A study of pore fluids and barite deposits from hydrocarbon seeps: Deepwater Gulf of Mexico.

摘要

The primary goals of this research were to (i) determine the factors controlling the chemical compositions of pore fluids in hydrocarbon seep sediments of the Gulf of Mexico, and (ii) delineate the nature, age and origin of barite deposits associated with seeps.; Three types of pore fluids from seeps have been discerned: (i) sediment-trapped seawater; (ii) brines derived by seawater dissolution of subsurface salt diapirs; and (iii) deep-seated formation waters enriched in Ba and Ra. The Ra-enriched formation waters advecting to the seafloor may explain why 226Ra levels in the Gulf of Mexico water column are higher than in the inflowing Caribbean Sea waters.; Microbial sulfate reduction exerts a major control on the origin and distribution of authigenic carbonates in seeps. Sulfate reduction using methane in sediments underlying mussel beds exhibits higher reduction rates than those using crude oil in bacterial mats. Consequently, pore fluids in mussel beds are highly supersaturated with respect to aragonite, calcite and high-Mg calcite, whereas most pore fluids from bacterial mats are undersaturated. These results are consistent with the observation that authigenic carbonates are often associated with mussel beds but rarely occur in bacterial mats.; Hydrocarbon seeps serve as a convenient natural laboratory for investigating stable isotope fractionations resulting from microbial sulfate reduction. The calculated fractionation factors in seeps range from 1.009 to 1.026 for sulfur and 1.002 to 1.014 for oxygen, and are controlled by sulfate reduction rates. In contrast to previous investigations which found a fixed 4/1 δ ₃₄S/δ18O ratio during sulfate reduction, this study indicates that the ratio also varies with reduction rates.; The seep barites occur as chimneys and crusts. The chimneys are 0.5 to 6.5 years old and their estimated growth rates vary from 4.4 cm/yr to 9.1 cm/yr, whereas the crusts are 9.0 to 23.1 years old. Sulfur, oxygen and strontium isotope compositions suggest that the chimneys formed at or above the sediment-water interface by mixing of sulfate-rich seawater with Ba-, Sr-, and Ra-rich formation fluids seeping with hydrocarbons. The crusts precipitated below the sediment-water interface by mixing of the formation fluids with residual seawater sulfate depleted by sulfate-reducing bacteria.