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Hydrocarbon occurrence in Strawn (Pennsylvanian) sandstones in West Tuscola field, near Abilene in Taylor County, Texas, is the result of stratigraphic entrapment in deltaic sandstones. The origin of the reservoir rock in the field area and the overall geometry and internal character of the deltaic complex were determined from the vertical sequence in numerous cores of the Gray sandstone and associated units and from numerous E-logs of uncored wells.
The vertical succession of deltaic facies consists from base to top of a progradational sequence (prodelta and delta front), an aggradational unit (delta plain-marsh and interdistributary bay), and an overlying "transgressive" shallow-marine interval. Reservoir sandstones are present within the delta-front facies as stream-mouth-bar deposits, known locally as the "Gray sandstone."
The stream-mouth-bar sandstones within the West Tuscola reservoir are lenticular, highly irregular in outline, and have varying trends of porosity; these are characteristics to be anticipated in deltaic deposits. Such features present problems in developing effective secondary-recovery methods and in predicting occurrences of other deltaic sandstones.
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