Seismic Evidence for Widespread Gas Hydrate Formation, Offshore West Africa

Abstract

Seismic reflection data recorded offshore west Africa reveal the presence of extensive bottom simulating reflectors (BSRs) on the continental slope off the Niger and Congo River Deltas. Cumulative surface areas encompassing the Nigeria and Congo BSR occurrences are approximately 11,000 km2 and 4000 km2, respectively. West African BSRs display many of the characteristics that have commonly been ascribed to submarine gas hydrates elsewhere in the world, including reflection polarity reversal relative to the sea floor reflection, consistency between depth of the BSR and base of the gas hydrate stability field, geometric relationships between BSR and bedding plane reflections, and a positive correlation between the subwater bottom depth of the BSR and the water depth. Widespread BSRs along the Nigeria and Congo continental slopes generally occur in areas of complex structure and where water depths are greater than 1200 m. The relationship between gas hydrate occurrence and structure exists, in part, because BSRs are more easily recognized on seismic sections where the BSRs cut across dipping stratigraphic reflections. However, faults and dipping stratigraphy also facilitate migration of biogenic or thermogenic gas to the base of the gas hydrate stability zone, enabling the formation of BSRs.