Time, Surfaces, and Rock Volume: A Four-Dimensional Re-evaluation of Reservoir Development in the Spiro Sandstone and Wapanucka Limestone, Arkoma Basin, Southeast Oklahoma

Brian W. Horn
BP, Houston, TX

The Wapanucka Limestone and Spiro sandstone are mapped as separate formations of late Morrowan and early Atokan age in the subsurface of the Arkoma basin and outcrops of the Ouachita Mountains. Regional stratigraphic correlations identify these rock units as two depositional sequences separated by the basal Atokan unconformity. Detailed correlations demonstrate the importance of identifying unconformities, understanding the stratigraphic relationships of surfaces and correlative rock volumes, and their time-space distributions.

The sub-Spiro shale that separates the Wapanucka Limestone from the overlying Spiro sandstone is time transgressive and correlative with Wapanucka carbonate facies in the Wilburton Field area and “Spiro” siliciclastic deposits in the Kinta Field. These strata are overlain by the basal Atokan unconformity. The down-dip (lowstand) deltas coeval to this erosional surface are observed in well log correlations in the hanging wall of the palinspastically restored Ouachita fold-and-thrust belt. Younger Spiro sandstone strata crop out along the Choctaw Fault and onlap this surface juxtaposing reservoir-quality sandstones of different ages in the sub-surface.

Deciphering the time-stratigraphic relationships interpreted from vertical facies successions of the Wapanucka and Spiro observed in cores explains the spatial relationships of reservoir-quality “Spiro” sandstones. They occur in distinct time-stratigraphic intervals that are correlative with both the Wapanucka and Spiro depositional sequences. These relationships help to predict future exploration potential for deep-water Spiro sandstone reservoirs and to identify undrained compartments in current producing fields.