Hydrocarbon occurrence in Strawn (Pennsylvanian) sandstones in the West Tuscola field, near Abilene, Texas, is the result of stratigraphic entrapment in deltaic sandstone. 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 reservoir 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 (pro-delta 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 "Grey sandstone."

The stream-mouth-bar sandstone in the West Tuscola reservoir is lenticular, highly irregular in outline, and has varied trends of porosity; these are characteristics common to deltaic deposits. Such features present problems in developing effective secondary-recovery methods and in predicting occurrences of other deltaic sandstones.

End_of_Article - Last_Page 869------------