Authigenic Carbonate Rock from Hydrate Environments in the Gulf of Mexico: Consideration of Thermodynamic Properties

Woodong Jung¹ and Roger Sassen^2
1Department of Geology and Geophysics, Texas A&M University, College Station, TX
²Geochemical and Environmental Research Group (GERG), Texas A&M University, College Station, TX

The formation and dissolution of authigenic carbonate rock at seeps and vents on the deep seafloor depend on complex microbial consortia that anaerobically oxidize hydrocarbons and reduce sulfate. Gas hydrate tends to be present in many of the environments where authigenic carbonate occurs, but the main source of carbonate-forming hydrocarbon is free hydrocarbon gas, either (1) microbial methane from the biosphere, or (2) oil-related hydrocarbon gases (methane through pentane) and oil from the deep subsurface petroleum system. The carbonate rock is dominated by calcite, but aragonite, dolomite, and siderite are also present. Pyrite is also formed by in situ microbial processes. Barite is often found in the rock. Selected minerals were studied by thermodynamic properties including Gibbs free energy, equilibrium constant, and activity coefficient. Venting of hydrocarbons from the seafloor is significant, part of the global methane cycle. Microbial processes sequester enormous volumes of methane before it can enter the water column and atmosphere.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas