In western Oklahoma the uppermost sand member of the Cherokee Group, the True sand, was first drilled and found productive in two discoveries, completed in 1980, in west-central Custer County and in central Roger Mills County, Oklahoma. For 1 ½ to 2 years these two discoveries, some 18 mi (29 km) apart, were thought to be stratigraphic equivalents of two separate sand bodies occurring parallel to the classic northwest-southeast-trending systems of the Anadarko basin. Energy Services has drilled eight productive Prue sand wells in this area and has been instrumental in establishing that the 1980 Prue discoveries are actually part of a predominantly east-west-trending system that extends some 40 mi (64 km) across the west-central part of western Oklahoma.

At present, some 40 productive wells will ultimately produce more than 100 bcf of gas and 3 million bbl of condensate from an average depth of 11,500 ft (3,500 m). Sand porosities range from 3 to 18% with most producing wells having porosities in the 12 to 15% range. Because Prue sand is slightly overpressured (a pressure gradient of .53 psi/foot), the reserves are generally better than normal-pressured wells at this depth.

Log and sample data from the 40 producing Prue wells and numerous surrounding nonproductive tests, along with the one core of the sand, give a fairly good picture of the sand geometry and depositional history.

The sand body is over 40 mi (64 km) in length, 1 to 1.5 mi (1.6 to 2.4 km) wide, and 60 ft (18 m) thick. The east-west trend of the sand is parallel to the present-day structure at the top of the Cherokee Group as well as to the interval isopach of the Cherokee Group. The majority of the open-hole logs and samples show a fining-upward sequence, where most of the clean productive interval is at the basal to middle part of the sand. Study of the core shows the interval to grade from a medium to fine-grained sand, highly laminated and cross-bedded with black shale, to a slightly coarser grained nonstructured interval and back into a highly laminated cross-bedded sandy black shale interval. The interval is topped by a 10 ft (3 m) thick black shale layer that is a predominant bed throughou the whole area. We interpret the sand morphology as that of a submarine offshore bar sand that was deposited in shallow water but never emerged above sea level to form a barrier island of beach-type environment.

The existence of a shaly limey interval for 500 ft (152 m) above the Prue sand along with the excellent preservation and consistency of the Prue sand geometry indicate that the Prue was never reworked and was likely deposited in a trangressive or at least a subsiding basin environment.

These conclusions have implications that may assist in the exploration of other Pennsylvanian sands in this area.

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