exhibit similar geographic distribution, indicating that some of the deep-water carbonate could have been derived from den­ sity currents, but that most of the carbonate was depo­ sited from suspension during periods when there was limited influx of elastics into the basin. The terrige­ nous turbidite fans are draped by calcareous shales and chalky or arenaceous limestones. Maps of net sand within operational units C and D (Figs. 53, 54) show that coalescent sublacustrine tur­bidite fans were deposited basinward of principal point sources; e.g., deltas and fan deltas. By comparing the distribution of net sand, it is evident that turbidite fans of unit D shifted basinward of the depocenter for unit C, and, unit D (Fig. 54) contains facies of the delta system that supplied sediment to the slope. Deltaic sands are represented by blocky to massive E-log f acies in a depocenter near each point where sediment entered the sub-basins. Also, in operational unit D, basinward of the Furado carbonate platform in the south­west part of Alagoas Sub-basin, an isolated subla­ custrine fan seems to be related to a destructional slope phase when a canyon was excavated into the plat­form near wells l-SAU-1-AL and l-FPB-3-AL in the Sumauma and Fazenda Pau Brasil area. COQUEIRO SECO FLUVIAL-DELTAIC AND FAN DELTA SYSTEMS In the Alagoas Sub-basin operational units D, E, F, and G are composed principally of coarse elastic sed­iments that were deposited by Coqueiro Seco fluvial­deltaic systems which prograded over Coqueiro Seco slope deposits and infilled the basin with a very thick section of terrigenous strata (Fig. 55). A relatively small fandelta system occurs in the southwest part of Alagoas Sub-basin. The fan prograded over the Furado carbonate platform (Fig. 55 and Plates I, IV). Elsewhere in the sub-basin, the upper part of the Coqueiro Seco Formation is composed of coarse elastic sediments (coarse to medium-grained sandstones) depo­sited by deltas that prograded across most of Alagoas Sub-basin. Diminished subsidence permitted the basin to fill, thus terminating Coqueiro Seco sedimentation. The fluvial-deltaic and fan delta systems dis­play a very high sand/shale ratio (greater than 60% sand) and are characterized by blocky and or massive E­log patterns. A map of fluvial-deltaic and fan delta ~ t~~~IA~f~j~~'iR' IAf.s ~SOURCE AREAS /V\/V" -EllOSIONA t LIMIT /,AUl.T-F Hlf-Nor ltU.CHlO I -EltOOIO • -WfU / / SUB BASIN SERGIPE-ALAGOAS BASIN FLUVIAL-DELTAIC &. FAN-DELTA. SYSTEMS NET COARSE CLASTIC C. I. -IOOm ,... J , ,10*'" 4 1,0mi Figure SS. Map of net coarse elastic facies, based on blocky and massive E-log patterns, Sergipe-Alagoas 0'£1lA'l'lt~/9\. i.1,. I T".l Basin, Brazil, showing source areas and principal depo­ bCFC. centers of fluvial-deltaic and fan delta systems. coarse elastic facies (Fig. SS} indicates that sediments supplied to the Coqueiro Seco slope system were derived directly from Coqueiro Seco fluvial-deltaic systems (compare Figures 38 and SS). More than 1,000 meters of sand accumulated in the principal depocenters of the Alagoas Sub-basin, and more than SOO meters is inferred to exist in the Sininbu Low. Net sand and sand percen­ tage values define high sand axes that extend landward to points where rivers entered the sub-basin. This relation contrasts with slope sand trends which pinch out in a landward direction. . . / Rio Jequia Fan Delta System--A fan delta is an alluvial fan that progrades into a body of water from adjacent highlands (McGowen, 1970). Friedman and Johnson (1966} called them tectonic deltas. Like allu­vial fans, fan deltas display small areas of drainage and flashy, high discharge. Sedimentation is accomp­lished by high-gradient braided streams that extend essentially to the toe of the delta. Distributaries are short and braided. The Rio Jequi; fan delta (Plates II and IV) was deposited on the Furado carbonate platform and is com­posed of thick conglomerates and conglomeratic, quart­zose to feldspathic, sandstones containing quartzite 128 pebbles and granite boulders in a sand or mud matrix. A relatively thin shale unit at the base of the fan delta system represents a distal fan and prodelta facies. This system is characterized by E-log patterns formed by massive conglomeratic sand stone deposits of braided stream origin (Fig. 27). Coqueiro Seco Fluvial-Deltaic System---Except for the Rio Jequia fan delta system, conglomerates rocks are rare in the Coqueiro Seco Formation of Alagoas Sub­basin. Coarse to medium-grained sandstones were commonly deposited by Coqueiro Seco fluvial-deltaic systems that prograded across the sub-basin. E-logs exhibit blocky to massive patterns, especially in the central part of the sub-basin (Plates I to IX). These characteristic log types become more common toward sediment point sour­ces where rivers entered the sub-basin. Delta systems have been classified as river­dominated (elongate to lobate) and marine-dominated (wave or tide) according to Fisher et al. (1969). Delta systems are supplied with sediment by a variety of mean­dering streams, unlike fan deltas which are associated with braided stream systems. In the absence of signifi­cant tidal or wave energy, lacustrine deltas are fluvi­ally dominated and exhibit the variety of delta plain 129 and delta front environments, processes and facies that typify this type of delta. A modern lacustrine river and delta is the Omo of East Africa (Butzer, 1971). Progradation of a delta results in a coarsening upward sequence of facies (prodelta, delta front, delta plain and fluvial, Frazier, 1967). This sequence typifies marine, fluvially-dominated deltas, as well as lacus­trine deltas. Principal differences result from homo­pycnal flow into lakes which greatly reduces the prodelta and delta front transition and produces abruptly grada­ tional coarsening upward sequences. Modern analogs are discussed elsewhere in this report. During deposition of the Coqueiro Seco fluvio­deltaic systems, sediment input was high because of intensity of tectonic activity that uplifted source areas. The ratio of bedload to suspension load in Coqueiro Seco rivers was high. Because the wave energy and tidal effects in the lakes were negligible, the deltas are medium-to coarse-grained, elongate to lobate, river-dominated systems. Sand distribution in opera­tional units E, F, G, and H (Figs. 56 to 59) indicates that the systems were high-constructive, elongate deltas. Sand distribution also documents progradation of the fluvial-deltaic systems across the entire basin ...... w ..... w ..... by the end of Coqueiro Seco time (operational units G and H). The geometry of sands indicates that each operational unit was supplied by rivers at approximately the same points along the margin of the sub-basin. Maps of net coarse cl~stic distribution in the operational units, when compared with the regional tectonic map (Fig. 10), show that the principal point sources (rivers) and the delta depocenters were consistently situated along the downthrown side of major faults, such as along Lagoa Jequia, Sinimbu, Coruripe and Tabuleiro dos Martins. Thus, the delta sites were controlled tec­tonically. Interdeltaic environments existed in struc­tural positive areas such as in Furado and Pilar areas. These are the same areas where Morro do Chaves carbonate platforms were established previously. Detailed compar­ison of net sand contour patterns for several opera­tional units document the shifting of the deltaic com­plexes by avulsion and abandonment of deltaic lobes. For example, in the Lagoa Manguaba Low where consider­able well data are available, it is possible to document the shifting depocenters in operational units F and G (Figs. 57, 58). During the deposition of operational units G and H (Figs. 58, 59), the sub-basin was almost entirely ...... VJ VJ filled with shallow-water sediments. These units are preserved only in the more negative areas (coincident with the overlying Ponta Verde Formation). Correlative sediments were eroded from the more positive areas during the Muribeca tectonic phase. A thick prodelta shale separating operational units G and H can be corre­lated over an extensive area (Plates I, II, III, VI, VII, and VIII). This shale is the only well developed prodelta facies recognized in the Coqueiro Seco fluvial­deltaic systems. Occurrence of the extensive, relati­vely thick, prodelta facies indicates that source areas at that time were almost eroded and the subsidence rate was again greater than sediment supply, permitting Coqueiro Seco deltas (unit H) with a lower sand/shale