Browsing by Subject "Geothermal resources -- Texas -- Gulf Region"

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Depositional setting, structural style, and sandstone distribution in three geopressured geothermal areas, Texas Gulf Coast
(University of Texas at Austin. Bureau of Economic Geology, 1983) Winker, C. D.
Three areas in the Texas Gulf Coastal Plain were studied using electric logs and seismic reflection data to interpret their depositional and structural history and to compare their potential as geopressured-geothermal reservoirs. The Cuero study area, on the lower Wilcox (upper Paleocene) growth-fault trend, is characterized by closely and evenly spaced, subparallel, down-to-the-basin growth faults, relatively small expansion ratios, and minor block rotation. Distributary-channel sandstones in the geopressured lower Wilcox Group of the South Cook fault block appear to be the best geothermal aquifers in the Cuero area. The Blessing study area, on the lower Frio (Oligocene) growth-fault trend, shows wider and more variable fault spacing and much greater expansion ratios and block rotation, particularly during early Frio time. Thick geopressured sandstone aquifers are laterally more extensive in the Blessing area than in the Cuero area. The Pleasant Bayou study area, like the Blessing area, is on the Frio growth-fault trend, and its early structural development was similar; rapid movement of widely spaced faults resulted in large expansion ratios and major block rotation. However, a late-stage pattern of salt uplift and withdrawal complicated the structural style. Thick geopressured lower Frio sandstone aquifers are highly permeable and laterally extensive, as in the Blessing area. In all three areas, geopressured aquifers were created where early, rapid movement along down-to-the-basin growth faults juxtaposed shallow-water sands against older shales, probably deposited in slope environments. Major transgressions followed the deposition of reservoir sands and probably also influenced the hydraulic isolation that allowed the buildup of abnormal pressures. Of the three areas, the Pleasant Bayou area has the best potential for geothermal energy production because of larger fault block area, greater thickness and lateral continuity of individual sandstones, and higher formation temperatures and pressures.
Geothermal resources, Frio Formation, middle Texas Gulf Coast
(University of Texas at Austin. Bureau of Economic Geology, 1975) Bebout, Don G.; Agagu, Olusegun Kokumo, 1948-; Dorfman, Myron
Knowledge of the regional sand distribution and its relationship to formation temperature and pressure is a preliminary step in evaluating the geothermal resources of the Frio Formation. At depths generally greater than 7,000 feet, the sands and shales of the Frio Formation are overpressured and undercompacted. The insulating effect of these overpressured and undercompacted sediments results in the accumulation of subsurface heat and, thus, high temperature water. The local variations of depth to top of geopressure are related to the distribution of sand and shale lithologies and to the location of growth faults. For more information concerning origin of geopressure or high temperatures, see Jones (1970) and Dorfman and Kehle (1974). Bruce (1973) discusses the nature of growth faults in detail. The resource in the geopressured zone consists of high-temperature water with relatively low salinity and with dis solved methane gas. The objectives of this study were to determine regional sand distribution of the Frio Formation, identify depositional environments, and delineate the geopressured zone and its relationship to sand/shale distribution, growth faults, and fluid temperatures in the Middle Texas Gulf Coast. This study is essentially an extension of that completed earlier for South Texas (Bebout, Dorfman, and Agagu, 1975); all correlation and mapping units are the same as those represented in the South Texas report. The Energy Research and Development Administration, through the Lawrence Livermore Laboratory, supported this study of the geothermal resources of the Frio Formation in Middle Texas Gulf Coast.
Geothermal resources, Frio Formation, upper Texas Gulf Coast
(University of Texas at Austin. Bureau of Economic Geology, 1976) Bebout, Don G.; Loucks, R. G.; Bosch, S. C.; Dorfman, Myron
The objective of this study is to identify major sand trends, which, along with subsurface temperatures and pressures, aid in evaluating the potential of producing geothermal energy from the Frio Formation, Upper Texas Gulf Coast. During the Tertiary, huge quantities of terrigenous sediments were deposited as gulfward-thickening sedimentary wedges along the Texas Gulf Coast. The sand and shale making up these wedges were transported across a broad fluvial plain and deposited in deltaic complexes or were reworked by marine processes into strandplains and barrier islands. Growth faults developed contemporaneously at the site of maximum deposition as a result of rapid loading of large quantities of delfaic and strandplain sands onto previously deposited prodelta and shelf muds. These growth faults allowed the accumulation of extremely thick sections of sand and also caused the isolation of many of these sand bodies from porous updip sands; pressured reservoirs developed after further loading and compaction (Bruce, 1973; Jones, 1975). This study is investigating geopressured geothermal reservoirs in this setting. Limited data obtained from deep wells drilled for oil and gas indicate that many of these large sand reservoirs are filled with water which has high temperature, is relatively low in total dissolved solids, and is saturated with methane gas. To be suitable for electric power generation, the reservoir should have a volume greater than 3 cubic miles (which is equivalent to 300 feet of sand distributed areally more than 50 square miles), permeability greater than 20 millidarcies, and subsurface temperatures higher than 300°F. This report reviews the results of the Bureau of Economic Geology regional study of the Frio Formation in the Upper Texas Gulf Coast. It is a continuation of two similar studies of the Frio in the Lower and Middle Texas Gulf Coast (Bebout, Dorfman, and Agagu, 1975; Bebout, Agagu, and Dorfman, 1975). The objective of these reports is to outline areas (fairways) which appear the most prospective for producing geothermal energy and which therefore deserve further, more detailed study.
