Abstract

Petrographic, geochemical, and fluid inclusion analysis of dolomite and calcite cements has been conducted on Mississippian carbonates collected from the surface and subsurface of the southern midcontinent of the United States (Oklahoma, Missouri, Kansas, and Arkansas). Fracture and vug, intergrain, and intragrain porosity are filled with calcite, authigenic quartz, and dolomite cements. Primary limestone porosity is filled partially by early marine and meteoric calcite cements. Equant (blocky) calcite cements were precipitated under seawater or mixed meteoric-seawater conditions in the phreatic zone and in the deep phreatic zone under late (burial) diagenetic conditions. Fracture- and breccia-filling saddle dolomite cements that were observed are late diagenetic and are likely related to the nearby Tri-State Mississippi Valley-type (MVT) mineral district.

Carbon and oxygen isotope values of dolomite cements range from δ¹⁸O_(VPDB) = −9.5 to −2.7‰ and from δ¹³C_(VPDB) = −4.0 to −0.4‰. Values for calcite cements range from δ¹⁸O_(VPDB) = −11.6 to −1.9‰ and from δ¹³C_(VPDB) = −12.2 to +4.6‰. These values are consistent with three types of diagenetic fluids: seawater, seawater modified by meteoric water, and evolved basinal water. Analysis of fluid inclusions in late calcite, dolomite, and quartz cements indicates the presence of both dilute and high salinity end-member fluids. Homogenization temperatures (T_h) of fluid inclusions range from 57°C to 175°C and salinities range from 0 to 25 equivalent weight % NaCl. Fluid inclusion T_h values and salinities are consistent with a saline basinal fluid variably diluted by fluids of meteoric or mixed seawater and meteoric origin. Petroleum inclusions were observed in late diagenetic calcite and dolomite cements.The late diagenetic cements filled porosity retained after early diagenetic cementation indicating that some original porosity in the Mississippian carbonate rocks remained open during petroleum migration. Elevated fluid inclusion T_h values over a broad region, not just in the Tri-State Mineral District, imply that the regional thermal maturity of rocks may be higher than believed previously.

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 diagenetic events affecting these rocks occurred during burial and basinal fluid migration through these strata.