Reservoir Heterogeneity at West 76 Field: An Opportunity for Increased Oil Recovery by Geologically Targeted Infill Drilling

HAMILTON, DOUGLAS STUART, Bureau of Economic Geology, University of Texas at Austin, Austin, TX

West 76 is a typical field of the Eocene Jackson-Yegua Barrier/ Strandplain sandstone play in south Texas. Located in northwest Duval County, the field has produced 4.6 million barrels (MMbbl) as of 1988 and has an estimated 6 MMbbl of remaining mobile oil. The Jackson-Yegua play, which includes nearly 300 fields, has produced 624 MMbbl (as of 1988) and has estimated remaining mobile oil in excess of 1 billion barrels (Bbbl).

Production at West 76 is from the Cole Sands of the uppermost Jackson Group at depths of 1300 to 1400 ft. The trapping mechanism is principally stratigraphic, but small normal faults influence production trends. Reservoir geometry is highly complex with numerous internal permeability barriers. The reservoir section was constructed by a progradational and overlying retrogradational barrier beach system that deposited discontinuous strike-oriented beach ridges, a strike-oriented tidal channel unit, and sand-poor lagoonal facies. The primary reservoir geometry was modified by minor dip-oriented, reservoir-quality washover sands, tidal-inlet fill, and fluvial sands. Production trends mirror the strike-aligned beach-ridge and tidal-channel facies but also bear a minor dip-oriented componen associated with the washover sand facies. Infill wells that produce from beach-ridge facies at rates an order of magnitude higher than existing, neighboring wells reflect the significant reservoir heterogeneity and highlight the considerable potential for untapped reservoir compartments.

Indentification of these compartments from detailed characterization of the reservoir provides targets for infill drilling and, ultimately, increased oil recovery. The geologic heterogeneity at West 76 has application to the entire Jackson-Yeuga Barrier/Strandplain sandstone play and increased recovery of the estimated 1 Bbbl of remaining mobile oil.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)