ratio to prograde across the central parts of the moderately deep sub-basin. Operational unit H (Fig. 59) is preserved only in more negative parts of the sub­basin. Following deposition of the fluvial-delta system that composes operational unit H, the sub-basin slowly subsided and drowned: the Ponta Verde shales then were deposited in central Alagoas Sub-basin. The Ponta Verde shales exhibit areal distribu­tion almost coincident with operational unit H. The Ponta Verde Formation was deposited in a moderately deep-water lacustrine environment as the Alagoas Sub­basin subsided in the absence of sediment supply. Small sublacustrine fans were deposited in the lake via tur­bidity currents emanating from small deltas along the margin. There is no evidence of density stratification or closed lake conditions at that time. The Ponta Verde environments gradually graded into a shallow carbonate shelf (Mundau Limestone, Maceio Member, Muribeca Formation). The sediments are composed of green to gray, splintery and blocky shales, which are non­calcareous, contain some brown, subbituminous basal sha­les, and are intercalated with thin fine-grained, quart­zose sandstones and dolomitic limestones. The Penta Verde Shale separates the elastic Coqueiro Seco Formation from the overlying Muribeca Formation, a coarse elastic, thick formation containing evaporites. An isopach map of the Ponta Verde Formation ex­hibits a maximum thickness of 250 meters (Fig. 60) and contains less than 50 meters of sand. The overall geometry of this formation conforms with the structural framework and paleobathymetric configuration of the sub­basin; thickest shale occurs in depocenters coincident with the Lagoa Manguaba and Paripueira Lows. Present limits of the Ponta Verde Formation, as well as the SERGIPE -ALAGOAS BASIN TOTAL ISOPACH PONTA VERDE FORMATION { ·'·i-50f"o I , ... , , km 5 llOmi 138 Morro do Chaves and Coqueiro Seco Formations, are essen­tially erosional. There is no stratigraphic equivalent of the deep water Ponta Verde Shale within the Rio Sao Francisco Sub-basin. Sinimbu Low--Fluvial-deltaic systems in the Alagoas Sub­ ~ basin are displayed on Figures 61 to 63. In the Jequia. area within the Sinimbu Low, coarse sandstones (Fig. 61) are well developed, especially in the basal part as evi­denced by the principally blocky and some massive E-log patterns. A channeled sequence was deposited over the thick sub-bituminous by-pass shale, which in some areas was deeply eroded. Three cycles of fluvial-deltaic sedimentation have been recognized by using prodeltaic shales to differentiate individual progradational sequences. These deltaic cycles probably were in response to fluctuations in tectonic and climatic con­ditions which affected the water budget and the level of the lake. Channel-fill deposits represent the dominant I fluvial-deltaic facies in the Jequia area but significant volumes of deltaic shale are also present. Prograda­tional patterns are not evident based on the electric logs, but this absence could be the result of erosion of fine-grained facies by the advancing channels of the TYPICAL ALAGOAS SUB-BASIN SECTION OF COOUEIRO SECO FLUVIAL/ DELTAIC JEQUIA AREA 1-JC-1-AL I FLUVIALthree shales, plain sand, of the ing fluvial Suspens i.on PRODELTA - Suspension careous ~ LITHOLOGY SANDSTONE, fine-to medium-grained, gray, poorly sorted, feldspatic and kaoll.nlc. SHALE. dark to medium brown and g ray, fi.s sile to blocky , silty, mi.caceous. and Laminated . SANDSTONE, as above. SHALE, as above, also black, bituminous. SANDSTONE, fine-to coarse-gra.ine::l, po­orly sorted, feldspat1c, kaolinic, cal­careous , and arg i Laceous. SHALE . gray to black, fissile. finely micaceous, bituminous . CONGLOMERATIC SANDSTONE,wh1te, feld­ spat1c , kaol1n1.t1c . SHALE gray to black as above . (.) CONGLOMERATIC SANDSTONE, quartzose, feld­spatic, subangular, poorly sorted, peb­bley and unconsolidated with excellent apparent porosity. SHALE dark brown to black, silty, blocky, bituminous , calcareous with thin inter­calations of limestone. SYSTEM INTERPRETATION -DELTAIC SYSTEM Lacustrine deltaic facies suc­cessively stacked in, at least, cycles, formed by prodelta delta front sand and delta "'nth good development cong lomeratic sandstones in operatioPal unit E represent­distributary fac1es. suspension prode 1t a shale fac ies . prode 1ta shale facies . UPPER SLOPE shale with thin cal-slope beds. Figure 61. Typical electric log pattern, Coqueiro Seco fluvial-deltaic system, Jequia area (Well l-JC-1-AL) I Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil. superimposed meanderbelt fluvial system. The final product of this f luvial destruction caused by downcut­ting and lateral migration of fluvial channels would be a coarse-grained channelized sequence. The upper part of the fluvial-deltaic systems in the Jequia area exhi­bits the best preserved fine-grained delta front and delta plain sandstones and shales in the Alagoas Sub-basin. Lagoa Manguaba and Paripueira Lows--In the Lagoa Manguaba and Paripueira Lows, fluvial-deltaic systems, as well as the slope system, are much sandier than in . ~ the J equia area. In these lows, typical sections (Figs. 62, 63) display massive, less blocky E-log patterns reflecting the predominance of sandy delta plain and fluvial facies, and limited finer grained elastics. Clastic facies are normally coarse to medium-grained; grain size increases toward source points. Also the section becomes entirely channelized toward source areas (Plates I, II, III, VI, VIII and IX). Because of very high sand/shale ratios, it is difficult to define operational units in fluvially­dominated areas, but careful correlations have permitted reasonable and confident separation of several deposi­ ALAGOAS SUB-BASIN TYPICAL SECTION OF COOUEIRO SECO LAGOA MANGUABA LITHOLOGY SHALE.gray to blue, blocky, VEfDE FM . spl1ntery . PON TA SANDSTONE . wh1te, fine-grained, m1caceous, calcareous, stone . 0 :;, SANDSTONE· fine-to 0 times granular to conglomerat1c, feldspatic, grading to ~ tic, mica.ceous and calcareous. SHALE. gray to green, blocky tery, rare dark brown, fissile . SANDSTONE, arkosic, coarseconglomeratic, chloritic, Conglomerate, kaoliniti.c. SHALE . gray and brow!) as SANDSTONE ,tan to light gray, coarse-grained, calcareous, kaolini1tic. SANDSTONE , fin~to very fine with some coarse beds, careous, may be SHALE .dark gray to brown, subbituminous calcareous , fissile. LIMESTONE,gray tan, locally chalky, (calcilutite) may be finely crystal­line. FLUVIAL/DELTAIC SYSTEM AREA INTERPRETATION f1sslle and Deep-water lacustrine bas1na 1 shale . h1ghly FLUVIAL -DELTAIC SYSTEM grading to silt­ Very sandy section formed of coarse-grained to conglomerat lC sandstone bodies, stacked in sev­eral fining-upward sequences. The sandstones are immature, arko­s1c, highly kaolin1tic, m1caceous. The section is representative of proximal facies, deposited close to the point sources of fluv1al­delta1c systems, with predominance coarse-grained, some of fluv1al. coarse-grained meander highly belt deposits (longitudinal and rkose, kaolini-, coarse-grained point bars)and delta plain facies. Fine-grained sediments representing levee and/or delta front facies, are and splin­ almost not pres•nt... Zonati.on of subbituminous, operational on1ts u very diffi­cult in th1s proximal area. -grained grading to above . fine-to feldspatic . SLOPE SYSTEM (?) Proximal turb1dite fan facies, feldspatic , cal­-grained, suppl1ed by the advancing fluvial­highly calcareous . deltaic system. Figure 62. Typical electric log pattern, Coqueiro Seco fluvial-deltaic system, Lagoa Manguaba area (Well 2-LMST-l-AL) Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil. TYPICAL SECTION COO SEGO SLO SYSTEM ALAGOAS SUB-BASIN OF COOUEIRO SECO BOCA DA CAIXA LITHOLOGY SHALF.. blue-gray, soft, blocky tery . SANDSTONE , fine-to medium-grained, m1caceous, may be thinly et with SHALE, brown, bttum1nous, DOLOMITE, microcrystal line. SHALE.dark brown, silty, platy, minous, laminated with limestone. SANDSTONE ,fine-to medium-grained , coarse-grai.n::!d, quaC"tzose, slightly calcareous, i.nter-calated with SiHALE gray and brown. SANDSTONE predominantly med1um-gra1ned and coarse-grained. as above. SHALE ,brown, subbetum1nous;. ish, Laminated and calcareous. SANDSTONE. predominantly coarse-grained, arkos1c, subrounded, regularly sorted, friable, with good apparent SHALE, greenish-gray, spll.ntery, careous, some brown. as above. SRALE , dark brown, Laminated, fissile . FLUVIALIDELTAIC SYSTEM A REA INTERPRETATION to splin­ Deep-water lacustrine