Mid-Continent Section

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Speculation or ‘spiculation’: Challenging the prevailing paradigms of secondary porosity development in the Mississippi Lime play


Long-lived paradigms surrounding the lateral and vertical heterogeneity of reservoir quality within the Mississippian section of Kansas and Oklahoma include: 1) preferential porosity development in localized spiculitic depofacies, generally referred to as the ‘Cowley facies’, and 2) multiple subaerial exposure episodes during the Mississippian deposition time, which lead to stacked and compartmentalized solution-enhanced porous zones through meteoric diagenesis and leaching of calcareous precursor. Though sponge spicules are locally present and meteoric processes have undeniably affected the unconformity-related deposits at top of the section, observations from conventional core, detailed petrography, and geochemical analysis provide evidence for a late-stage hydrothermal fluid alteration overprint on porosity development. Characteristic circular to tubular pores in chert, often misinterpreted as spicule molds, occur in all lithofacies and reservoir types within the Mississippian interval, including non-spicule-bearing facies. The spicule-like porosity fabric is inherent to replacive chert, and results from successive episodes of dissolution and re-precipitation of silica as dense chert is progressively altered to microporous tripolitic chert. Cross-cutting relationships indicate that such intricate, low permeability micropore network develops post-stylolitization, in association with hydrocarbon migration and late diagenetic ferroan baroque dolomite and calcite spar cements.