Hydrogeochemistry of saline fluids and associated water and gas.

Abstract

This dissertation integrates elemental and isotopic analyses of saline formation waters and associated natural gas in sedimentary basins to provide constraints on their origin and geochemical evolution. The Michigan Basin and a modern evaporite environment, the Salina Ometepec, Baja California, were examined as analogs to assess the importance of early versus later diagenetic alteration of minerals and organic matter. The Devonian Antrim Shale, an economic natural gas deposit in the Michigan Basin, produces methane that is dominantly of microbial origin. Microbial methanogenesis was identified by H-isotope analysis of gas and co-produced water. Microbial activity was further established by extremely high $\delta\sp{13}$C values for dissolved inorganic carbon (DIC) and for gaseous CO$\sb2.$ Stable isotopic compositions of H$\sb2$O, $\sp3$H determinations and $\sp{14}$C dating of DIC in the formation waters, suggest the presence of Pleistocene-age groundwaters, modern groundwaters and basinal brines. Major solutes in formation waters are derived from halite dissolution, clay-mineral cation exchange, and basinal CaCl$\sb2$-type brines in subjacent strata. Localized microbial activity within gas productive reservoirs further modify concentrations of CO$\sb2$ and CH$\sb4$ gases and dissolved sulfate, acetate and bicarbonate. Chemical heterogeneity of formation waters is even more pronounced from the perspective of the many reservoir rock systems in the basin. Each main aquifer has a distinct suite of chemical properties, which requires not only different solute sources but hydrologic compartmentalization within the basin itself. Formation waters from the Michigan Basin commonly have equimolar concentrations of Ca$\sp{2+}$ and Na$\sp+.$ This relation has been observed in many sedimentary basins and has been explained by Na$\sp+$ replacement of Ca$\sp{2+}$-feldspar. However, Sr isotopic composition and Br$\sp-$ concentrations in formation waters from the Michigan Basin coupled with its tectonic setting suggest otherwise. The modern evaporite system studied is stressed by extreme fluctuations in temperature and salinity, leading to unusually low organic productivity, inhibited sulfate reduction and limited carbonate precipitation. Early diagenesis of detrital aluminosilicates and authigenic evaporite minerals modify marine brines, significantly with regard to elements considered conservative during seawater evaporation. The combination of studies from modern and ancient environments demonstrate how original marine fluids can be modified soon after deposition, when the chemical system is open to elemental exchange, and continue evolving with buried sediments in basins.