ABSTRACT Exploration for hydrocarbons in Mississippian strata in Kansas and Oklahoma began in the 1900s. Early production came from open-hole completions in vertical wellbores at the apex of structural and stratigraphic traps. In the mid-20th century, cased-hole completions and hydraulic fracture stimulation allowed development of lower permeability zones. Recently operators began to explore and develop transition zones and low-permeability facies with horizontal drilling. The petroleum system that includes these accumulations consists of two hydrocarbon kitchens in the Arkoma and Anadarko basins, which have been generating oil and gas from the Woodford Shale since the beginning of the Pennsylvanian. Hydrocarbons charged out of the basins and along the fractured terrain of the Cherokee platform into reservoirs from Kinderhookian to Chesterian age across the carbonate facies belt. The distribution of these reservoirs, including limestones, dolomites, and cherts, along with structural configuration, governs the relative abundance and location of oil, gas, and water in each trap. The past decade saw over four thousand laterals targeting Mississippian reservoirs, including shales in unconventional traps, and the greatest rise in oil production in the region since the 1920s. High associated water volumes have created escalating operational costs and are correlative with earthquake activity.

ABSTRACT Mississippian rocks in north-central Oklahoma were deposited on a ramp-shelf system that trended along an approximate northeast–southwest strike and that deepened to the southeast and southwest into the Arkoma and Anadarko basins. The system is bounded on the east by the Ozark uplift. Structure in this area is dominated by extensional and transverse faulting associated with the Transcontinental arch (Nemaha uplift). Shallower water (shelf) depositional settings dominate in the northern part of the study area and deepen toward the south into the Anadarko and Arkoma basins. Sedimentary rocks on the carbonate ramp are dominated by cyclic, partially dolomitized, argillaceous mudstones interbedded with fine-grained wackestones to grainstones. Intergrain pore space is filled by bladed, isopachous, and syntaxial marine calcite cements followed by blocky calcite cements. Limestone is commonly replaced by chert with intergrain open space filled by fine crystalline quartz (chert) cement. Late diagenetic fracture, breccia, and vug (FBV) porosity are filled by calcite and less commonly, by quartz cement that displays a coarse, blocky habit. Carbon and oxygen isotope values for limestones and replacive dolomite are consistent with precipitation from Mississippian seawater and mixed seawater–meteoric water; values for FBV-filling calcite cements indicate precipitation from evolved basinal waters. The 87 Sr/ 86 Sr values of calcite micrite, replacement dolomite, and fracture-filling calcite range from 0.7077 to 0.7112. The lower values are consistent with equilibration with Mississippian seawater through most of the study area. More radiogenic 87 Sr/ 86 Sr values for fracture-filling calcite cements in the northeast part of the study area indicate interaction with continental basement rocks or siliciclastic rocks derived from continental basement. Two-phase (liquid plus vapor) aqueous and petroleum inclusions were observed in FBV-filling calcite and quartz cements. The aqueous inclusions have homogenization temperatures of 48°C to 156°C and salinities ranging from 0 to 25 equivalent weight % NaCl equivalent, and reflect the presence of distinct dilute and saline fluid end-members. Calculated equilibrium δ 18 O water values (VSMOW) for fluids that precipitated fracture-filling calcite cements are variable, ranging from –0.3 to +14.5‰ and do not reflect a single end-member water. Early diagenesis was dominated by seawater-involved cementation, with modification by meteoric water during sea-level low-stands. FBV-filling calcite and quartz represent a later stage of diagenesis associated with petroleum generation and migration. Formation of fractures in the Mississippian section in north-central Oklahoma likely is related to fault movement along the Nemaha ridge instigated by Ouachita tectonism during the Pennsylvanian and extending into the Permian. This timing corresponds with regional flow of saline basinal fluids associated with the orogenic activity. These fluids ascended along faults and contributed to precipitation of FBV-filling cements. Calculated δ 18 O water values for calcite cement in some areas of north-central Oklahoma suggest that cement-depositing fluids approached isotopic equilibrium with the host carbonate rocks. In other areas, however, cement-depositing fluids have oxygen isotope signatures that reflect nonresident fluids whose flow was restricted to fault and fracture pathways, which did not permit isotopic equilibration with the host limestone. In particular, fracture-filling calcite veins from Osage County, with high 87 Sr/ 86 Sr (>0.710) and low δ 13 C values (–2.3‰ to –4.1‰), reflect fluids that retained isotopic characteristics that were derived through interaction with subjacent shale source rocks.

Abstract Identifying distinct facies shifts within mudrocks has made it difficult to build sequence stratigraphic frameworks within fine-grained lithologies. Three cores from Lincoln, Pottawatomie, and Pontotoc counties and two outcrops at the Hunton anticline quarry in Murray County cover proximal and distal regions of the Arkoma basin within southern Oklahoma. Chemostratigraphic and gamma-ray profiles supplemented with lithologic descriptions can be used to build sequence stratigraphic interpretations within mudrock systems. Detrital sediment input is associated with Ti, Zr, Al, and K. The degree of basin restriction correlates with Mo and V concentrations, barring certain mineralogical affinities. Silicon is found in biogenic quartz, detrital quartz, feldspars, and clays. However, evaluating Si as a ratio between Si/Al, in conjunction with the Ti and Zr concentrations, the Si/Al ratio provides a rough approximation for the amount of biogenic quartz present within a sample. At several horizons in the Woodford, the Si/Al value spikes, and the Ti and Zr values drop; these spikes are interpreted as planktonic blooms. Stratigraphic successions with ambiguous gamma-ray profiles correlations can be correlated accurately by utilizing surfaces that are recognized within chemostratigraphic profiles. Within the Arkoma basin, the chemostratigraphic profile of the Woodford Shale is interpreted within a sequence-stratigraphic framework using the following general criteria. Progradational packages record increasing concentrations of Ti, Zr, Al, and K. Retrogradational packages record a declining trend in these elements, indicating the transgressive systems tract. Lowstand system tracts and highstand system tracts can be distinguished by the degree of bottom-water restriction. Low base level correlates to a greater degree of basin restriction.