Abstract

Seismic reflection data from the South Caspian Sea, offshore Azerbaijan, prove evidence for buried gas hydrates wellbeneath the seafloor (~300 m), with implications for drilling hazards and seafloor instability. Some of the seismic characteristics of these gas hydrates are (1) a shallow zone of pronounced high velocity (Vp=2.1 km/s, Vs=0.8 km/s) as compared with the surrounding sediments (Vp=1.55-1.60 km/s, Vs=0.36 km/s), (2) seismic blanking, (3) a bottom reflector with high negative impedance contrast (-Rc=0.123), and (4) a top, sealing reflector with a pronounced positive impedance contrast (+Rc=0.198). The interpreted thickness and depth of these hydrates match well with the hydrate stability field predicted from thermobaric modeling. The presence of both thermogenic and biogenic gas, identified from coring at the seafloor, suggests that gas hydrates in the South Caspian Sea may be stable in water depths as shallow as ~150 m, much shallower than other areas reported worldwide. The maximum predicted thickness is 1300 m, considerably thicker than other known hydrate occurrences. Accumulation of these hydrates near the base of the continental rise appears to control a large region (>200 sq. km) of shallow deformation, including shallow faulting evident on the detailed bathymetry of the seafloor. The gas hydrates of the South Caspian region prove to be widespread features of the deepwater of the South Caspian Sea, buried deposits well beneath the seafloor, and accordingly, they may represent significant and previously underestimated geo-hazards. Primary among these hazards are (1) uncontrolled release of free gas trapped beneath the hydrate seal, (2) disruption of the gas hydrate stability field leading to either explosive dissociation of the gas hydrate, or reduction in sediment strength, and (3) slope instability, and mass sediment transport. The association of gas hydrates with active mud volcanoes in the South Caspian Sea increases the potential for offshore flaming eruptions.

Introduction

Gas hydrates are quasi-stable solid substances, composed of rigid cages of hydrogen-bonded molecules of water that entrap molecules of natural gas, mainly methane (e.g. Kvenvolden, 1993; 1995). Four principal elements are required in order to form gas hydrates, which are hydrocarbon gas, water, high pressure, and low temperature (Kvenvolden, 1993; Bagirov and Lerche, 1997).

Gas hydrates have been under the close attention of both the academia and industry mainly due to their three main characteristics, such as (1) potential drilling hazards, (2) considerable fuel resource for the future, and (3) likely role in global climate change. Gas hydrates are quasi-stable structures that can dissociate slowly or explosively, and such they can affect the strength of the sediments in which they reside (Kvenvolden, 1995; Sloan, 1998). Therefore, they can play a significant role in sediment transport in marine sediments that can be triggered by either high sedimentation rates or sea level fluctuations or other processes that can produce changes in the pressure-temperature regime.

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