SEQUENCE STRATIGRAPHY AND RESERVOIR CHARACTERISTICS OF BOOCH SANDSTONES (MCALESTER FORMATION), IN SOUTHEASTERN OKLAHOMA

The Booch stratigraphic interval, as defined in the subsurface by the Oklahoma oil and natural gas industry, is equivalent to the lower three quarters of the lower Desmoinesian (middle Pennsylvanian) McAlester Formation. Recognized on the Cherokee Platform, the Booch becomes thickest in the Arkoma Basin, where it can be seen to be composed of eight, northerly sourced progradational sequences bounded by flooding surfaces. On well logs each of these eight parasequences exhibit a distinct coarsening upward profile as the environment of deposition progresses from distal marine, to delta front, to delta plain, to in some cases, incised valley. The depositional environments represented by sandstone reservoirs occurring at the top of Booch parasequences include distributary-mouth bars, tidal channels, distributary channels, over bank (crevasse) splays, and/or multi-story channel-fills (incised valleys).

The middle Booch consists of three parasequences that record the period of maximum valley incision, progradation, and sand deposition. From an oil and gas perspective, this interval contains the best reservoirs. It separates the two lower Booch parasequences, which are sandstone poor and represented by mostly marine shale, from the three upper Booch parasequences that record periods of both widespread marine inundation and the subsequent progradation of large delta systems.

The Booch was deposited over a period of roughly two million years; a timeframe that provides an average of 250,000 years between the flooding surfaces that initiate the deposition of each parasequence. Although the cyclicity observed may have a eustatic component, it is thought that local variations in sediment influx and subsidence, mostly through sediment loading, were the primary factors driving changes in sea level. Large, post-depositional folds generated by Ouachita compression have brought the Booch to the surface and allowed a direct correlation of outcrops to nearby well logs. The recognition of Booch cyclicity through regional well log analysis has provided a framework into which unnamed and incorrectly identified sandstone outcrops can be placed.