Browsing by Subject "Carbonate rocks"

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Acidizing of naturally-fractured carbonate formations
(2001-08) Dong, Chengli; Hill, A. D. (A. Daniel)
The lab experimental studies of acidizing of fractured carbonate cores show three etching patterns under different conditions: wormholing, channeling and surface etching. Based on the observations in experiments, a mathematical model of acidizing of naturally-fractured carbonates is developed. Rough-surfaced fractures are numerically generated in the model. The model predicts the same acid etching patterns with the same dependencies on fracture properties as observed in the experiments. When large single fractures are implemented into the model, the acid etching and penetration in single fracture as well as the effects of fracture properties are observed. The studies of acid behavior at intersection points of fracture networks indicate that acid flow and transport is concentrated in a main flow path in the network, which is formed by a system of intersected fractures. A method is developed to directly numerically generate the main flow path without generating the network. The model is combined with the generated main flow path to predict the result of acidizing natural fracture networks. A leakoff model that includes the effects of both matrix leakoff and the leakoff from the tail fractures is integrated into the model. The simulation results illustrate the acid penetration and the etching of the fracture walls along the main flow path and the effects of the network properties. The model predicts deeper acid penetration in acidizing of naturally-fracture carbonate formations than would be possible with only matrix flow.
Deposition of prograding carbonate sand shoals and their subsequent diagenesis, lower Glen Rose (Cretaceous), South Texas
(1980) Bay, Annell Russell; Debout, D. G.; Folk, Robert L.
The Glen Rose Formation is a thick (300 to 900 m), carbonate unit that was deposited on a broad, shallow-marine shelf during the early Cretaceous. Three cyclic, shoal-water complexes of the lower Glen Rose Formation developed over the Pearsall Arch in Frio and Medina Counties of South Texas. These complexes, consisting of high-energy grainstone and coral-stromatoporoid-caprinid boundstone and packstone, trend east-west for at least 125 km and are located approximately 80 km inland of the Cretaceous shelf edge and 70 km seaward of the Cretaceous shoreline. Three major, depositional environments have been recognized within the lower Glen Rose of South Texas: open shelf, shoal-water complex, and protected lagoon. The facies and their respective depositional environments, from the base to the top of the upward-coarsening sequences are (1) sandy mudstone/wackestone, fossiliferous quartzarenite, mollusc-miliolid wackestone, and echinoid-mollusc wackestone deposited in an open-shelf environment; (2) echinoid-mollusc and oncolite-caprinid packstone deposited in intertidal shoals and subtidal grainflats; (3) coated-grain and echinoid-mollusc grainstone deposited in sandflats, tidal channels, spits, and bars; (4) coral-stromatoporoid-caprinid boundstone and packstone deposited as patch reefs and associated flanking debris. Toucasiid-oyster-miliolid wackestone, toucasiid boundstone, and burrowed and laminated mudstone deposited in shallow-water, protected lagoons and tidal mudflats interfinger with upward-coarsening sequences on the landward side. The lagoonal deposits overlying the shoal-water sequence indicate the seaward progradation of the complex. Eventually each complex was transgressed by open-shelf facies and the cycle began again. The patch reefs and associated biostromes may have prograded out across the shelf and formed the initial build-up of the Stuart City shelf margin. A petrographic study of grainstone diagenesis indicates there are four gradational phases of diagenesis: (1) early submarine, (2) early mixing-zone and meteoric, (3) late meteoric-to-early subsurface, and (4) late subsurface. Early cementation in the marine environment is evidenced by micrite envelopes and isopachous crusts around some allochems. Subaerial exposure of bars and spits allowed early invasion of meteoric and mixing-zone water as indicated by aragonite dissolution or neomorphism to calcite; precipitation of finely crystalline, equant, isopachous cement; syntaxial cement; and medium crystalline, nonferroan, equant, calcite cement. During early burial of the rocks coarser crystals of nonferroan, equant calcite precipitated, indicating continual flushing with meteoric water. Later burial of rocks to the subsurface lead to the growth of zoned ferroan and nonferroan calcite, replacement of mollusc shells by lutecite and megaquartz, precipitation of euhedral quartz overgrowths, anhydrite precipitation and replacement, precipitation of baroque dolomite, and stylolitization. Petrography of the mud-supported facies indicates there is replacement of micrite by both limpid and inclusion-rich dolomite. The limpid dolomite is interpreted to form in a schizohaline environment whereas the inclusion-rich dolomite formed in both schizohaline and hypersaline environments.
Modeling reflux dolomitization
(2005) Fullmer, Shawn M.; Lucia, F. Jerry
The process of reflux dolomitization is commonly invoked to explain dolostone occurrence and distribution. The time required for this process to take place is not well understood. Volumetric flow rates are the dominant control on the time requirement. Published platform scale numerical models of reflux require unrealistically high permeability values and reaction efficiencies to explain the mapped dolomite volume. Approaching the problem at the high-frequency-cycle scale allows tighter constraints on the variables and a simpler geologic setting to investigate the permeability and reaction efficiencies required to form a dolostone unit. The high-frequency cycle used as a model in this study is located near the top of the Cretaceous age upper Glen Rose Formation in central Texas. It is an upward shallowing, Albian age cycle that consists of a subtidal mud-dominated packstone overlain by an evaporitic tidal-flat cap. The dolostone extends 1.5 meters down from the cycle top. Permeability values reflecting conditions at the onset of dolomitization were reconstructed by, 1) obtaining current porosity and permeability values from outcrop samples, 2) approximating porosity loss due to burial diagenesis, 3) interpreting pre dolomitization rock fabrics from thin sections, and 4) calculating permeability using the uncompacted porosity and rock-fabric information. The data was entered into a variable density flow model and the results suggest that the time required to form this 1.5 m dolostone unit is between 300 and 1,000 years using a harmonic mean permeability of 260 md (2.6 x 10⁻¹³ m²) and magnesium exchange efficiencies between 10-40%. These more realistic permeability and efficiency values are much lower than the 100 - 10,000 Darcy's (10⁻¹¹ - 10⁻⁹ m²) permeability and 100% reaction efficiency values used in current published reflux models
Petrology of Sedimentary Rocks
(Hemphill Publishing Company, 1980) Folk, Robert L.
Structural diagenesis and spatial arrangement of fractures in carbonate rocks
(2023-08-17) Corrêa, Rodrigo dos Santos Maia; Laubach, Stephen E. (Stephen Ernest), 1955-; Kerans, Charles; Stockli, Daniel F; Gale, Julia F; Lamarche, Juliette; Pyrcz, Michael
Fractures are among the most intriguing features of geology. They are commonly associated with important activities in contemporaneous society, including essential energy and infrastructure development projects. However, the distribution and formation of fracture networks is still an important subject of debate, since we still do not fully comprehend formation mechanisms that would allow us to accurately predict fracture patterns in the subsurface or, in some instances, we lack adequate descriptive tools for portraying them. This dissertation focuses on the issue of fracture description and predictability by creating new statistical methods to analyze fracture arrangement and by describing petrographical and geochemical characteristics of fracture cements, also known as structural diagenesis, which allow insights into timing and conditions of fracture formation. I show how my new algorithm can discriminate several fracture spatial arrangements in 2D. Using examples from carbonate rocks, I demonstrate how a range of structural and geochemical tools together help unravel the history of fractures in two key settings: adjacent to faults and in deep-seated settings beneath salt. The results advance how to characterize fracture patterns and our understanding of how fractures form.