Regionally the O.S.A. and Gillian pools, producing

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from the upper Simpson Sand, lie on a prominent northeast-southwest-trending anticline that plunges toward the southwest. This 25-mi.-long structural trend is an asymmetric fold bordered on the western flank by a normal, down-to-the-west fault possessing an average throw of 180 ft. at a Simpson datum. Discontinuous normal faulting, downthrown toward the east, exists on the more gentle eastern flank of the trend.

The Gillian pool accumulation is attributed entirely to structural closure, whereas that of the O.S.A. pool is primarily stratigraphic, because of the presence of the updip beveled edge of the upper Simpson Sand which regionally circumvents the northwestern portion of the buried pre-Mississippian Chautauqua arch. Locally in the O.S.A. sector, this sandstone edge strikes nearly north-south and parallels the crest of the anticline, thus providing a preserved sandstone section on the western flank.

The combined structural-stratigraphic relationship of these pools is significant. It is estimated that a reserve in excess of 3,500,000 barrels of recoverable oil has been proved to date.

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