Gas Hydrates of the South Caspian Sea, Azerbaijan: Drilling Hazards and Sea Floor Destabilizers

摘要

Seismic reflection data from the South Caspian Sea, offshorernAzerbaijan, prove evidence for buried gas hydrates wellbeneathrnthe seafloor (~300 m), with implications for drillingrnhazards and seafloor instability. Some of the seismicrncharacteristics of these gas hydrates are (1) a shallow zone ofrnpronounced high velocity (Vp=2.1 km/s, Vs=0.8 km/s) asrncompared with the surrounding sediments (Vp=1.55-1.60rnkm/s, Vs=0.36 km/s), (2) seismic blanking, (3) a bottomrnreflector with high negative impedance contrast (-Rc=0.123),rnand (4) a top, sealing reflector with a pronounced positivernimpedance contrast (+Rc=0.198). The interpreted thicknessrnand depth of these hydrates match well with the hydraternstability field predicted from thermobaric modeling. Thernpresence of both thermogenic and biogenic gas, identifiedrnfrom coring at the seafloor, suggests that gas hydrates in thernSouth Caspian Sea may be stable in water depths as shallow asrn~150 m, much shallower than other areas reported worldwide.rnThe maximum predicted thickness is 1300 m, considerablyrnthicker than other known hydrate occurrences. Accumulationrnof these hydrates near the base of the continental rise appearsrnto control a large region (>200 sq. km) of shallowrndeformation, including shallow faulting evident on therndetailed bathymetry of the seafloor. The gas hydrates of thernSouth Caspian region prove to be widespread features of therndeepwater of the South Caspian Sea, buried deposits wellrnbeneath the seafloor, and accordingly, they may representrnsignificant and previously underestimated geo-hazards.rnPrimary among these hazards are (1) uncontrolled release ofrnfree gas trapped beneath the hydrate seal, (2) disruption of therngas hydrate stability field leading to either explosiverndissociation of the gas hydrate, or reduction in sedimentrnstrength, and (3) slope instability, and mass sedimentrntransport. The association of gas hydrates with active mudrnvolcanoes in the South Caspian Sea increases the potential forrnoffshore flaming eruptions.