Hydrocarbon occurrence in Strawn (Pennsylvanian) sandstone bodies in West Tuscola field, near Abilene in Taylor County, Texas, is the result of stratigraphic entrapment in deltaic sandstones. Cores and numerous electric logs show that 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, very 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.