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Abstract: Depositional and Diagenetic History of the Mississippian Chat, North Central Oklahoma


The Mississippian Chat, present at the unconformity between the Pennsylvanian and Mississippian, is a weathered and/or detrital interval of tripolitic chat or dense chert at the top of the Osagean.

The depositional environment of the Chat reflects uplift and both erosion and weathering-in-place of Osagean Mississippian cherty limestone. Fossiliferous clasts found in Chat cores were likely eroded in a high energy environment such as that found above wave base in Mississippian shallow seas. These clasts were transported by small scale debris flows into a lower energy environment and deposited in a lime mud matrix.

Examination of thin sections indicates late diagenesis partially replaced calcite shells and cement with silica subsequent to the debris flow. Well preserved original fossil structures suggest that "force of crystallization" was the method of molecule by molecule calcite replacement by silica. Dissolution of remaining calcitic fossils by intrusion of meteoric water created secondary porosity and a potential hydrocarbon reservoir.

The Chat appears on well logs as a low resistivity zone with low density and high porosity. Analysis of producing fields suggests production from structural highs, pinchouts and diagenetically formed stratigraphic traps. Trend analysis suggests a relationship between positive structural residual values and Chat production. Seismic reflections resulting from the porosity contrast of the Chat with adjacent zones indicate the presence of the Chat.

AAPG Search and Discovery Article #90944©1997 AAPG Mid-Continent Section Meeting, Oklahoma City, Oklahoma