GEOPHYSICAL EVIDENCE FOR GAS HYDRATES IN THE DEEP WATER OF THE SOUTH CASPIAN BASIN, AZERBAIJAN

Abstract

New 2-D seismic reflection data from the South Caspian Sea, offshore Azerbaijan, document for the first time in the deep water (up to 650m) of this area, the presence of gas hydrates. Geophysical evidence for gas hydrates consists of a shallow (300–500m below seafloor) zone of pronounced high velocity (~2,100m/s) as compared with the surrounding sediments (1550–1600m/s). This zone appears on the seismic data as a depth-limited (~200m thick) layer extending down the flank of an elongate structural high, and displays seismic blanking effects on the sedimentary section. A strong positive-polarity (Rc≈0.123) reflector marks the top of this velocity anomaly, and is interpreted as the top of the gas hydrate layer. Similarly, a high-amplitude (Rc≈0.11), negative polarity reflector coincides with the base of the high-velocity layer, and is interpreted as the base of the hydrate zone. Both the top and bottom of the hydrate layer approximately parallel the seafloor bathymetry, and cut discordantly across the stratigraphic section, suggesting that the two reflectors are thermobaric and not stratigraphic interfaces. Decreasing amplitude with offset at the base of the gas hydrate layer may indicate the accumulation of free gas beneath this interface. These gas hydrates fall within the hydrate stability field predicted from thermobaric modeling for the South Caspian Basin, but typically in thinner layers than would be expected from theoretical calculations. The minimum predicted water depth that allows hydrate formation is ~150m, and the maximum predicted thickness of the gas hydrate stability field is ~1350m.