basinal shale. inte rcalat­and FLUVIAL -DELTAIC SYSTEM Deltaic sect.ion from prodelta sha­les, delta fr.::int channel mouth­bars and distr.ibutary channels , represented by Operat l.onal Unit H. bitu­Prodelta shales and distal delta front sands. some Operational Units E, F, and G friable, Very hl.9°1 sand/ shale ratio sectl.on formed by stacked and coalescent distr.ibutary channel-flll dne . SHALE , gray, lam1nated, calcareous. LIMESTONE . very flne-gra1ned (Cal ­cilutite) white, soft . chalky , --= slightly dolomitic . --= -c:=::::::::=:: LIMESTONE, meaium-to coarse gra1n­bioclast1c, mas­ ,? =~v~ ~lcarenite). DELTA SYSTEM INTERPRETATION FAN DELTA SYSTEM Proxi.mal fan aelta fac1es -conglo­ mer:ateanOCong lomer:at"'lCsa na s torie deposited by braided streams and debris flow (sheet-wash deposits). Medial fan oelta facies -Med tum­ toCOarse-graTr;@d~lone and shale depositeo by braided to coarse-grained mea noerbelt fluvial sys t ems . Fine-grained seo1ments repro::ser.t overba nk deposits. Distal fan aelta fac1es -Delta front san'Ust'O'neS anciS'hates "''ith reworked bars and mouth bars . Prooelta facLes -Suspension clays with minor sands (offshore bars) . CARBONATE PLATFORM SYSTEM Modera te to hioh energy, b1oclastic (pelecypods, oysters , etc. )carbon­ate plat torm . GENERALIZED FAN DELTA AND CARBONATE PLATFORM 1"()0EL (From McGowen . 1918) Figure 71. Typical electric log pattern, Coqueiro Seco fan delta system, Praia de Santana area (Well l-PS-1-SE), Rio sg'o Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil. "" but fan deltas developed in the Rio Sao Francisco Sub-basin, while fluvial-deltaic systems operated in Alagoas Sub-basin. These differences may reflect different distances from and sizes of the source areas, as well as different river gradients. The boundary between slope and fan delta depo­ • ,v sits in the Rio Sao Fancisco Sub-basin is relatively sharp and the E-log facies transition from serrate (slope) to blocky and massive (fan delta) patterns is well defined in this sub-basin. This contrast with the more complex cyclic relationship exhibited by these systems (slope and fluvial-deltaic) in the Alagoas sub- basin. SERGIPE -ALAGOAS BASIN COMPARATIVE STRATIGRAPHIC COLUMN MORRO DO CHAVES, COQUEIRO SEGO AND PONT A VERDE FORMATIONS SUB BASINS: RIO SAO FRANCISCO ALAGOAS SEISMIC STRATIGRAPHIC EVALUATION General Considerations Seismic profiles were of fundamental importance in defining the basic geometry of the Coqueiro Seco and Morro do Chaves Formations in the Sergipe-Alagoas Basin, especially in those areas where well information is very sparse or absent, such as the northern part of Alagoas Sub-basin and offshore areas of Alagoas and Coruripe sub-basins (See Methods and Procedures). Unfortunately, the generally poor quality of seismic lines precluded a conventional seismic stratigraphic approach in this investigation. However, some seismic profiles in Alagoas and Rio sa'o Francisco sub-basins were usable and provide some useful information about the sedimentary sequence that infilled the sub-basins. As defined by Brown and Fisher (1979), seismic stratigraphy permits the recognition, delineation, mapping, and interpretation of fundamental basin-fill stratigraphic units. Interpretation of the nature and distribution of successive depositional systems and the evolution of tectonic and stratigraphic style of basin history is possible using adequate seismic data. 159 Seismic sequences are unconformity-bounded units composed of relatively conformable stratal reflections (Vail, et al., 1976). Each sequence is composed of one or more contemporaneous depositional systems (Brown and Fisher, 1977). Unconformities bounding seismic sequen­ces are identifiable by erosional truncation or by lapout relationships. Those reflections which are not unconformities are inferred to represent isochronous stratal surfaces which may pass through various facies. Within the seismic sequence are reflection units called seismic facies, which are presumed to be the seismic response to lithofacies. Seismic facies are three­dimensional and differ from overlying, underlying, and laterally equivalent seismic facies by a unique com­bination of geometry, reflection configuration (parallel, divergent, mounded, draped, progradation), amplitude, frequency, interval velocity, reflection con­tinuity and bounding unconformities (if any). It is the role of seismic stratigraphic interpreters to recognize, delineate and map seismic sequences and component seis­mic facies, and to infer the component lithofacies and depositional systems represented by the seismic facies. With these principles in mind, several seismic profiles will be discussed, realizing that to date the poor qual­ity of seismic profiles of the rift phase precludes establishing seismic sequences and interpretating seismic facies throughout the sub-basins. Alagoas Sub-basin In Alagoas Sub-basin, a seismic dip profile pro­file (composed of profiles 42-RL-7, 42-RL-4, and 28-RL-336) across the entire sub-basin (Fig. 73) inter­ # cepts the Furado carbonate platform system, the Sao Miguel dos Campos High area, the Jequi~ area (well 3-JA-2-AL) and extends to the offshore limit of litho­genetic sequence II (well l-ALS-7). Several intercepted wells permit good control of several facies defined in this investigation. The base of sequence II is well defined by a reflector exhibiting low frequency and good continuity, generated by the sharp contrast in velocity and density (acoustic impedance) between the porous Penedo sandstones and the carbonate section of Morro do Chaves Formation. Because of the superposed style of Coqueiro Seco deposition, lap-out evidence of the uncon­formity was not recognized. Similarly, erosional trunc­ation of the underlying Penedo reflectors was not observed. This seismic profile documents offshore \ thining of the Coqueiro Seco and Morro do Chaves Figure 73. Dip seismic profile composed of profiles 42-RL-7, 42-RL-4, and 28-RL-336, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil, showing Furado carbonate platform system, scfo Miguel dos Campos High, Jequia' area, and extending to offshore limit of sequence II (Well l-ALS-7). I •. i I •• Ii~ Q) ·-c: -J I 8 1~ I .. I : : ,,J..,.1 to .. ..,... t I ' Q) I c: .. ·­ ...J TOP 1.1.CHAVESt C. SECO ••Fl.IS . \ TRANSITION SLOPE/DELTA SYSTEMS ' - = MASSIVE CARBONATE M. CHAVES ·:~ 1-ALS-7 BASE M CHAVESIC. SECO ·~ GJ,,, f 1' p.. '"'"II"" . ' 1-.:.... ~ • "' . "' . I 'I Ii I I !I ii'' I !1 i' ALAGOAS SUB-BASIN Seismic Dip Section Lines• 42-7 42-4 28-336 sequence; that is, the top of sequence II is erosionally truncated in the direction of well l-ALS-7. The poor quality of this seismic profile does not permit seismic facies interpretation, but discontinuous, parallel reflections are visible in the basal part of Coqueiro Seco slope system, especially in the area between the wells l-LSM-1-AL and 3-JA-2-AL. A more recent seismic profile (27-RL-412) of better quality in the same area of the former seismic profile clearly displays a parallel reflection configu­ration, moderate continuity, and low amplitude (Fig. 74). On this latter line there is no suggestion of progradational configuration, which agrees with my sub­surface stratigraphic interpretation that defines an uplap depositional model generated by tectonic sub­sidence. This schematic model is illustrated by Figure 75, model I (Brown and Fisher, 1977). A third dip seismic profile (Fig. 76, 42-RL-22) in the Alagoas Sub-basin is located within Lagoa Manguaba Low. The Coqueiro Seco slope system is repre­sented by parallel reflections overlain by an almost reflection-free section representative of the fluvial­deltaic system. Near a source area (well l-VA-1-AL) several discontinuous, erosional, channelled reflections = TOP M CHAVESIC _ SECO FMS TRANSITION SLOPE/DELTA SYSTEMS l - = BASE M CHAVESIC SEGO FMS ~. . ~ 0 .