Paul W. Smith

Factors Controlling Simpson Group Production in Central Oklahoma

ABSTRACT The area of study includes Townships 7 through 9 North, Ranges 1 through 3 West, Cleveland and McClain Counties, Oklahoma. Sandstones of the five Simpson Group Formations have produced more than 65,000,000 barrels of oil from this 324 square mile area. Production is from structural traps, generally small faulted anticlines ranging in size from 80 to 750 acres. Cumulative oil production per field ranges from 36,000 to 11,760,000 barrels of oil. The distribution of oil production from within the Simpson Group has been considered enigmatic because, although all of the sandstone members are present throughout the study area, some fields have attained production from all of the sandstone members and others produce from only one horizon. The Simpson shales are not capable of generating hydrocarbons and the oil produced from the Simpson Group probably originated from the Devonian/Mississippian Woodford Shale. The McClain County fault juxtaposed the Woodford Shale with the Simpson Group enabling lateral oil migration from the Woodford into the Simpson sandstone. Also, tortuous migration pathways allowed Woodford oil to ultimately accumulate in Simpson reservoirs which involve downthrown younger rocks adjacent to upthrown older rocks. Slight structural movements occurred contemporaneously with the deposition of the Simpson Group creating semi-parallel northeast-southwest oriented thick and thin trends. Local structural movements were active during part or all of Simpson deposition. This created stratigraphic variations in the individual Simpson Group Formations which caused the apex of a field to migrate (or even vanish) with depth. Consequently, lower Simpson structures may not be reflected by upper Simpson structures, and conversely, upper Simpson structures may not continue with depth. Variations in Viola deposition enhanced the vertical discontinuity of structures within the study area so that mapped Viola structures may not reflect underlying Simpson structures. Furthermore, a dramatic change in the orientation of the structural grain began to appear in late Ordovician (Viola) time. Subsequent movements enhanced this later structural orientation producing two acute structural trends which control the entrapment of oil. This study demonstrates that the dual structural imprint is probably the single most important factor in controlling the distribution and accumulation of hydrocarbons within the area. The study also identified stratigraphic conditions within the Simpson Group sandstone which may affect their productivity.