Geology of hydrocarbon seeps on the northern Gulf of Mexico continental slope.

摘要

The geology of four hydrocarbon seeps on the Louisiana continental slope of the Gulf of Mexico was studied. The study area included GC 184/185, GC 234, GB 386, and GB 425.; The data base includes 3.5 kHz, 12 kHz, and 25 kHz reflection profiles, 77 kHZ side-scan sonar records, piston cores, push cores, grab samples, video tapes, and bathymetry, which were obtained by the submarine NR-1, the submersible Johnson Sea-Link, and the R/V Gyre. Industry 3.5 kHz reflection records, minisleeve exploder records, and selected multi-channel seismic lines were also used.; Hydrocarbon seeps were concentrated along active faults associated with salt movement. Seep sediments included authigenic carbonates, gas hydrates, and mud that was either oil- or gas-charged. Each of these substrates cause acoustic anomalies in high-frequency reflection profiles.; 25 kHz reflection profiles showed six echo types. Types I and II consisted of hard substrates either exposed or buried, respectively. Type III featured acoustic turbidity caused by gas bubbles or disseminated carbonates or both. Type IV showed acoustic reverberation resulting from scattered shell fragments and carbonate nodules. Type V exhibited zones of acoustic wipeout caused by gas in sediments. Type VI consisted of undisturbed mud layers.; Authigenic carbonates were concentrated along hydrocarbon conduits. Chemosynthetic organisms concentrated on authigenic carbonate substrates, suggesting that these zones have high concentrations of hydrocarbons and provide solid substrates for benthic organisms.; Round-top and flat-top seep mounds exist in the study areas. The round top of Bush Hill is believed to have been controlled by faults, which acted as fluid conduits. Sediments from the conduits began to deposit along the steep slope caused by fault scarps. As sediments accumulated, they were eventually deposited over the uplifted seafloor, forming an asymmetrical mud mound. In contrast, the flat-top mounds of GB 386 and GB 425 exhibited lateral deposit of fluidized sediments at the seafloor. Widespread fluid venting caused the formation of a carbonate crust and/or gas hydrates. Finally, a flat-top mud mound, covered by carbonates and/or gas hydrates, was formed, as in the GB 386 mound. At GB 425, fresh mud provided by vents eventually covered the mound's hard substrates.