~\ 0 ALAGOAS SUB-BASIN Seismic Dip Section Line, 27 -412 Figure 74. Dip seismic profile 27-RL-412, Alagoas Subbasin, Sergipe-Alagoas Basin, Brazil, showing parallel reflection configuration typical of uplap deposition controlled by tectonic subsidence. n OFFLAP: SEDIMENT CONTROL Q SLOPE FAN I FACIES (sediment control)7 111--0nlap (sediment starved)7 and IV--Carbonate shelf-slope. (From Brown and Fisher, 1977). ...... O'\ O'\ --IOP M CKllVES1C SECO f MS ., ,R,.NSHION s10PE DHI" sr s rEMS c=,J &<.SE I.I CffllVE SC SECO f t.IS ALAGOAS SUB-BASIN Section llne 1 42-22 Figure 76. Dip seismic profile 42-RL-22, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil, showing parallel reflections representing slope facies overlain by almost reflection-free section representing fluvial-deltaic facies. In the Lagoa Manguaba Low, discontinuous, erosional reflections occur corresponding to channel-fill f luvial facies. occur corresponding to channel-fill fluvial facies. The best quality strike seismic profiles across the Alagoas Sub-basin occur near the present coast line (seismic lines 42-RL-169, 42-RL-157, 42-RL-156, 42-RL-153, and 42-RL-151) where topographic problems are almost absent. The strike line (Fig. 77) extends along the basin axis from the northernmost end of the Sergipe­Alagoas Basin to the Paripueira Low. Where reflection quality permits, it is possible to observe regional thickening of the Morro do Chaves and Coqueiro Seco sequence. Two seismic facies are recognized on the pro­file: basal parallel reflections displaying good con­tinuity, and an upper almost reflection-free facies. These are interpreted to represent slope and fluvial­deltaic systems, respectively. Quality of the seismic profile is poor in the Paripueira Low, where the Morro do Chaves and Coqueiro Seco sequence is overlain by more than a thousand meters of Muribeca elastic and evaporite sediments. In summary, in the Alagoas Sub-basin only a few seismic profiles permit application of elementary seis­mic stratigraphic analysis. The limited data available suggest that the Coqueiro Seco slope system is charac­terized by more or less continuous and parallel Figure 77. Strike seismic profile composed of profiles 42-RL-169, 42-RL-157, 42-RL-156, 42-RL-153, and 42-RL-151, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil, along the basin axis, from northernmost end of Sergipe-Alagoas Basin to Paripueira Low. Slope and fluvial-deltaic systems are labeled in the seismic line. \ -TOP M. CHAVEStC SECO FMS TRANSITION SLOPE1DE1 TA SYSTEMS c:::J BASE M CHAVEStC SECO FM! ~BASEMENT '\.__.. • 0 () ~­ . . Q) ·c:­ -J ALAGOAS SUB-BASIN Seistt1i&: Strike Section Lines • 42 -169 42-157 42 -156 42 -153 42-151 reflectors, which fill the sub-basin in an uplap or onlap fill configuration, characteristic of deep-water sedimentation in a subsiding marine or lacustrine basin. Fluvial-deltaic facies are recorded as reflection-free seismic facies suggesting the absence of areally extensive acoustic impedance contrasts within the heterogenous facies of channel-fill and deltaic sandstones. • I\/ • • Rio Sao Francisco Sub-basin v In the Rio Sao Francisco Sub-basin, the quality of on-land seismic profiles in the rift section is poor because of the presence of a thick marine carbonate sec­tion and interpretations could not be attempted. Rough topography and thick weathered sections added to the data quality problems. A dip seismic profile (seismic lines 42-RL-100, 7-RL-406, 37-RL43, and 28-RL-176) illustrates the onshore quality problem (Fig. 78). In the offshore and nearshore part of this profile, it is possible to define a thick Morro do Chaves carbonate platform (Alagamar carbonate platform) covered by Coqueiro Seco fan-delta deposits, as well as the pre-Aptian erosional truncation or unconformity on top of sequence II. A parallel dip seismic profile (Fig. 79, seismic lines 37-RL-117, and 48-668) crosses the Figure 78. Dip seismic profile composed of profiles 42-RL-100, 7-RL-406, 37-RL-42, and 28-RL-176, Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil, from onshore well 1-MB-1-SE to offshore limit of sequence II, showing Alagamar carbonate platform system, Coqueiro Seco fan delta and slope systems and erosional trun­cation of strata by pre-Aptian unconformity. 0 Q) . . s ...J 2 I •. I I •. •. I i .. ;. 1 I •. .. I I 1 I ;:::; •• II •• 2 Q) c: - 8 ..c:: · -0 ~ ~ TOP M. CHAVESIC. SECO FMS. \ \ -= TRANSITION SL.OPE/DELTA SYSTEMS = AIASS/VE CARBONATE M. c'H~vEs = BAs.E M. CHAVESIC. SEco FMs. . \ RIO SAO FRANCISCO SUB-BASIN Seismic Dip Section Lines• 42 -100 7-406 37 -42 28 -176 Alagamar carbonate platform in a sublacustrine canyon area, where several anomalies confirm the presence of the canyons: the canyons also are verified by stra­ tigraphic interpretation. Careful analysis of seismic lines in this area might permit detailed mapping of this important sublacustrine erosional feature. In conclusion, the widespread but poor quality of seismic profilea in the rift section of Sergipe-Alagoas Basin precludes a normal seismic strati­graphic approach, but careful analysis of better quality profiles permits recognition of some important aspects of the depositional systems, geologic erosional events and other features in the sub-basins. These document and confirm the general understanding of basin evolution inferred from limited well control. ANCIENT STRATIGRAPHIC ANALOGS The best ancient stratigraphic examples anala­gous to the Coqueiro Seco and Morro do Chaves sequence, and other Brazilian rift sequences, are rift basin sequences along the West African Coast (Fig. 80). The Uinta and Green River Basins and the Ridge Basin of the u.s. also exhibit stratigraphic similarities to Sergipe­Alagoas rift strata. West African Basins Gabon Basin, the African counterpart of Sergipe­Alagoas Basin (Fig. 97) was studied by Reyre et al. (1966), Belmont et al., (1966), Brink, (1974), and Lehner and Ruiter (1977). According to these authors, subsidence and sedimentation in the Gabon Basin started during Late Jurassic or Early Cretaceous. Since its inception, the dabon Basin has remained a half-graben structure but on a smaller scale the basin is charac­terized by a fabric of horst and grabens. A marine transgression occurred in the Gabon Basin shortly after the late Aptian, and a thick salt layer was deposited over earlier non-marine sediments, followed later by 176 ~~ ~ .. . 178 typical passive-margin marine sedimentary sequences. The pre-salt sequence in the Gabon Basin, which is known as the Cocobeach Group (Fig. 81) is composed of (1) a basal section of alternating sandstones, shales, and a thick succesion of bituminous shales which are genetic equivalents of the Barra de Itiuba Formation (sequence in Sergipe-Alagoas Basin: (2) a medial section composed of about 50% fine-to coarse-grained sandstones, sandier than the basal section, and about 50% of brown, gray and black shales deposited by alluvial fan, delta and subla­custrine slope systems genetically equivalent to the Coqueiro Seco Formation (sequence II) in Sergipe-Alagoas Basin: and (3) and an upper section equivalent geneti­cally to the Muribeca Formation (sequence III) in Sergipe-Alagoas Basin. The upper section in the Gabon Basin overlies an erosional surface similar to the pre-Aptian unconformity on the Sergipe platform area. Oil shows exist in almost all sedimentary units in the Gabon Basin. Oil production in Gabon is from the upper Aptian Gamba Formation, genetically equivalent to the Carrnopolis Member of the Muribeca Formation on the Sergipe platform of Sergipe-Alagoas Basin. Petroleum is also produced from open marine Upper Cretaceous and Tertiary sequences in turbidite reservoirs. '-•­ w :.""~o.­ ,__o._ ........,.,_ ·=­ '-°"·­ IW NI i.:=:::J =­ r-:--=i ---...­ ~---,.._ g=­ ~..­ -l-·---·--'•' Figure 81. Northeast-southwest and east-west cross sec­tions of Gabon Basin, Africa, showing stratigraphic and structural relationship of lacustrine elastic, evapo­rites and marine sediments. {From Brink, 1974). Oil has been found in the Congo-Cabinda Basins in rocks equivalent in age and facies to the Cocobeach Group, but the main production in Cabinda Basin comes from the Lucula Formatin (Malongo Field), equivalent to the N' Dembo Formation of Gabon Basin and Serraria Formation of Sergipe-Alagoas Basin (Sampaio, et al., 1973 and Ponte and Asmus, 1978). Cabinda Basin (Brice and Pardo, 1980) is a rift basin located in offshore Angola and is a prolific oil producer, which was infilled by more than 2,500 meters of non-marine sedi­ments and covered by Aptian salt. Pre-rift and rift strata are separated by a major unconformity and both are oil producers. Rift sediments are inferred to have filled a deep lacustrine basin produced by initial rift faulting. Organic-rich dolomitic shales, which are the best source rocks in the Cabinda Basin, grade upward into shallow-lacustrine green shale, turbidite sand­stones, and carbonate rocks. The shallow-water non­marine carbonate rocks which are genetically equivalent to the Morro do Chaves Formation in Sergipe-Alagoas Basin, are important reservoirs in the Cabinda basin. In the Congo Basin (fig. 80), a small field (Point Indienne) produces from coquinoidal limestone and inter­calated sandstones, similar to