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High Resolution Sequence Stratigraphy and Reservoir Characterization of the "mississippian Limestone" in North-central Oklahoma

Leblanc, Stephanie Leigh
Mississippian limestone reservoirs are significant unconventional hydrocarbon reservoirs in central and northern Oklahoma and southern Kansas that are found at relatively shallow depths (3,000-6,000ft; 915-1,825m). Over 14,000 vertical wells have been drilled in the "Mississippian Lime" reservoirs, but recent activity has focused on developing the reservoir using horizontal drilling methods. Despite historic and recent drilling activity, little is understood about the properties and distribution of many of the productive Mississippian reservoirs.Detailed facies analysis suggests deposition on a regionally extensive, carbonate ramp to distally steepened carbonate ramp. Facies stack into shoaling upwards packages consisting of glauconitic sandstones and weakly calcareous mudstones to wackestones at the base, followed by progressively higher energy, traction-dominated grainstone facies. The sequence stratigraphic hierarchy of the "Mississippian Limestone" can be defined as an overall 3rd order sequence (100's of meters thick) containing 4th order high frequency sequences (10's of meters thick) and 5th order cycles (few meters thick), which form due to variations in eustatic and relative sea level.The stratigraphic hierarchy plays a major role in controlling the overall quality and vertical heterogeneity of the reservoir units. Core and thin section analyses demonstrate that the "Mississippian Limestone" is characterized mostly by fracture, moldic and vug porosity, and that the highest reservoir quality units exist in the higher energy, traction current facies. Reservoir quality appears to be mainly controlled by the 4th order high frequency sequences. Impermeable mudstones and cemented grainstones associated with these high frequency sequences likely cause vertical compartmentalization of the reservoir. Incorporating a detailed sequence stratigraphic framework into the reservoir characterization of the "Mississippian Limestone" provides an enhanced understanding of the complex lateral and vertical variability of the subsurface reservoir facies, and leads to better reservoir prediction at the exploration and production scales.