Trapping and migration of methane associated with the gas hydrate stability zone at the Blake Ridge Diapir: new insights from seismic data

The Blake Ridge Diapir is the southernmost of a line of salt diapirs along the Carolina trough. Diapirs cause faulting of the superjacent sediments, c reating pathways for migration of fluids and gas to the seafloor. We analyz ed reflection seismic data from the Blake Ridge Diapir, which is located in a region with known abundant gas hydrate occurrence, A striking feature in these data is a significant shallowing of the base of gas hydrate stabilit y (BGHS) over the center of the diapir: The seafloor is warped up by about 100 m above the diapir, from about 2300 m to about 2200 m. The BGHS, as ind icated by a bottom simulating reflection (BSR), is about 4.5 s off the flan ks of the diapir, rising to about 4.15 s at the center. Above the diapir, a fault system appears to rise vertically from the BGHS to about 0.05 s belo w the seafloor (40-50 m); it then diverges into several steeply dipping fau lts that breach the seafloor and cover an area similar to 700 m in diameter , Other secondary faults diverge from the main fault or emerge directly fro m the BGHS near the crest of the diapir. Gas and other fluids may migrate u pward through the faults. We performed complex trace analysis to compare th e reflection strength and instantaneous frequency along individual reflecti ons. A low-frequency anomaly over the center of the diapir indicates high s eismic attenuation. This is interpreted to be caused by migration of fluids (probably methane) along fault pathways. The migration of gas (i.e. probab ly mainly methane) through the gas hydrate stability zone is not yet unders tood. We speculate that pore fluids in the faults may be too warm and too s alty to form gas hydrate, even at depths where gas hydrate is stable away f rom the diapir. Alternatively, gas hydrates may seal the fault walls such t hat water supply is too low to transform all the gas into gas hydrates. The shallowing of the BSR may reflect increased heatflow above the diapir eith er caused by the high thermal conductivity of the underlying salt or by adv ective heat transport along with fluids. High pore water salinity shifts th e gas hydrate stability to lower temperatures and may also play a significa nt role in BSR shallowing. We, therefore, investigated the possible effect of pore water salinity on shallowing of the BSR. We found that BSR shallowi ng may theoretically be entirely caused by increased salinity over the diap ir, although geologically this would not be reasonable. This observation de monstrates the potential importance of pore water salinity for lateral vari ations of BSR depths, in particular, above salt structures. (C) 2000 Elsevi er Science B.V. All rights reserved.