Geophysical Characterization and Identification for a Gas Hydrate and Free Gas Coexisting System Using Seismic-Well Integration

Gas hydrate have been widely identified using different geophysical exploration techniques in the world's continental margins showing different morphologies and properties. The gas hydrate system is governed by temperature-pressure conditions and influenced by tectonic-sedimentary processes， which undergoes dynamic adjustments on geological timescales， and characterized as various types of bottom-simulating reflectors (BSRs). The dynamic gas hydrate systems show complex contacts between gas hydrate and free gas and the coexistence between them controlled by different geological factors. Five types of coexistence exhibiting distinct spatial anomalies have been identified by drilling data and seismic anomalies， corresponding to the BSR adjustment and different controlling factors. The coexistence of gas hydrate and free gas has been confirmed from geophysical anomalies of seismic data， well log and other methods due to the influences of fluid migrations， tectonic evolution and sedimentary processes. We emphasize that the coexistence types are closely related with the controlling factors and are easily misinterpreted as free gas-bearing sediments. In general， double BSR， multiple BSR， plumbing or feature BSRs are seismic indicators for gas hydrate occurrence， and the shifts of BSRs are often observed from the seismic amplitude and attribute anomalies.