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Many existing and undiscovered hydrocarbon reservoirs in Permo-Pennsylvanian carbonates of the Permian basin are stratigraphic traps in various shallow-marine depositional facies. Paleoenvironmental interpretations and an understanding of the causal relations among facies occurrence, mappable paleogeologic features, and regional stratigraphy provide predictive models in the exploration for similar traps in the Permian basin.
Some of the depositional environments recognized in shallow-shelf carbonates in this area include strandline beaches, tidal channels and barrier bars, lagoonal and innershelf patch reefs, and shelf-marginal oolitic or bioclastic grainstone shoals and organic buildups. The areal occurrence, geometry, and reservoir-trap configurations of each of these facies and, hence, the strategy and model-approach toward their exploration, are dictated by an understanding of the interplay between several factors, including paleobathymetry, relative rates of subsidence and sedimentation, regional stratigraphy and history of transgression or regression, and complexities of diagenesis. The coincidence, or lack thereof, of preexisting structure or bottom topography and the predictability of occurrence o a given depositional facies are probability potentials dependent on the nature of regional sedimentation patterns and the types of sediments and/or organisms present during deposition.
Porosity evolution in these facies may or may not be related to and mappable together with depositional facies. Porosity formation or occlusion may occur in a spectrum of diagenetic environments from eogenetic (submarine and meteoric exposure) to mesogenetic (deep burial). Porosity types and reservoir permeabilities are dependent on original facies textures and timing of porosity formation.
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