Large quantities of natural gas may be stored in gas hydrate-bearing sediments because as much as 164 times the saturation concentration of gas at STP conditions exists in these solid phases per unit volume (Kvenvolden, 1988a; Sloan, 1989). It is estimated that there are about 104 Gt (Gt = 1015 gm) of carbon stored in gas hydrate in sediments, which is about twice the estimate of carbon in all other fossil fuel deposits (Kvenvolden, 1988b). Moreover, there may be considerable volumes of free gas trapped beneath the overlying gas hydrate-cemented zones associated with the bottom simulating reflector (BSR), as well as dissolved gas in the pore fluids.
Gas hydrate is believed to be common in continental margin sediments because seismic reflection data have indicated its presence in every ocean basin (Kvenvolden and Barnard, 1983; Kvenvolden, 1988a, 1988b). Gas hydrate usually is detected in seismic reflection data by the presence of a BSR. The BSR often cuts across sediment bedding planes, thus clearly distinguishing itself as an acoustic response to a diagenetic change rather than a depositional horizon. The BSR is believed to represent the base of the gas hydrate stability zone, which occurs at depths between about 200 and 600 m below seafloor (mbsf) on continental rises. The pore spaces of sediments above the BSR are partly filled with gas hydrate, which may increase the sediment density, whereas deeper sediments may contain free gas, resulting in a sharp contrast in acoustic impedance and a strong reflector at the base of the gas hydrate stability zone. The Carolina Rise, particularly along the Blake Ridge, was the area where marine gas hydrate was first identified on the basis of a BSR (Fig. 1) and is an area where gas hydrate appears to be especially extensive (Markl et al., 1970; Tucholke et al., 1977; Shipley et al., 1979; Paull and Dillon, 1981; Dillon and Paull, 1983; Markl and Bryan, 1983).
ODP Leg 164 was devoted to refining our understanding of the in situ characteristics and amounts of natural gas hydrate stored in marine sediments. The program involved drilling three sites on the Blake Ridge to 750-m depths that extend through the zone where gas hydrate is stable and into the sedimentary section below (Fig. 1). Short holes (50 m) were drilled at four sites on the crests of two diapirs on the Carolina Rise where gas hydrate-bearing sedimentary sections have been disturbed by the intrusion of diapirs (Fig. 1). Because of the ephemeral nature of gas hydrate, emphasis was placed on downhole measurements and sampling strategies that allow in situ conditions of gas hydrate to be reconstructed.
The objectives of Leg 164 included:
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