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Reservoir-Scale Stratigraphy, Sedimentology and Porosity Characteristics of Mississippian Reservoirs, Northeastern Anadarko Shelf, Oklahoma

Abstract

The “Mississippi Lime” has been a productive carbonate play for several decades. The onset of horizontal exploration as well as hydraulic fracturing has increased its viability as a play. However, geologic controls on production and reservoir quality have frequently been ambiguous to geologists. The Mississippian limestone varies significantly in reservoir quality, with some characterized by higher porosity tripolite or spiculite, other parts characterized by lower porosity chert, and some characterized by unaltered limestone. In northern Oklahoma, the Mississippian limestone formed on the east-west trending margin of the Anadarko Shelf. This carbonate system is characterized by a shallowing upward character as well as high-order transgressive-regressive cyclicity. These shallowing upward high-order cycles have been established in Kansas and are also observed in northern Oklahoma. Unconformities in the area are caused by relative falls in sea level in addition to regional tectonics. Pre-Pennsylvanian tectonics created the Nemaha Uplift, the cause of subaerial exposure in the area, which then led to alteration. The Mississippian-aged limestones demonstrate a variety of alteration types, such as silicification, dolomitization, brecciation, and fracturing. The dominant lithologies include chert-breccia, bedded chert-breccia, grainstone and mudstone, with more alteration and brecciation occurring near the top of the Mississippian interval. This variability (both vertical and lateral) is established through core, thin section, and XRF analysis. Lithologies, lithofacies, and pore types, observed in core, thin sections, and XRF data are calibrated to open-hole logs to predict and map their spatial distribution as well as stratigraphic and lateral variability.