Horizontal drilling activity in the Mississippian section is moving east of the Nemaha Ridge in northern Oklahoma. Exploration for oil accumulations the Osagian low-permeability ramp sediments has produced promising results as operators begin to unravel the details of reservoir facies and their deliverability. Regionally, this multi-state and complex stratigraphic trap rides over the low-relief structural rise that has channeled fluid movement through its brittle, fractured carbonates since Pennsylvanian time. Fluid migration culminated in the deposition of Mississippi Valley Type lead-zinc deposits of the Tri-State district and left scattered and breached oil and gas accumulations in its wake. The fluid carrier beds are comprised of crinoidal sediments, carbonate mudstones, and inter-bedded cherts. These rocks have been altered through time by a variety of processes ranging from early cementation and silicification, through regional migration of fluids from the basins to the east, west and south, multiple episodes of fracturing, and finally vertical migration of hydrothermal fluids along basement involved faults. Structural grain at all scales, from regional basement faulting to cm-scale intra-chert fracturing, provides a natural context for the interpretation of reservoir facies. Observations related to lineaments, faults and fractures enable the geologists who are working the play to begin to unravel the interlocking controls on the complex diagenetic history of the region. The spatial distribution of the reservoir units is reflected by historical and current drilling patterns. These conduits define a plumbing system that continues to deliver high fluid volumes from the reservoir today.
Structural History
In the Mississippian-aged reservoirs east of the Nemaha Ridge, old vertical accumulations have been drilled and produced since the 1920s. These traps are typically small structural closures aligned along NNE-trending features that are sub-parallel to the Nemaha Ridge (Figure 1). Trapping trends truncate along orthogonal lineaments. Northeast-trending features, including the Nemaha Ridge, are rooted in basement-involved faulting that is compressional in origin (Gay, 2003a, 2003b). Transpressional movement along these faults prior to and during Mississippian and Pennsylvanian time created pop up structures, as well as exposure of the Mississippian limestone and chert section at the end of Mississippian time. Relaxation and extension coincident with and following the Ozark uplift and dip reversal allowed for movement of fluids from the basement into the Mississippian section along deep-seated fault zones, as well as the migration of oil from Devonian source rock kitchens to the east and south. During this phase, traps were also breached, rotated, and refilled. Movement of surface groundwater into the system from outcrop continues today.
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