ABSTRACT The St. Joe group (Lower Mississippian, Tournaisian) is petrographically and isotopically analyzed using δ 13 C and δ 18 O bulk sample stable isotopes in central, northeastern Oklahoma, and southwestern Missouri. Determined to be conformable in Oklahoma, this group represents deposition in the mid- to outer-ramp setting during one long-term depositional cycle and can be used as a reference section for geochemical chronostratigraphy. Minor diagenetic alteration did not overprint the initial isotope signal, and the resulting curve is similar to those from previous studies and is integrated with published conodont biostratigraphy. The resulting correlation indicates that the St. Joe group was deposited in the upper Tournaisian Stage.

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).

ABSTRACT The Permian marks a time of substantial climatic and tectonic changes in the late Paleozoic. Gondwanan glaciation collapsed after its earliest Permian acme, aridification affected the equatorial region, and monsoonal conditions commenced and intensified. In western equatorial Pangea, deformation associated with the Ancestral Rocky Mountains continued, while the asynchronous collision between Laurentia and Gondwana produced the Central Pangean Mountains, including the Appalachian-Ouachita-Marathon orogens bordering eastern and southern Laurentia, completing the final stages of Pangean assembly. Permian red beds of the southern midcontinent archive an especially rich record of the Permian of western equatorial Pangea. Depositional patterns and detrital-zircon provenance from Permian strata in Kansas and Oklahoma preserve tectonic and climatic histories in this archive. Although these strata have long been assumed to record marginal-marine (e.g., deltaic, tidal) and fluvial deposition, recent and ongoing detailed facies analyses indicate a predominance of eolian-transported siliciclastic material ultimately trapped in systems that ranged from eolian (loess and eolian sand) to ephemerally wet (e.g., mud flat, wadi) in a vast sink for mud to fine-grained sand. Analyses of U-Pb isotopes of zircons for 22 samples from Lower to Upper Permian strata indicate a significant shift in provenance reflected in a reduction of Yavapai-Mazatzal and Neoproterozoic sources and increases in Grenvillian and Paleozoic sources. Lower Permian (Cisuralian) strata exhibit nearly subequal proportions of Grenvillian, Neoproterozoic, and Yavapai-Mazatzal grains, whereas primarily Grenvillian and secondarily early Paleozoic grains predominate in Guadalupian and Lopingian strata. This shift records diminishment of Ancestral Rocky Mountains (western) sources and growing predominance of sources to the south and southeast. These tectonic changes operated in concert with the growing influence of monsoonal circulation, which strengthened through Permian time. This resulted in a growing predominance of material sourced from uplifts to the south and southeast, but carried to the midcontinent by easterlies, southeasterlies, and westerlies toward the ultimate sink of the southern midcontinent.