Gravity-driven faults: Migration pathways for recycling gas after the dissociation of marine gas hydrates

Three dimensional seismic data from offshore of Mauritania (West Africa) reveal tiers of normal and tear faults that are approximately parallel to the strike and dip of the slope respectively. They formed as a result of multiple episodes of down-slope creep and have propagated through 60-140 m of sediment. Some intersect the seabed while others are buried and formed during earlier down-slope translation. Gas hydrates, the base of which is evidenced by aligned amplitude terminations or a bottom simulating reflection, form within the faulted succession. There are spatial relationships among the fault locations, breached gas accumulations and the location of shallower free gas sealed by hydrates. These are consistent with the upward-resetting of the base of the hydrates, the migration of dissociated gas up the faults to either become trapped below its reset base in a new free gas zone or potentially recycled back into a hydrate phase. There is little evidence for venting of dissociated gas at the seabed. Gravity-driven faulting is common on continental and island slopes, where gas hydrates are expected to occur, therefore this model for the utilization of gravitationally driven faults for the migration and recycling of gas should be generally applicable. (C) 2012 Elsevier B.V. All rights reserved.