INFLUENCE OF LATE DIAGENETIC FLUIDS ON MISSISSIPPIAN CARBONATE ROCK PROPERTIES IN THE SOUTHERN MIDCONTINENT

Petrographic, geochemical, and fluid inclusion studies were conducted on Mississippian carbonates in the southern Midcontinent of North America (Oklahoma, Missouri, Kansas and Arkansas). These rocks currently comprise an active unconventional hydrocarbon play. Possibly the most important, and the least understood, diagenetic events affecting these rocks occurred during burial and basinal fluid migration through these strata. Limestone porosity is partly 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. Late diagenetic saddle dolomite and equant calcite cements possibly are related to the nearby Tri-State Mississippi Valley-type mineral district, which in turn is genetically associated with brine and petroleum migration in the region. Carbon and oxygen isotope compositions of dolomite cements range from δ¹⁸O_(VPDB) = -2.7 to -9.5‰, and δ¹³C_(VPDB) = -0.4 to -4.0‰. Calcite cements range from δ¹⁸O_(VPDB) = -1.9 to -11.6‰, and δ¹³C_(VPDB) = 4.6 to -12.1‰. 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 indicates the presence of both dilute and high salinity fluid end members (calculated values ranging from 0 to 25 equivalent wt % NaCl) and homogenization temperatures ranging from 50° to 175°C. These salinities and temperatures 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 imply that the thermal maturity of Mississippian carbonate rocks may be higher than previously believed. The existence of warm basinal fluids in the southern Midcontinent may be related to regional brine migration out of the Arkoma basin during the Ouachita orogeny. This study further indicates that the Mississippian carbonate rocks in the southern Midcontinent have a complex diagenetic fluid history, continuing long after early diagenetic cementation.