Abstract

The Mississippian Limestone formed through complex structural, stratigraphic, and diagenetic processes involving subsidence, tectonic uplift leading to periodic subaerial exposure, changes in ocean chemistry, variability inherent with carbonate cyclicity, as well as postdepositional alteration. These geologic complexities led to significant heterogeneity and compartmentalization within Mississippian mid-continent reservoirs, obscuring stratigraphic relationships. A novel log-based approach, called derivative trend analysis (DTA), is used to identify and correlate depositional cycles associated with five major stratigraphic zones. In the absence of abundant and complete core data, DTA serves as a rudimentary, yet informative, tool to effectively develop a sequence-stratigraphic framework. Classifying electrofacies, especially those constrained to core observations, can elucidate key relationships between depositional environments and reservoir properties, as well as provide an improved understanding of spatial heterogeneity. Three methods of electrofacies classification (artificial neural network, K-means clustering, and K nearest neighbor clustering) provide varying accuracies when used to create predictive lithology logs based only on the combined signatures of open-hole well logs in noncored wells. Stratigraphic models produced from the integration of these lithology logs with an interpreted stratigraphic framework reveal a relatively uniform, flat-lying basal Kinderhookian section, overlain by prograding clinoforms with internally shoaling-upward cycles of limestone, shales, and spiculites deposited during the Osagean and Meramecian stages. The sequence is capped by a high-porosity unit comprised mostly of brecciated chert associated with subaerially exposed strata underlying the sub-Pennsylvanian unconformity. Toward the south and east of the Hardtner field area, Osagean strata thin significantly and are covered by Meramecian spiculites of the Cowley Formation. Spatial porosity distributions reveal high reservoir quality deposits associated with regressive phases of third-order cycles, with the highest porosity intervals occurring up-section and toward the northeast of Hardtner field.

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