Minor non-carbonate authigenic components as indicators of late-stage diagenetic processes in the smackover limestones

Abstract

Variations in mineralogy and composition of late authigenic sulfur-bearing minerals in upper Smackover limestones recorded diagenetic events associated with hydrocarbon migration. This suite of minerals occurs in samples taken from 18 subsurface cores of the upper Smackover from northern Louisiana and southern Arkansas. Spatial variations in cadmium concentration of sphalerite, Ba concentration of celestite, and the δ[^34]S values of galena and sphalerite suggest that fluids associated with hydrocarbons in the lower Smackover migrated into the upper Smackover along faults at the Louisiana-Arkansas border. A second generation of sulfide mineralization and a δ[^34]S = +10.8 [parts per thousand] CDT of a wurtzite-sphalerite mixture suggests that thermochemical sulfate reduction occurred after hydrocarbon migration resulting in the formation of the "sour gas belt". Uranium concentration, as revealed by fission track imaging and instrumental neutron activation analyses, varies on a microscopic and regional scale in the upper Smackover limestones. Uranium is locally concentrated in stylolites and fine grained opaque material. Comparison of uranium and thorium concentrations in stylolites suggests these elements are insoluble residue of the pressure solution process. Regionally, uranium concentration varies with clay content of the host limestone indicating that sorption plays a major role in the geochemical behavior of uranium in the j subsurface. Comparison of U/Th ratios of upper Smackover limestones to those of black shales from the Gray Sands tentatively suggests that uranium was redistributed from hydrocarbon source rocks to reservoir rocks. However, better understanding of the association of uranium with organic-rich rocks and the role of sorption are required before uranium can be used reliably as a pore fluid tracer. The concentration of sulfur-bearing minerals and uranium along stylolites demonstrates the importance of these pressure solution seams to late-stage diagenesis in the Smackover...