--> Influence of Diagenetic Fluids on Mississippian Carbonate Rock Properties in the Southern Midcontinent

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Influence of Diagenetic Fluids on Mississippian Carbonate Rock Properties in the Southern Midcontinent

Abstract

Petrographic, geochemical, and fluid inclusion analyses of dolomite and calcite cements have been made on Mississippian carbonates collected from the surface and subsurface of the southern Midcontinent (Oklahoma, Missouri, Kansas and Arkansas). Limestone porosity is largely occluded by early marine and meteoric calcite cement. Fracture and vug porosity are filled with calcite, chert, and dolomite cements. Both early and late blocky ferroan calcite cements were formed in the deep phreatic zone. Saddle dolomite cements are late diagenetic, possibly related to the nearby Tri-State Mississippi Valley-type mineral district, which in turn is genetically associated with petroleum migration in the region. Carbon and oxygen isotope compositions of dolomite cements range from δ18O(VPDB) = -2.7‰ to -7.7‰, and δ13C(VPDB) = -0.4‰ to -2.1‰. Calcite cements range from δ18O(VPDB) = -1.9 ‰ to -11‰, and δ13C(VPDB) = 4.6‰ to -4.8‰. Isotope values are consistent with three diagenetic waters: meteoric water, seawater modified by meteoric water, and basinal water. Analysis of two-phase fluid inclusions (water and vapor) in late calcite and dolomite cements indicate the presence of both dilute and high salinity fluid end members (calculated values ranging from 0 to 25 equivalent weight % NaCl) at homogenization temperatures ranging from 57° to 170°C. These temperatures and salinities indicate a saline basinal fluid possibly mixing with a dilute fluid of meteoric or mixed seawater/meteoric origin. Elevated fluid inclusion temperatures over a broad region, not just in the mineral district, imply that the thermal maturity of Mississippian carbonate rocks may be higher than previously believed. This study indicates that the Mississippian carbonate resource play on the southern Midcontinent has a very complex diagenetic history, continuing long after early diagenetic cementation. Possibly the most important, and the least understood, diagenetic events affecting these rocks occurred during burial and basinal fluid migration through these strata. Extension of this study into north central Oklahoma will provide a better understanding of the porosity development (cementation history) and thermal maturity of Mississippian carbonate reservoirs in this area and should lead to more effective exploration strategies.