Wilcox sandstone reservoirs in the deep subsurface along the Texas Gulf Coast : their potential for production of geopressured geothermal energy
(University of Texas at Austin. Bureau of Economic Geology, 1982) Bebout, Don G.
Regional studies of the lower Eocene Wilcox Group in Texas were conducted to assess the potential for producing heat energy and solution methane from geopressured fluids in the deep-subsurface growth-faulted zone. However, in addition to assembling the necessary data for the geopressured geothermal project, funded by the U.S. Department of Energy, this study has provided regional information of significance to exploration for other resources such as lignite, uranium, oil, and gas. Because the focus of this study was on the geopressured section, emphasis was placed on correlating and mapping those sandstones and shales occurring deeper than about 10,000 ft. The Wilcox and Midway Groups comprise the oldest thick sandstone/shale sequence of the Tertiary of the Gulf Coast. The Wilcox crops out in a band 10 to 20 mi wide located 100 to 200 mi inland from the present-day coastline. The Wilcox sandstones and shales in the outcrop and updip shallow subsurface were deposited primarily in fluvial environments; downdip in the deep subsurface, on the other hand, the Wilcox sediments were deposited in large deltaic systems, some of which were reworked into barrier-bar and strandplain systems. Growth faults developed within the deltaic systems, where they prograded basinward beyond the older, stable Lower Cretaceous shelf margin onto the less stable basinal muds. Continued displacement along these faults during burial resulted in (1) entrapment of pore fluids within isolated sandstone and shale sequences and (2) buildup of pore pressure greater than hydrostatic pressure and development of geopressure. Regional electric log correlation markers made possible the subdivision of the Wilcox into lower, middle, and upper parts. The net-sandstone map of the lower Wilcox indicates a dominantly lobate pattern northeast from De Witt County to the Sabine River. Fisher and McGowen (1967) interpreted the lower Wilcox in that area to have been deposited in a high-constructive delta system, which they named the Rockdale Delta System. To the south, the lower Wilcox net-sandstone map indicates a narrow, elongate trend; Fisher and McGowen interpreted these sediments to have been deposited in strandplain and barrier-bar systems and named them the San Marcos Strandplain and Cotulla Barrier-Bar Systems. The dominantly shale section of the middle Wilcox was deposited as a result of a marine transgression over the lower Wilcox. Thus, the shale is an offshore equivalent both of middle Wilcox sandstones far updip and of sandstones of the lower part of the upper Wilcox. The net-sandstone map of the upper Wilcox indicates abrupt thickening along the Lower and MiddleTexas Gulf Coast, resulting from deposition contemporaneous with faulting in major deltaic systems. In contrast, along the Upper Texas Gulf Coast, abrupt downdip thickening does not occur, and these strata are not extensively growth faulted. Information on subsurface pressures and pressure gradients was obtained from (1) bottom-hole shut-in pressure data from drill-stem tests, (2) shale resistivity data from electric logs, (3) mud weights from well log headings, and (4) shale transit times from sonic logs. The top of geopressure was picked at approximately the depth where a pressure gradient of 0.7 psi per foot occurs. This depth is considered the "operational" top of geopressure because it is somewhat higher pressure than the 0.465 psi per foot hydrostatic pressure gradient. The zone where pressure gradients are between 0.465 and 0.7 psi per foot is transitional and difficult to identify consistently. The resulting map indicates that the top of geopressure occurs between depths of 8,000 and 13,000 ft along the Wilcox trend; in general, the top of geopressure is deeper in high-sandstone areas and shallower in high-shale areas. Formation temperatures, corrected to equilibrium values, were determined throughout the Wilcox trend to permit calculation of the amount of methane dissolved in the water. Knowledge of subsurface temperature is also essential in studying diagenesis of sandstone, dewatering of shale, and maturation of organic material and generation of hydrocarbons. As a result of this regional study of the Wilcox Group, six geothermal fairways were identified--Zapata, Duval, Live Oak, De Witt, Colorado, and Harris. Thick sandstone units with formation temperatures greater than 300�F occur in all fairways; however, high porosity and permeability occur in only the De Witt Fairway (Cuero area), making it the only one having high potential for geopressured geothermal energy production.