high energy facies of Morro do Chaves Formation (Geoservices, 1966). Organic­rich lacustrine source rocks (Brice et al., 1980) are inferred to exist in all West African marginal basins. The geometry and sedimentary character of the source beds in several basins along the West African coast indicate the presence of a number of deep Early Cretaceous lakes which extended along the South Atlantic Rift System, probably similar to present Lake Tanganyika and to ancient lakes where the Morro do Chaves and Coqueiro Seco sequence was deposited. Green River And Uinta Basins, U.S. Other lacustrine basins which have been studied in detail are the Green River and Uinta Basins of Utah, Colorado, and Wyoming (Fig. 82). The Green River Forma­tion is a complexly intertonguing, intermontane, lacus­trine sequence composed of more than 10,000 feet of sed­iments deposited in the Uinta Basin (Ryder, 1980) during the Paleocene and Eocene. Bradley (1931) and Ryder (1976) believe that much of this section was deposited in a large fresh-water lake, but other authors, such as Eugster and Surdan (1973), believe in a playa-lake ori­gin for the Green River Formation. Only the Wilkens Peak Member at the base of Green River Formation is accepted by all authors as a saline lake sequence of 112· 110• 1oe• 42" 40• 38" 1oe• 112· Figure 82. Generalized geologic map showing Uinta and Green River Basins separated by Uinta Uplift, Utah and Wyoming, U.S.A. (From Ryder, 1976). possible playa-lake environment. Stratigrahic analysis of these lacustrine sediments by Ryder (1976) indicate three major facies: (1) an open lacustrine facies occu­ pying primarily the distal or central basin areas and consisting mostly of organic-rich, mud-supported car­ bonate and claystone containing minor amounts of sand­stones and siltstones deposited in a relatively deep, stratified lake. Some of these open lacustrine rocks contain fossils (mollusks and ostracodes) and may have been deposited in nearshore environments at or near the hypolimnium; (2) a marginal-lacustrine facies composed of gray-green, calcareous claystone, channel-fill sand­stones, and grain-to mud-supported carbonates. Deposi­tional environments are inferred to have been deltaic, interdeltaic, and carbonate flats. Streams that supplied the deltaic system were probably perennial and provided the water needed to maintain the lake level; and (3) alluvial facies of alluvial fan, lower deltaic plain and high mud-flat origin are composed of thick sandstone and conglomerate beds displaying poorly devel­oped horizontal stratification. Red claystone, minor isolated channel-fill sandstones, and thin fossiliferous gray-green claystones characterize deposits of the mud­ flat environment. Isopach maps of major stratigraphic intervals in the Green River Formation of the Uinta Basin show that the thickest sedimentary section was not deposited around the periphery of the lacustrine basin, as is the case in most marine basins, but was deposited in the central part of the lake (Gould, 1951 and Visher, 1965) as in Sergipe-Alagoas Lower Cretaceous rift lakes. Lacustrine deltas, however, exhibit local thickening along the basin margins. Ridge Basin, U.S. Ridge Basin, California is an ancient analog of the Sergipe-Alagoas Basin. The basin, which was studied by Link and Osborne (1978), displays facies similar to the Coqueiro Seco Formation. Ridge Basin is a wedge­shaped trough 15 by 40 kilometers in size, which contains over 30,000 feet of lacustrine sediments. The basin was formed by subsidence along the San Gabriel strike-slip fault. Stratigraphic sections, electric logs from bore­holes, petrography, and maps document three non-marine sedimentary facies or sequences: (1) alluvial fan­fluvial facies, (2) marginal lacustrine facies, and (3) offshore lacustrine facies (Figure 83). Lacustrine sedimentation was initiated by deposition of a thick turbidite-deltaic sequence in a relatively deep-lake East ~A l.. LU¥1AL r&M Olll'OS1TS ~S LUlll" Jl' OLO[O STJIATA ~' LOii'( O• OU.TA "RONT CHAHN[LS ~1.. AC UST•U•( ..UO LJtu111e101rrs 0 SH OIU L.IN[ SANO Figure 83. Schematic diagram illustrating deep-water lacustrine sedimentation model, Ridge Basin, California. (From Link and Osborne, 1978). environment. Stratification of the water column is documented by ferroan dolomite beds (3.8%) associated with organic-rich offshore shale facies. Thick, inter­ bedded sandstones and organic-rich shales and mudstones are typical offshore facies. Strata are commonly graded, contain mudstone rip-up-clasts, dish structures, sole-marks and slump structures. Laterally continuous amalgamated, conglomeratic, sandstones which overlie the turbidite sequence contain channel-fill, cross-bedded strata that are interpreted as distributary channel deposits which supplied sediments to the underlying slope system. Later, the delta is inferred to have prograded over the deeper distal f acies like the Coqueiro Seco deltas in Sergipe-Alagoas Basin. Marginal lacustrine facies in the Ridge Basin are complex, and they represent shoreline, nearshore, mud flat, sand flat, and f luvial-deltaic environments along both sides of the basin where coarse elastic and carbonate facies were intercalated with transitional lacustrine mudstone facies. Coarse-grained fluvial and alluvial fan sedi­ments were deposited in a series of coaslesced fans along the margins of the basin. They are composed of poorly stratified and sorted conglomerates which grade into distal sandstones displaying tabular crossbeds. Fan deltas were also deposited along the margin of the Ridge Basin lake. The fan complexes clearly built into the shallow-water lake, prograding over initial bottom­set and foreset strata. Locally, thick fluvial sequen­ces cap these complexes: with delta shifting or abandon­ment, thick deposits of interdistributary mud and sand accumulated over the fan delta facies. In conclusion, Ridge Basin lake deposits infilled a narrow, tec­tonically active basin in which a great quantity of coarse-grained sediments accumulated in alluvial fan, deltaic, and sublacustrine environments, similar to the sedimentary evolution inferred for the Coqueiro Seco sequence of Sergipe-Alagoas Basin, Brazil. HOLOCENE DEPOSITIONAL ANALOGS Great African Rift System The Morro do Chaves and Coqueiro Seco Formations of Sergipe-Alagoas Basin were deposited in an Early Cretaceous rift-lake environment during the opening of the South Atlantic Rift. The modern East African Rift System and its lakes provide an excellent recent example of this style of deposition. According to Matzuzawa (1969) tectonic belts and rift-valleys or fracture belts are the weakest and most unstable zones of the earth crust where forces generated by convection currents are focused. The Great African Rift System is a recent rift-valley within a faulted block of the crust extending north-south along the Arabian Peninsula and the eastern part of African continent (Figure 84). The rift exhibits a uniform width of about 30 to 60 kilome­ters and a total length exceeding 7,000 kilometers. The rift system is a long zone of depressed continental crust bounded by several discontinuous fault systems. Volcanic activity is associated with this geotectonic feature and a high geothermic flux is shown by the stu­dies of Van Herzen (1972). 188 OMO DELTA, LAKE RUDOLF A great number of studies of the structural evo­lution of the Great African Rift System have been made: Dixey (1956), McKenzie et al. (1970), Baker et al. (1972), Lowell et al. (1975), Freund (1966), Girdler (1972), and Milanovsky (1972). Only a few workers have reported on the sedimentological and stratigraphic character of this major feature, especially the lacus­trine and sublacustrine sedimentation that occurs in the many giant lakes formed along the structural system (Fig. 84). Butzer (1971) described the geomorphology, geometry and recent surficial sediments of the Omo delta area in Lake Rudolf. No descriptions of the sedi­mentation in the lake itself is presented because of the absence of data. Lake Rudolf (Fig. 85) is a non-outlet lake covering 7,500 square kilometers. The total catch­ment area is 146,000 square kilometers. About 80-90% of the annual water influx appears to be derived from the Ethiopian Plateau, via the Omo River. Its drainage basin equals 73,000 square kilometers. The Omo delta is situated in a tectonic depression at the northern end of Lake Rudolf (Fig. 85). Compared with marine estuarine deltas the Omo delta is small, but it is a classic example of a delta prograding into a non-outlet lake. ~ ( 2) 100 ~ ~~ llhmO IJIO#Q Jr [ . 