Exploration programs that rely on the bottom simulating reflector as the primary hydrate marker may be missing significant accumulations of gas hydrate on continental slopes. I propose that, in at least some reservoirs, gas hydrate forms primarily as dikes, which are not imaged by conventional seismic surveys. Development of the dikes is controlled by free gas pore pressure, reservoir stresses, and the cohesion and friction angle of the sediment. Multiple dikes are expected to be parallel, or to develop in chevrons, but may not be equally spaced.

Nature and Distribution of Gas Hydrates

Gas hydrates are a class of clathrate compounds in which individual small molecules occupy sites within a solid crystalline matrix of water molecules [Sloan, 1998]. In natural hydrate reservoirs, guest molecules are either pure methane or a mixture of compounds comprising natural gas. Gas hydrates form in a restricted range of elevated pressure and reduced temperature. Typically they are found in sediments within a few hundred meters of the seafloor, in water depths of about 1000 m or more [Kvenvolden, 1993]. For a gas hydrate deposit to form, a source of gas is required, and seeps of natural gas are common in many parts of the world. Therefore it is not surprising that gas hydrates appear to be widespread in deep water marine environments. A compilation [Kvenvolden and Lorenson, 2001] documents more than one hundred occurrences of hydrates on continental margins and in inland seas.

In most cases, the location and areal extent of deposits are estimated from a peculiar seismic signature of gas hydrate presence, the bottom simulating reflector (BSR) [Shipley et al., 1979]. The BSR is seen in marine seismic images running parallel to, and several hundred meters below, the seafloor, approximately coincident with the base of the gas hydrate stability zone (GHSZ). Unfortunately, the BSR is often a poor predictor of hydrate occurrence. For example, at Blake Ridge, little hydrate was found in a well drilled to a strong BSR, whereas there were hydrate shows in a well drilled in a locale where the BSR was absent [Paull et al., 2000]. Major BSR-directed drilling campaigns on Blake Ridge offshore South Carolina [Paull et al., 2000], on Hydrate Ridge offshore Oregon [Trehu et al., 2004], and elsewhere have shown gas hydrate to be generally dilute throughout the gas hydrate stability zone. Sometimes significant concentrations are found in limited depth intervals, but these can be far above the depth of the BSR.

There is in fact no reason to expect a strong BSR to indicate a rich hydrate deposit. The BSR is commonly thought to be caused by an accumulation of free gas trapped under sediment rendered relatively impermeable by accumulation of hydrate at the base of the GHSZ. In fact, very little gas is required to produce a strong seismic reflector [Domenico, 1977; Dai et al., 2004], and free gas is usually not found at the BSR in quantities detectable by logging tools.

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