Minor and trace authigenic components as indicators of pore fluid chemistry during maturation and migration of hydrocarbons. Final report.

摘要

Variations in mineralogy and composition of late authigenic sulfur-bearing minerals in upper Smackover limestones recorded diagenetic events associated with hydrocarbon migration. 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 subsurface. Comparison of uranium 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. Late authigenic sulfur-bearing minerals are excellent indicators of late-stage diagenetic events and evolution of pore fluids associated with hydrocarbons. Study of these minerals in the upper Smackover limestones has provided the first concrete evidence of migration of late-stage pore fluids into these rocks along faults and has shown that this flow was focussed in specific areas. This knowledge allows a clearer and more specific interpretation of previous authors observations and data.