Geologic structures associated with the carbonate/hydrate mound in Mississippi Canyon Block 118, Gulf of Mexico

A carbonate/hydrate mound in Mississippi Canyon Block 118 has been chosen by the Gulf of Mexico Hydrates Research Consortium to be the site of a sea-floor observatory. It will include seismo-acoustic, geochemical and micro-biologic sensors to monitor ambient noise, fluid venting and environmental conditions. The observations are expected to promote a better understanding of how fluids migrate within the mound and affect the formation/dissociation of gas hydrates. A number of preliminary studies have been done in preparation for installing the observatory. The mound is approximately one kilometer in diameter and is located on the continental slope in about 900m of water. Its surface has been imaged by multi-beam bathymetric sonar from an AUV 40m above the sea floor and by cameras at, or a few meters above, the sea floor. Also, direct visual observations have been made from manned submersibles. The interior of the mound and the underlying hydrate stability zone have been imaged seismically, electromagnetically and by direct-current resistivity. Proprietary 3-D seismic volumes show nearly vertical normal faults that connect deep salt formations with soft fine-grained sediments near the sea floor. It is hypothesized that these faults act as conduits for brines and hydrocarbon fluids, including petroleum and natural gas, to migrate upward and form the carbonate and hydrate constituents of the mound. Gas samples have been collected from vents and outcropping hydrate. Chemical analyses show the vent gas to be thermogenic from deep hot source rocks and to average 95% methane, 3% ethane 1% propane with minor other gases. There is no significant biogenic component. The outcropping hydrate is Structure II with gas composition 70% methane, 7.5% ethane, 15.9% propane with minor other gases. The difference between gas compositions from vents and hydrate appears to be due to molecular fractionation during hydrate crystallization. Results of geochemical studies indicate that vents formed at different times in various places on the surface of the mound. It is hypothesized that hydrate accumulations fill the faults to the point of blocking the flow and then divert the fluids into other migration routes. The mound may also be influenced by geologic structures on a larger scale than fluid migration. On 18 April, 2006, an earthquake occurred in the vicinity of Mississippi Canyon Block 122, about 15km east of the observatory site. It was characterized by anomalously weak P-waves and was not detected by USGS algorithms that rely on P-wave first arrivals to trigger them. It did produce long-period surface waves which allowed it to be detected and studied by other researchers. On the basis of surface waves, its magnitude was 4.8, a significant event. And it was not an isolated event. A similar earthquake occurred on the continental slope near Green Canyon Block 210, about 200km south-west of the observatory site, on 10 February, 2006. It had a 5.3 surface-wave magnitude. The weakness of P-wave arrivals is thought to indicate that such events are generated by gravity-driven slumping in soft sediments. If so, they may be symptoms of the instability of the continental slope offshore of the Mississippi River delta.