0 It is probably very similar to deltas that infilled the Sergipe-Alagoas sub-basins during late Coqueiro Seco time. The Omo delta is situated in a complex tectonic depression known as the Lower Omo Basin, an extension of the Lake Rudolf Trough. The Omo drainage displays a striking geometric arrangement of its principal stream course because of tectonic control. Geomorphologic units of the Omo delta (Fig. 86) are a meanderbelt, delta flats, delta fringe, and prodelta. The meander­belt zone comprises about 175 square kilometers of allu­vial sediments composed of bed-load and medium-grained point bar sands. Finer channel-fill sediments are depo­sited whenever the flood surge wanes temporarily. The very low relief delta flats cover 370 square kilometers, and the delta fringe, now partially submerged, is the transitional fluvial-lacustrine environment of the Omo delta plain. The birdfoot shape exhibited by the con­temporary shoreline is largely due to the Dielerhiele distributary extending 12 kilometers into the lake (Fig. 86). The delta lobes actually are lobate in geometry, but recent rises in lake level have resulted in sub­mergence of large areas of the lobes exposing prin­cipally the leveed distributaries. The prodelta zone in front of the delta fringe is characterized by turbid waters in all seasons indicating fluvio-lacustrine sedi­ mentation. Persistent wave activity assures deep mixing of the lake water, so that oxygen is abundant at all depth levels. Lake level is now higher than in the recent past; consequently, the Omo channels are now drowned by estuarine conditions and suspended mud is the only sediment reaching the river mouth. Bed load sands are being trapped in the estuaries. Hence, the present Omo delta is atypical and is not prograding. When the estuaries are filled, it will again prograde a normal delta sequence. Studies of lake levels show that Lake Rudolf fluctuations probably correspond to climatic variations. These changes of lake level resemble other closed lakes of the Great African Rift System (e. g. Victoria, Tanganyika). These changes are caused by major climatic variations with duration of several deca­des, basically influencing the lake level and, in turn, affecting sedimentation and f acies distribution. LAKE KIVU AND LAKE TANGANYIKA Staffers and Hecy (1978) studied Lake Kivu and Lake ·Tanganyika, two deep freshwater lakes in the west­ ern branch of rift zone of East Africa (Fig. 84). These lakes have a catchment area of 7,140 and 231,000 square kilometers, and a lake area of 2,060 and 32,000 square kilometers, respectively. Lake Tanganyika is thermally stratified and exhibits a perennial thermo­cline at approximately 100 meters, below which condi­tions are anoxic. Cores from the bottom of Lake Kivu and Lake Tanganyika were studied, but maximum penetra­tion was only 2.55 meters (Fig. 87). Most of present sediments are clays containing smectite and kaolinite and siderite-rich, or pyrite-rich clays containing bet­ween 6 to 12% organic content (Degens et al., 1971). It is possible that present sedimentation in these anoxic lakes could be analogous to deposition of basal bituminous shale (operational unit B-1) of the Coqueiro Seco slope and basin system. Alpine Lakes, Europe Several Alpine lakes have been studied in much more detail than the East African lakes, and substantial data are available describing the lacustrine and subla­custrine processes and sediments of these lakes. LAKE BRIENZ, SWITZERLAND. Strum and Matter (1978) studied Lake Brienz (Switzerland) which is 14 kilometers long and 261 meters deep. It is an oligotrophic lake with a distinctive K 15 K 13 K 1 K 12 K 10 K 4 LOST LOST LOST LOST LOST 0 100 ? ! 200 ' l 254m l 300 440m .~ 110 229m ~o;o;o;o}:'\'.:.:&3 Ar•90"it• •oo Monohydro · 118m k -=..,-:H e.lcit• Mg · C•kite / !:·:·:-::-:-:-::·:·:·:] ProtodotornUe Sldetlte Rich C=:J Sedtment r---1 Pytite Rich ? STRATIGRAPHY OF LAKE KIVU SEDIMENTS L___J-· 90m (-.,Dei>t•I 700 BOO ___ _J 402m Figure 87. General stratigraphy of Lake Kivu bottom sediments. (From Stoffer and Hechy, 1978). 197 thermal stratification during the summer, similar to other temperate-region lakes. Sedimentation in this lake is entirely elastic, and is dominated by two rivers which enter at opposite ends. Turbidites from both deltas interfinger at the center of the lake; small deltas are being built at the river-lake interface. Three major types of sediments are being deposited: (l} massive and lenticular deltaic sand and silts; (2} laminated and homogenous by-pass muds on slopes; and (3} laminated muds interbedded with graded sands and silts on the basin plain. Massive turbidite beds up to 1.5 meters thick apparently are the product of high density tur­bidity currents that occur only once or twice per cen­tury as a result of catastrophic flooding or landslide. Such turbidites cover most of the basin plain with coarse sand which grades distally with silty layers. A second type of turbidite deposit is composed of thin, dark layers of very fine sand apparently deposited by low-density turbidity currents which occur one or more times each year in association with spring meltwaters and heavy rainfalls. Mud on sublacustrine basin flats apparently results from annual suspension sedimentation cycles related to thermal stratification in the lake waters and seasonal input of sediment-laden river waters. LAKE GENEVE, SWITZERLAND Houbolt and Jonker (1968) studied recent sedi­mentation in Lake Geneve (Lac Leman), a deep freshwater, oligotrophic lake. Seismic profiles and cores were ana­lyzed, off the mouth of the Rhone River, showing the presence of a sublacustrine channel with natural levees extending to a depth of about 200 meters. Below this depth the natural levees disappear and the channel disappears at 280 meters. Where the channel disappears, a fan-shaped sublacustrine sand body has been deposited: the fan extends into the deepest part of the lake (309) meters). The main sublacustrine channel was filled with fine to medium sand and the levees consist of mud con­taining fine sand and silt layers. The central part of the sublacustrine fan is composed of massive sand; laterally the sands grade into alternating sand and mud. The part of slope outside the channel is a muddy by-pass area. The Rhone River is building an offlapping deltaic section into the lake: a muddy zone of prodeltaic facies separates the slope sediments from the sandy deltaic sediments (Fig. 88). In summary, the following ver­tical sequence is being deposited in Lake Geneve: (1) central basinal plain composed of muddy deposits con­taining very little sand: (2) lower slope and basinal sand deposits forming sublacustrine fans containing intercalations of mud; (3) upper slope and prodelta mud deposits containing minor amounts of sand deposited by migrating sublacustrine channels; and (4) deltaic sand deposits (top-, fore-, and bottomset units). HYDROCARBONS OCURRENCES AND ANALYSIS General Considerations Presently, Sergipe-Alagoas Basin produces about 50,000 bbl/day of oil { 8,000 cubic meters). Total cumulative production of oil in June, 1980 was 190 million barrels (30,000 million cubic meters) and 121 billion cubic feet of associated gas. This production is mostly (90%) from rift reservoirs supplied by rift source rocks, principally from rift sequences III and {Carmopolis Member of the Muribeca Formation and Barra de Itiuba Formation, respectively). The pre-rift Jurassic Serraria Formation is also a principal reservoir where faulted into contact with rift source beds. The Coqueiro Seco and Morro do Chaves Formations which comprise rift sequence II, the subject of this study, have produced only small amounts of oil and gas. Production has been principally from Alagoas {Cidade de Sao Miguel dos Campos, Fazenda Pau Brasil, and Coqueiro Seco Fields) and Rio Sao"" Francisco sub-basins {Robalo N , Field). Presently, only the Cidade de Sao Miguel dos Campos {gas) and Fazenda Pau Brasil {oil) Fields are 201 being developed. Cumulative daily production of all fields producing from rift sequence II is about 575 bbl/d (90 cubic meters): gas production data are una­ vailable. Other small oil and gas fields such as Jequi~, Vila Jequi~, Pia~abu9u and Rio Sao Francisco Fields have one or two depleted oil and gas wells and constitute non-economic occurrences of hydrocarbons. In terms of petroleum exploration, lacustrine rocks historically have not been considered very impor­tant targets, particularly by North American and European companies. However, recognition