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Porosity and permeability in the Ellenburger Group of west Texas result from a complex interaction of early to late diagenetic processes. Porosity formation occurred in at least seven stages: (1) early marine phreatic calcite cementation, (2) fabric-selective (mixing-zone?) dolomitization, which created intercrystalline porosity, (3) episodic subaerial exposure and karstification, which created vuggy, cavern, solution-channel, moldic, fenestral, breccia, and fracture porosity, (4) mineralogy-selective meteoric phreatic or mixing-zone silicification, which preserved existing porosity by preventing further carbonate cementation, (5) deep-burial xenotopic dolomite recrystallization, which destroyed nearly all of the precursor intercrystalline porosity, (6) deep-burial dolomi e, calcite, and anhydrite cementation of some vugs and fractures, and (7) late-stage tectonic fracturing, which created most of the reservoir permeability.
The Ellenburger Group consists of numerous vertically stacked subaerial exposure cycles 1-20 ft thick. Porosity within each cycle is laterally discontinuous and patchy. The complete cycle is composed of four zones (from top to bottom): (1) glauconitic shale, which is interpreted to be a paleosoil horizon, (2) brecciated dolomite, cherty dolomite, or chert, which formed from solution collapse, (3) nonbrecciated dolomite containing abundant dissolution-generated porosity, and (4) nonporous dolomite, which was largely unaffected by karstification. Zones 1 and 4 are nonporous with very low permeability; zones 2 and 3 have high porosity and permeability. The presence of subaerial exposure cycles throughout the Ellenburger Group has resulted in numerous vertical permeability barriers, which may be the cause of reservoir stratification in some fields.
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