m),
high acoustic transit-time velocities (~3.6 km/s), high neutron porosities
(~67%), and low apparent densities (~1.05 g/cm3).Gas hydrate has been inferred to occur at about 50 locations throughout the world (reviewed by Kvenvolden, 1988). Previous to Leg 164, however, gas hydrate had been sampled and surveyed with downhole logging devices at only three locations: (1) North Slope of Alaska (Collett, 1993), (2) Middle-America Trench off the Pacific coast of Guatemala (Shipboard Scientific Party, 1985), and (3) Cascadia continental margin off the Pacific coast of Canada (Shipboard Scientific Party, 1994). The downhole log data from the North Slope of Alaska and Middle-America trench have been the focus of several published studies (Collett et al., 1984; Mathews, 1986; Collett, 1993); however, the available downhole log data from the Cascadia continental margin have not been examined in detail. In the next section of this report, we have reviewed the results of the completed gas hydrate downhole log studies in northern Alaska and offshore Guatemala.
The only confirmed gas hydrate occurrence on the North Slope of Alaska was obtained in 1972, when a core containing this substance was recovered from a depth of about 657 m in the Northwest Eileen State-2 well (Collett, 1993). The confirmed gas hydrate occurrence in the Northwest Eileen State-2 well is characterized by relatively high electrical resistivities and acoustic velocities. In Collett et al., (1984) it was assumed that a Pickett crossplot could be used to determine the degree of gas-hydrate saturation in a gas hydrate- and water-bearing rock unit. The accuracy of this procedure to determine gas-hydrate saturation is not known. In Collett (1993), a series of Pickett crossplots were used to calculate gas-hydrate saturations in four gas-hydrate occurrences overlying the Prudhoe Bay and Kuparuk River oil fields on the North Slope of Alaska. Mathews (1986) also used the Pickett crossplot technique to estimate gas-hydrate saturations in the Northwest Eileen State-2 well.
The confirmed gas-hydrate occurrence in the Northwest Eileen State-2 well presents itself as an ideal starting point for the development of gas-hydrate downhole-log evaluation techniques. The responses of the commonly available downhole logs within the confirmed gas-hydrate interval of the Northwest Eileen State-2 well are summarized below (modified from Collett, 1983).
In most gas hydrate studies, only two downhole logging devices are consistently used to identify potential gas hydrate: they are the electrical resistivity and acoustic transit-time logs.
In 1982, while conducting
research coring operations on Leg 84 of the Deep Sea Drilling Project (DSDP), a
1.05-m-long core of massive gas hydrate was recovered at Site 570 in the
Middle-America Trench off the Pacific coast of Guatemala. The cored gas hydrate
sample was determined to be from the interval between 247.4 and 251.4 m
sub-bottom depth. Downhole log surveys indicated that the actual thickness of
the massive gas hydrate occurrence was about 3 to 4 m (Mathews, 1986). The
massive gas hydrate was characterized by high electrical resistivities (~155 m),
high acoustic transit-time velocities (~3.6 km/s), high neutron porosities
(~67%), and low apparent densities (~1.05 g/cm3).
Mathews (1986) estimated the amount of methane gas within the gas hydrate occurrences at Site 570 by using a log normalization technique. At Site 570, the resistivity log "plateaus" in the massive gas hydrate zone, indicating a 100% gas-hydrate saturated interval. Mathews (1986) normalized the resistivity data and assumed that a value of 1.0 indicates a 100% pure gas hydrate. Therefore, any deviation from 1.0 indicates that gas hydrate has been replaced by rock matrix material and/or formation water.
