ABSTRACT

Pore fluids associated with hydrocarbon seeps in deepwater Gulf of Mexico were sampled and analyzed for chemical and isotope compositions in order to determine their nature and origin. Three types of fluids have been discerned. Fluids pertaining to type I have normal Gulf of Mexico deepwater salinities (about 38) and their ⁸⁷Sr/⁸⁶Sr ratios (0.70916, n=3) are identical to the overlying seawater (0.70917). These fluids originated as seawater trapped in the sediments whose chemical and isotope compositions were modified by microbial sulfate reduction, authigenic carbonate precipitation and gas hydrate sublimation. Fluids of the second type are highly saline (116 to 182 salinity range), with ⁸⁷Sr/⁸⁶Sr ratios (0.70797 to 0.70897, n=3) below that of seawater and elemental chemistry compatible with halite dissolution. This type of fluids is interpreted to be derived by dissolution of shallow subsurface salt diapirs during seawater convective circulation. The third type fluids are characterized by high salinity (range from 52 to 155), high concentrations of Ba, Sr and Ca and lower than seawater ⁸⁷Sr/⁸⁶Sr ratios (0.70843 to 0.70862, n=8). These fluids are likely to be derived from deep-seated formation waters advecting on the seafloor. Whereas the first and second type of fluids occur in authigenic carbonate deposition areas, the third type is exclusively encountered in barite deposition sites.