PETROGRAPHY AND GEOCHEMISTRY OF MISSISSIPPIAN CARBONATES IN THE SOUTHERN MIDCONTINENT, PRELIMINARY RESULTS

Preliminary petrographic and geochemical analyses have been made on Mississippian carbonates collected from the surface and subsurface of the southern Midcontinent area. Limestones are extensively cemented by early marine and meteoric calcites. Some limestones display partial to complete dolomitization. Fracture and vug porosity are filled with calcite, chert, and dolomite cements. Saddle dolomite cements, observed in this study, likely are the result late diagenesis, possibly related to basinal fluid migration through Mississippian strata. These fluids in turn may be related to the Tri-State Mississippian Valley Type (MVT) mineral district. Mineralization in the Tri-State area is genetically associated with petroleum.

Stable carbon and oxygen isotope analyses of Mississippian carbonates have been made. The δ¹⁸O and δ¹³C values for the Compton Formation in Jane and Stone Counties, MO average -1.25‰ to -8.3‰ and 1.93‰ to 4.56‰, respectively. The δ¹⁸O and δ¹³C values average -1.43‰ to -9.2‰ and 1.62‰ to 3.5‰, respectively for the Pierson Formation in Benton County, AK. The isotopic behavior of the samples suggests that three diagenetic waters may be involved: meteoric water, seawater modified by meteoric water, and basinal water. Two-phase fluid inclusions (water and vapor bubble) were observed in both late stage calcite and dolomite cements. The presence of vapor bubbles in these inclusions, ranging in size from 2µm to 8µm, suggest that the cements were precipitated from warm (>60°C) fluids.

The Mississippian carbonates resource play on the southern Midcontinent is very complex in terms of its reservoir properties. Possibly the most important diagenetic events affecting these rocks, and the least understood, occurred during burial and basinal fluid migration through these strata. Continuation of this study will address these questions. Better understanding of this will lead to more effective exploration strategies.