of the many world wide basins which contain lacustrine strata and the many potential reservoirs/source beds contained therein has generated new interest in rift prospects. Chinese oil fields contain several billion barrels of reserves in non-marine sediments (Chen, 1983: Meyerhoff, 1970). As stated by Ryder (1980), it was only after Hedberg's paper (1968) that lacustrine shales were widely accepted as a source of petroleum, especially the black shales of the deep lacustrine offshore facies. Lacustrine source rocks and the oil derived from them are normally different from marine counterparts: non­marine crude oil has high wax paraffin content because it is derived from continental, lignitic organic matter or non-marine algae. Brice et al. (1980), who studied the Melania Formation in the Gabon Basin of West Africa showed that the principal source rock of this and other basins along the West African coast was deposited in a deep strati­fied lake. This permitted the preservation of organic matter in a stable, low energy, anaerobic environment. Consequently, the Melania Formation contains up to 20% bulk volume of organic content in dolomitic shale. This is similar to recent sediments found in Lake Tanganyika. Similar deep anoxic lake conditions probably existed during rift-phase sedimentation in the basins along the Brazilian East Coast. For example, the basal part of the Coqueiro Seco Formation (operational unit B-1) is composed of black, fissile, bituminous shale probably deposited in a relatively deep, starved lake showing high organic productivity and high degree of preser­vation. Therefore, these shales are considered to be excellent source rocks. Nevertheless, only small amounts of petroleum have been discovered in the Morro do Chaves and Coqueiro Seco strata in Sergipe-Alagoas Basin. At present, the total production of all oil fields (Fig. 89) in sequence II is around 575 bbl/day, but some fields have not been fully developed. Fernandes et al. (1981) recognized three major types of prospective plays in the Morro do Chaves and Coqueiro Seco sequence: (1) Morro do Chaves carbonate plays: (2) Coqueiro Seco slope sand plays: and (3) Coqueiro Seco fan delta and fluvial-deltaic sand plays. Plays in the Morro do Chaves and Coqueiro Seco sequence are characterized by unique structural and stratigraphic relationships of reservoir and source bed related to the nature of the principal depositional systems operative at that time. Morro do Chaves Carbonate Plays There is no current conunercial production from this play, but some good oil and gas shows have been . '\/ . detected in the Alagoas and Rio Sao Francisco Sub- basins. In Alagoas Sub-basin mostly high energy car­ bonates (grainstones and packstones) of the Furado and Pilar carbonate platforms are covered by interdeltaic or fluvial-deltaic sediments. These sediments normally contain inunature source beds because of relatively shallow burial. Some sparse geochemical data in the Furado area confirm inunaturity of these rocks (Fiqueiredo et al., 1974). Therefore, these areas are 20S *-Oil. SHOW "tl-GAS SHOW rvvv--EROSIONAL uw1r /FAULT NR-NOT REACHED E -ERODED • -WELL ( :.: -OIL FIELD not prospective for this play. Nevertheless, basinward, especially in the Jequia area ( wells l-JA-1-AL and other), upper slope or shallow water limestones composed of argillaceous wackestones and packestones are covered by black, bituminous shale, probably a good source rock for petroleum. Some oil was produced from these carbonatesr well l-JA-1-AL produced an estimated 50 MCF/day of gas during a formation test. ~ In Rio Sao Francisco Sub-basin, the Morro do Chaves carbonate platform play (Alagamar and Marituba carbonate platforms) provides better conditions to trap petroleum. In this sub-basin, the play can be subdivided in two types: (1) a play in which the source rock is provided by overlying Muribeca evaporitic shales depo­sited unconform.ably over the carbonate reservoirsr and (2) a play in which the source rocks are slope or distal shales of fan delta and slope systems which were depo­sited on and lateral to the carbonate platform. A good example of the first type of play is in Pacatuba area {well l-PTA-3-SE) and Marituba area {1-well-MT-.l-AL) where good oil shows were detected. Here oil production was obtained in a formation test in reservoirs beneath the unconformity in well l-PTA-3-SE. The Ponta dos Mangues area (well l-PDM-1-AL) and the Sergipe Submarine #47 area (well l-SES-47, Fig. 28) are good examples of the second type of play1 little oil was produced in both areas in formation tests from near the top of the car­bonate section beneath distal fan delta shales. In summary, Morro do Chaves carbonate platform plays are probably economically marginal in the Alagoas Sub-basin. They are associated with paleohighs where porous carbonate facies overlain by basinal shales may ;v be present. In the Rio Sao Francisco Sub-basin, best prospects are in areas where carbonate reservoirs are either overlain by the pre-Aptian unconformity or by distal fan delta facies. It is recommended that maps be prepared to show where the Morro do Chaves limestones are unconformably overlain above the pre-Aptian uncon­formity by Muribeca black shales which can serve as both source and seal. Further, maps should be made to show where faults may provide pathways to bring petroleum upward from deeper source beds (slope facies) to accumu­ late in porous reservoir rocks of the carbonate plat­ form. In areas where the Morro do Chaves carbonate rocks are in contact with overlying Muribeca evaporitic rocks, along the pre-Aptian unconformity,dolomitization can be expected. This ought to improve the quality of reservoir rocks, such as in the Pia9abu~u and Marituba areas (Fig. 25-G). Coqueiro Seco Slope Plays Extensive turbidite deposition occurred in the central parts of the Alagoas and Rio S~o Francisco Sub-basins. Good quality turbidite reservoir rocks and source beds exist in the Alagoas Sub-basin where opera­tional unit B-1 serves as a source bed. In this type of play, petroleum is produced within slope and basinal shales, flushed directly into adjacent turbidite reser­voirs and accumulated in updip pinchout areas against contemporaneous highs or in traps associated with cc.n­temporaneous faults. Oil and gas fields and noncommer­cial shows associated with the Coqueiro Seco slope system are shown in Figure 90. In the Alagoas Sub-basin three small fields were IV • discovered in this play: Cidade de Sao Miguel dos I Campos, Fazenda Pau Brazil, and Jequia Fields. Only the first two fields are presently being developed: the third one is depleted. The Cidade de Sao"' Miguel dos Campos Field is associated with an updip pinchout of operational unit D (Figs. 54, 90, 91): the sandstone reservoirs onlap the carbonate slope on the margin of the Furado carbonate platform (Fig. 92). Recently l ! PRINCIPAL AREAS OF TURBIDITE 0E""1SITION -tl-Oil SHOW tr-GAS SHOW A/V'\/"-EROSIONAL LIMIT /FAULT HR-NOT 1'EAC11EO E -ERODED • -WELL ,•) -OIL FIELD ()-OASFIELO / I I i i llOm;, _JI tial turbidite reservoirs. ALAGOAS REPRESENTATIVE CORES OF COOUEIRO SECO CIDADE DE SAO MIGUEL 3-CSMC 8-AL • HC PRODUCTION SUB-BASIN FLUVIALIDELTAIC & SLOPE SYSTEMS DOS CAMPOS AREA SHALE ,stlty, parallel laminated. CONGLOMERATE ,with cobbles of shale, quartz. granite, sandy matrix, calcareous. SHALE, dark gray, carbonaceous, with small lenses of coal, parallel laminated. CONGLOMERATE ,as above. SANDSTONE; fine-to medium-grained, cross-bedded. arkos1c, ca tcareous . SHALE ,dark gray, massive, with plant fragments. SHALE ,as above . CONGLOMERATE ,grading to con s "' "' ('lO " 0 0 '.', v '.>) in 0 \ 0 o;: ~~ EXPL ANA Tl ON cpO"' 500 D ~v "'_,_ "'lf, Operational Sand Units --... Limit between Slope and c z Delta Systems (by-poss shale) 400 Ci,~ of> ~ 300 ~ ~ Shale Units SCALE -I Fault w " "' c D ~ "' "'ii 200 "' 1Z 0 E D s Carbonate Platform SP Spontaneous Potential ~ 100 Ties well ~ 3-3' 0 ' . ' SN Resistivity (short normal) 4 3 2 I 0 \ Kilometers ' . ALAGOAS SUB BASIN STRATIGRAPHIC DIP CROSS SECTION B-8' Plate VI. Stratigraphic cross section c-c•, dip sec­tion, Coqueiro Seco and Morro do Chaves Formations, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil, showing depositional systems and operational units distribution. 