Multiple resolution seismic imaging of a shallow hydrocarbon plumbing system, Woolsey Mound, Northern Gulf of Mexico

摘要

The northern Gulf of Mexico is dominated by salt tectonics, resulting fracturing and numerous seafloor seeps and vents. Woolsey Mound, site of the Gulf of Mexico Hydrates Research Consortium's seafloor observatory, has been investigated extensively via surveys, direct sampling and seafloor instrument systems. This study presents an innovative approach to seismic data interpretation, integrating three different resolution datasets and maximizing seismic coverage of the complex natural hydrocarbon plumbing system at Woolsey Mound. 3D industry seismic data reveal the presence of a salt body at in the shallow subsurface that has generated an extended network of faults, some extending from the salt body to the seafloor (master faults). Higher resolution seismic data show acoustic wipe-out zones along the master faults with expulsion features - seafloor pockmarks and craters - located immediately above them and associated, in the subsurface, with high-amplitude, negative anomalies at constant depth of 0.2 s TWTT b.s.f., interpreted as free gas. Since pockmarks and craters provide pathways for hydrocarbons to escape from depth into the water column, related sub-surface seismic anomalies may indicate free gas at the base of the gas hydrates stability zone (GHSZ). Fluid flow and gas hydrates formation are segmented laterally along faults. Gas hydrates formation and dissociation vary temporally in the vicinity of active faults, and can temporarily seal them as conduits for thermogenic fluids. Periodic migrations of gases and other fluids may perturb the GHSZ in terms of temperature and pressure, producing the observed lack of classical BSRs.