The geologic occurrence of gas hydrate has been known since the mid-1960s, when gas-hydrate accumulations were discovered in Russia (reviewed by Makogon, 1981). Gas hydrate is widespread in permafrost regions and beneath the sea in sediment of outer continental margins (reviewed by Kvenvolden, 1993). Cold surface temperatures at high latitudes on earth are conducive to the development of onshore permafrost and gas hydrate in the subsurface. Onshore gas hydrate are known to be present in the West Siberian Basin (Makogon et al., 1972) and are believed to occur in other permafrost areas of northern Russia, including the Timan-Pechora province, the eastern Siberian Craton, and the northeastern Siberia and Kamchatka areas (Cherskiy et al., 1985). Permafrost-associated gas hydrate is also present in the North American Arctic. Direct evidence for gas hydrate on the North Slope of Alaska comes from a core-test in gas hydrate exploration well (the Northwest Eileen State-2 well), and indirect evidence comes from drilling and open-hole industry well logs that suggest the presence of numerous gas hydrate layers in the area of the Prudhoe Bay and Kuparuk River oil fields (Collett, 1993). Well-log responses attributed to the presence of gas hydrate have been obtained in about one-fifth of the wells drilled in the Mackenzie Delta, and more than half of the wells in the Arctic Islands are inferred to contain gas hydrate (Judge and Majorowicz, 1992). The recently completed Mallik 2L-38 gas hydrate research well, confirmed the presence of a relatively thick, highly concentrated, gas hydrate accumulation on Richards Island in the outer portion of the Mackenzie River Delta (Dallimore et al., 1999).
The presence of gas hydrate in offshore continental margins has been inferred mainly from anomalous seismic reflectors (i.e., BSRs) that coincide with the predicted phase boundary at the base of the gas-hydrate stability zone. Gas hydrate have been recovered in gravity cores within 10 m of the sea floor in sediment of the Gulf of Mexico (Brooks et al., 1986), the offshore portion of the Eel River Basin of California (Brooks et al., 1991), the Black Sea (Yefremova and Zhizhchenko, 1974), the Caspian Sea (Ginsburg et al., 1992), and the Sea of Okhotsk (Ginsburg et al., 1993). Also, gas hydrate have been recovered at greater sub-bottom depths during research coring along the southeastern coast of the United States on the Blake Ridge (Kvenvolden and Barnard, 1983; Shipboard Scientific Party, 1996), in the Gulf of Mexico (Shipboard Scientific Party, 1986), in the Cascadia Basin near Oregon (Shipboard Scientific Party, 1994), the Middle America Trench (Kvenvolden and McDonald, 1985), offshore Peru (Kvenvolden and Kastner, 1990), and on both the eastern and western margins of Japan (Shipboard Scientific Party, 1990, 1991).
3. HOW DOES GAS HYDRATE OCCUR IN NATURE?
Little is known about the nature of gas hydrate reservoirs. For example, do hydrate occur as pore-filling constituents or are they only found in massive form. Information about the nature and texture of reservoired gas hydrate is needed to accurately determine the amount of gas hydrate and associated gas in a given gas hydrate accumulation. The textural nature of gas hydrate in the reservoir also controls the production potential and characteristics of a gas hydrate accumulation. The physical and chemical conditions that result in different forms (disseminated, nodular, layered, massive) and distributions (uniform or heterogeneous) of gas hydrate are not understood (reviewed by Sloan, 1998). It is necessary; therefore, to systematically review descriptions of known gas hydrate occurrences and evaluate existing gas hydrate reservoir models in order to assess the nature of gas-hydrate-bearing reservoirs.
For this review of the nature of gas hydrate occurrences, I have relied extensively on the offshore gas hydrate sample database published by Booth et al. (1996). In this database, Booth et al. (1996) systematically review and describe more than 90 marine gas hydrate samples recovered from 15 different geologic regions. In general, most of the recovered gas hydrate samples consist of individual grains or particles, which are often described as inclusions or disseminated in the sedimentary section. Gas hydrate also occur as, what has been described as, a cement, nodules, or as lamina and veins, which tend to be characterized by dimensions of a few centimeters or less. In several cases, thick, pure gas hydrate layers measuring as much as 3- to 4-m-thick have been sampled (DSDP Site 570; Shipboard Scientific Party, 1985). In both marine and terrestrial permafrost environments, the thickness of identified gas-hydrate-bearing sedimentary sections varies from a few centimeters to as much as 30 m (Collett, 1993; Booth et al., 1996; Dallimore et al., 1999). Most pure gas hydrate lamina and layers, however, are often characterized by thicknesses of millimeters to centimeters (Booth et al., 1996; Dallimore and Collett, 1995; Dallimore et al., 1999). Booth et al. (1996) conclude that gas-hydrate-bearing sedimentary sections tend to be tens of centimeters to tens of meters thick, but thick zones of pure hydrate are relatively rare and only represent a minor constituent of potential gas hydrate accumulations.
The Booth et al. (1996) review along with recently published gas hydrate sample descriptions from the Mackenzie Delta (Dallimore and Collett, 1995; Dallimore et al., 1999) and the Blake Ridge (ODP Leg 164, Shipboard Scientific Party 1996), confirm that gas hydrate are usually uniformly distributed within sediments as mostly pore-filling constituents.
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