2 41 c' I-SAU-I-AL 1-FPB-2-AL I-RN-I-AL l-BSM-1-AL SP SN DATUM -­ -­ .,,,,. - - ---­ TOP OF COQUEIRO SEGO FORMATION - -- _.. -- .,,,,,.. ,,,,,,,.. i! ?J MORRO 00 CHAVES FORMATION FURADO CARBONATE PLATFORM 500 Ties well 2-2' LAGOA MANGUABA LOW EXPLANATION Limit between Slope andOperational Sand Units 400 D / Delta Systems (by-poss shale) ~ 300 j SCALE Shale Units Fault D ~ 200 100 Carbonate Platform SP Spontaneous Potential D o+-~~~~~~~~~~~~~~~ D Mixed Carbonate I 0 2 3 SN Resistivity (short normol) Kilometers Clostic Unit GR Gamma ray ALAGOAS SUB BASIN STRATIGRAPHIC DIP CROSS SECTION c-c' Plate VII. Stratigraphic cross section D-D', dip sec­tion, Coqueiro Seco, Morro do Chaves, and Ponta Verde Formations, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil, showing depositional systems and operational units distribution. D o' 2-LMST-1-AL l-TP-1-AL I-NF-I-AL SP SN DATUM TOP OF COO. SEGO OR VERDE FORMATIONS 500 400 a;"' 300 ~ 200 IOO -- __§ __ _...,.,.­ ..,,,,...,~ -::::------­ / / / / / EXPLANATION LAGOA MANGUABA LOW Limit bet ween Slope end Operotionol Send Units DeIto Systems (by-poss shale) Shale Units Foul! D / D ~ SCALE Mixed Corbonote I SP Spontuneous Potential Clos tic Unit SN Resis11vity ( short nor mo I ) D o+-~~~.,.-~~~r-~~~..--~~--, 0 3 4 Kilometer s ALAGOAS SUB BASIN STRATIGRAPHIC DIP CROSS SECTION D-01 Plate VIII. Stratigraphic cross section E-E', dip sec­tion, Coqueiro Seco, Morro do Chaves, and Penta Verde Formations, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil, showing depositional systems and operational units distribution. 2 .5 E E' I-NEP -I -AL I-MD-I-AL l-MD-2-AL DATUM TOP OF COO. SECO OR P VERDE FORMATIONS PILAR CARBONATE PLATFORM 500 400 ~ 300 EXPLANATION ~ SCALE 200 Limit between Slope and Operational Sand Units ---.. Delta Systems (by-poss shale) D 100 O+-__,__,__,..----,--,--,...---,--,--.--,--,--,--. Shale Units Fault D '\ 0 2 3 4 Kilometers Carbonate Platform SP Spontaneous Potential D Mixed Carbonate I SN Resistivity (short normal) ~ Clostic Unit ALAGOAS SUB BASIN STRATIGRAPHIC DIP CROSS SECTION E-E' Plate IX. Stratigraphic cross section F-F', dip sec­tion, Coqueiro Seco, Morro do Chaves, and Ponta Verde Formations, Alagoas Sub-basin, Sergipe-Alagoas Basin, Brazil, showing depositional systems and operational units distribution. F F' Plate x. Stratigraphic cross section 1-1', strike section, Morro do Chaves Formations, Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil, showing depositional systems distribution. 1' l-VV-1-SE 2-BAST-1-SE I-TO-IA-SE l-PTA-3-SE l-PTA-4-SE SP SN SP SN SP. DATUM TOP OF MORRO DO OiAVES FORMATION Ties well ALAGAMAR CARBONATE PLATFORM A-A' Ties well 500 B-81 400 300 ~ IP SCALE - IP 200 ~ EXPLANATION 100 ~Fan Delta System ~ Fault --~~--~~~--~~~.--~~-+-O D 4 3 2 0 Kilometers Carbonate Platform SP Spontaneous Potential Ties well c-c' SN Resistivity (short normal) - RIO SAO FRANCISCO SUB BASIN STRATIGRAPHIC STRIKE CROSS SECT ION 1-11 Plate XI. Stratigraphic cross section 2-2', strike section, Coqueiro Seco and Morro do Chaves Formations, Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil, showing depositional systems distribution. 2 251 2' I-PR-I-SE 1-PTA-2-SE 1-so-1-sE 1-PIA-2-AL I-Pl A-I-AL SP SN 12.1 km DATUM MORRO DO CHAVES TOP OF M. CHAVES/ FORMATION COQUEIRO SEGO FORMATIONSDO CHAVES I FORMATION Ties well A-A' ALAGAMAR CARBONATE PLATFORM ./~::UBA CARBONATE PLATFORM ~ I SERGIPE ~ PLATFORM /~ // Uplifted oreo EXPLANATION Limit between Slope andCarbonate Platform System Delta Systems (by pass shale) D / Slope/ Basin System Fault D '\ D Fan Delta System SP Spontaneous Potential SN Resistivity (short normal) o.J_~~~..-~~--,r-~~......,...~~~~ 2 3 4 Kilometers Plate XII. Stratigraphic cross section 3-3', strike section, Coqueiro Seco and Morro do Chaves Formations, Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil, showing depositional systems distribution. 3' area 1-SES-47 SP SN Plate XIII. Stratigraphic cross section 4-4', strike section, Coqueiro Seco and Morro do Chaves Formations, Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil, showing depositional systems distribution. 4' 3-RB-3-SES 1-SES-28 3-RB-17-SES 3-RB-2A-SES 1-SES-23 1-SES-39 1-SES-22 GR SN GR SN SP SN GR SN SP SN GR SN SP SN -------11.0km--------t------2.4km--------+------0.8km---------l-------l.8km-------t-----­ _... DATUM TOP OF COQUEJRO SEGO / FORMATION _.... ---­ MORRO DO Ties well EDRMATION ------------- c-c' - \---=~==--=== ---? ---~="""'""==-:::::::::'""'.::::=--=-? ---­ SERGIPE --==-=­ PLATFORM Uplifted area 500 Ties well ILHA DAS FLORES A-A' EXPLANATION LOW 400 ALAGAMAR CARBONATE PLATFORM D Slope I Basin System 2 ~ 300 Fault SCALE :;;:"' 200 Fan Delta System SP Spontaneous Potential ~ 100 - RIO SAO FRANCISCO SUB BASIN o+----.....----...----.------. Carbonate Platform SN Resistivity (short normal) 0 2 3 4 D STRATIGRAPHIC STRIKE CROSS SECTION Kilometers Limit between Slope and GR Gamma Ray / Delta Systems 4-41 Plate XIV. Stratigraphic cross section A-A', dip sec­tion, Coqueiro Seco and Morro do Chaves Formations, Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil, showing depositional systems distribution. A I-PX-I-SE l-PX-2-SE 2-BAST-l-SE I-PR-I-SE 1-ALG-1-SE I-SIS-I-SE l-SES-27D 3-RB-17-SES SP SN SP SN SP SN SP SN SP SN SP SN SP SN SP SN '----2.3km----...______ _ _ _____._______6.8km------....--'"-------­ O~P~O~F~M~ORRODOCHAVES/ _............."""t:>..::w::~~-.£o1:::::...::::..o::::::.:::::::::::::::::::::::::::::::::~ ~c:::::::::::::::::--...............--~.....................::::;;~~~;;;:::::---------........................-.................--........................---:-~~r'"--....--..-...............................-...................................___..................--11~;------,_..~~~"'\'"-,~-r......._,..\"~tt~"\'............"'\..........\"._.._\'"'\°"-........'f'.........~-1\ft~:---~---"'\ ............."\'-..-"\,..._,,..'"\_....~............."\............'"\'.........."\ilr--....._..~...._...,..-;;:-_...."\°...__~.........."\---"IC-"';--"\..........\'"._.._\'°'__,~~.............."":---~-..-'\°...........,\;=~;7,,_....-;T~ JAPOATA ­PLATFORM Plate xv. Stratigraphic cross section B-B', dip sec­tion, Coqueiro Seco and Morro do Chaves Formations, Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil, showing depositional systems distribution. B I-TO-IA-SE SP SN l-PTA-2-SE SP SN l-PDM-1-SE SP SN l-SES-39 SP SN B' FAN DELTA MORRO TOP OF M. CHAVES / COO. SECO FORMATIONS ALAGAMAR CARBONATE PLATFORM SLOPE I BASIN SYSTEM Uplifted area ILHA DAS FLORES LOW EXPLANATION D ---Limit between Slope and Carbonate Platform Delta Systems 500 Slope I Basin System 400 D '\ Fault "' v 300 D ~ SCALE Fan Delta System SP Spontaneous Potential ~ 200 100 SN Resistivity (short normal) o._______~------~------.--------. 0 2 3 4 Kilometers RIO SAO FRANCISCO SUB BASIN STRATIGRAPHIC DIP CROSS SECTION 8-81 Plate XVI. Stratigraphic cross section c-c•, dip sec­tion, Coqueiro Seco and Morro do Chaves Formations, Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil, showing depositional systems distribution. ~ 61 c' c 1-IDA-SE I-CG-I-SE l-IDA-1-SE 1-SES-22l-PTA-3-SE I-SO-I-SE SP SNSP SNSP SN SP SN SP SN SP SN DATUM TOP OF M. CHAVES I COQ. SECO FORMATIONS ALAGAMAR CARBONATE PLATFORM Uplifted area EXPLANATION Limit between Slope and Carbonate Platform Delta Systems Slope I Basin System Fault D -­ D '\ Fan Delta System SP Spon·toneous Potential SN Resistivity (short normal ) FRANCISCO SUB BASIN DIP CROSS SECTION c-c' Plate XVII. Stratigraphic cross section D-D', dip sec­tion, Coqueiro Seco and Morro do Chaves Formations, Rio Sao Francisco Sub-basin, Sergipe-Alagoas Basin, Brazil, showing depositional systems distribution. D 263 I-MT-I-AL l-PIA-18-AL I-PIA-I-AL o' l-RSF-6-AL l-RSF-1-AL SP SN SP SN SP SN SP SN SP SN DATUM TOP OF M. CHAVES I COQ. SECO FORMATIONS PALMEIRA PLATFORM () 0 :;o c 2-2' Uplifted area I) rn ILHA DAS FLORES -n 500 ---.... Limit between Slope and D ~ Carbonate Platform J::> LOW Delta Systems c 400 ~ (/) D Slope /Basin System w300 " Fault Qj SCALE \ ::::? 200 0 Fan Delta System SP Spontaneous Potential 100 0+-~~~.,-~~~r-~~-y~~~--. SN Resistivity (short normal) 0 2 3 4 - RIO SAO FRANCISCO SUB BASIN Kilometers STRATIGRAPHIC DIP CROSS SECTION o-o' BIBLIOGRAPHY Asmus, H. E., and Porto, R., 1980, Diferen~as nos esta­gios iniciais da evolupao da margem continental Brasileira: Possiveis causas e implica~oes, in Anais do XXXI Cong. Bras. Geol., Santa Catarina, V. 1, p. 225-239. Baker, B. H., Mohr, P. A. and Williams, L.A. J., 1972, Geology of the Eastern rift system: Geol. Soc. America Spec. Pub. 136, 67 p. Bates, c. c., 1953, Rational theory of delta formation: Am. Assoc. Petroleum Geologists Bull., v. 37, p. 2119-2162. Bell, H. s., 1942, Density currents as agents for transporting sediment: Jour. 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