ABSTRACT

A. W. P. Olmos Field, McMullen County, Texas was extended 6 miles southward in 1984. Gas production was established from the Olmos sand (Cretaceous, upper Campanian) from 10,810 ft to 10,912 ft in the Little Oil and Gas, Inc. et al No. 1-A Huff. The stepout production proved a 2,000 ft productive column for the field which is a stratigraphic trap.

The Olmos has produced at the present A. W. P. Field area since 1975, but improved stimulation techniques resulted in the actual field discovery well, the Burnett Oil Co. No. 1 Elliot, in 1981. As of January 1, 1987, there were 267 producing wells; the field covers approximately 100 square miles.

The Olmos is a fine grained to very fine grained sandstone that was deposited oblique to the ancient shoreline and present day strike. Deposition was the result of a dramatic lowering in sea level that resulted in the progradation of a delta front to the shelf margin. Gravity flows on the shelf, slope, and basin plain resulted in turbidite deposition; however, the Olmos maintains its integrity throughout the field and was deposited as a basically continuous unit.

The A. W. P. Olmos Field sand can be categorized into a depositional model for ramp facies turbidites. In the south or downdip A. W. P. area, slope deposition occurred at what is now a downthrown fault block of a large depositional fault. During Olmos deposition the fault was a plane of movement, or ramp, upon which previously deposited sediment moved basinward. The upthrown fault block represents an area where unstable sediment accumulation on the Olmos delta front margin resulted in slumping, as evidenced by well logs with poorly developed Olmos sections.

Deepwater deposition also occurred to the south, or downdip, in the A. W. P. Field. Decreased permeability, resulting from increased clay in the downdip Olmos indicates a depositional environment of gradually decreasing energy. This condition suggests deposition basinward from the slope as gravity flows lose their energy. The presence of benthonic forams in well cuttings also indicates an upper bathyal environment. The Olmos section expands from 50 ft in the area of delta front and slope deposition to 110 ft in the area of distal ramp deposition on the basin plain.