Abstract

Mississippian limestone and chert reservoirs at Tonkawa field in north-central Oklahoma formed on a regionally extensive carbonate ramp. The deposits commonly form shoaling-upward lithofacies successions that stack into high-frequency transgressive–regressive cycles and form reservoir zones. Localized uplift, subaerial exposure, and associated diagenetic processes have significantly impacted lithology and reservoir-quality distribution. Tonkawa field is on the eastern margin of the Nemaha uplift and exhibits an upthrown western block and a downthrown eastern side, which are offset by as much as 500 ft (150 m) of vertical displacement. Erosion of the western block has removed over 450 ft (135 m) of the Mississippian and Woodford Shale such that, locally, Pennsylvanian shales lie directly on the Ordovician Wilcox sandstone. On the eastern side of the field, greater than 400 ft (120 m) of Mississippian strata are present. Mississippian lithologies include (1) porous chert conglomerate, (2) porous tripolitic chert, (3) massive-to-laminated dense chert, (4) dense chert breccia, (5) bioturbated limestone, (6) limestone breccia, and (7) nodular-to-bedded mudstone (shale). The main reservoir rock, tripolitic chert, primarily formed by in situ karst development of subaerial highs (e.g., sponge bioherms and cherty limestone) followed by silica replacement of calcite and partial to complete dissolution of the remaining calcite to form secondary porosity. Tripolitic chert is most common at the top of the Mississippian, but deeper cycles within the Mississippian are also capped by high-porosity, low-resistivity chert. Detailed 3-D lithology and porosity models that are constrained to core, well-log, and seismic-inversion-derived P-Impedance data illustrate the heterogeneous character of the deposits. In general, wells drilled in areas of thin tripolitic chert reach peak-oil production early, but production declines rapidly because of limited reservoir volume. Areas with greater tripolitic chert thickness require more time to reach peak-oil production but produce at higher rates for longer periods and therefore have higher long-term cumulative production. Cumulative oil production is variable even where tripolitic chert is relatively thick; therefore, factors other than tripolitic chert thickness must impact oil production (e.g., karst, fractures, water saturation).