Evolution of Lower Permian Oolite Shoal in Northwest Anadarko Basin: ABSTRACT

Abstract

The Lower Permian (Wolfcamp) Council Grove B-zone in the northwest part of the Anadarko basin in Ochiltree County, Texas, is represented by two carbonate rock types: (1) bioturbated oolitic bioclastic wackestones; and (2) cross-stratified oolite grainstones. These oolite and oolitic facies are underlain and overlain by bioturbated argillaceous bioclastic wackestones. A Council Grove B-zone isopach map indicates that the oolite shoal has an east-west depositional strike. The presence of north-south-trending tidal bars and channels superimposed on the oolite shoal suggest that the tidal currents responsible for the formation of the oolite flowed north and south. A transverse cross section reveals that the base of the oolite facies is stratigraphically higher in a southward direction, indicating that the direction of maximum tidal flow and/or storm surge and direction of oolite progradation was to the south. Paleogeographic time slice maps from a lower datum reveal that the oolite shoal initially formed as two isolated shoals which were superimposed on prominent structural highs. These shoals later merged and prograded southward. After deposition, the shoal was exposed to early, freshwater phreatic diagenesis, as indicated by oomoldic porosity and equant calcite cementation. Later diagenesis resulted in bladed anhydrite and coarse baroque dolomite partly filling oomoldic porosity. Oomoldic porosity results in conventional log-derived water saturations (SW) that are often overly optimistic. Therefore, to adequately evaluate oomoldic reservoirs using logs, the Production Ratio Index (PRI = SW sonic × ^phgrneu-den) should be used to predict the ratio of hydrocarbon to water production. End_of_Article - Last_Page 892------------