Gas hydrates are crystalline substances composed of water and gas, in which a solid water lattice accommodates gas molecules in a cage-like structure, or clathrate. Gas hydrates are widespread in permafrost regions and beneath the sea in sediment of outer continental margins. While methane, propane, and other gases can be included in the clathrate structure, methane hydrate appears to be the most common in nature (Kvenvolden, 1988). The amount of methane sequestered in gas hydrate is probably enormous, but estimates of the amounts are speculative and range over three orders of magnitude from about 3,114 to 7,634,000 trillion m3 (reviewed by Kvenvolden, 1993). The amount of gas in the hydrate accumulations of the world greatly exceeds the volume of known conventional gas reserves. Gas hydrates also represent a significant drilling and production hazard. Russian, Canadian, and American researchers have described numerous problems associated with gas hydrate, including blowouts and casing failures (reviewed by Yakushev and Collett, 1992). Recent studies indicate that atmospheric methane, a greenhouse gas, is increasing at a rate such that the current concentration will probably double in the next 50 years (Kvenvolden, 1988). Because methane is 21 times more radiatively active than carbon dioxide, it is predicted that methane will surpass carbon dioxide as the predominant atmospheric greenhouse gas in the second half of the next century. The source of this atmospheric methane is uncertain; however, numerous researchers have suggested that destabilized natural-gas hydrate may be contributing to the build-up of atmospheric methane (reviewed by Kvenvolden, 1988).
One of the fundamental problems that links the gas hydrate resource, hazard, and climate issues is the need for accurate assessments of the gas volumes within gas hydrate accumulations. Most of the published gas hydrate resource estimates have of necessity been made by broad extrapolation of only general knowledge of local geologic conditions. Gas volumes that may be attributed to gas hydrate are dependent on a number of reservoir parameters, including the areal extent of the gas-hydrate occurrence, reservoir thickness, reservoir porosity, and the degree of gas-hydrate saturation (Collett, 1993). Two of the most difficult reservoir parameters to determine are porosity and the degree of gas-hydrate saturation. Well logs often serve as a source of porosity and hydrocarbon saturation data; however, downhole-log calculations within gas hydrate-bearing intervals are subject to error. The primary reason for this difficulty is the lack of previous quantitative laboratory and field calibration studies.
Leg 164 of the Ocean Drilling Program (ODP) was designed to investigate the occurrence of gas hydrate on the Blake Ridge (Paull, Matsumoto, Wallace, et al., 1996). The presence of gas hydrate in this area has long been suspected because of seismic reflection data showing a strong bottom-simulating reflector (BSR), which often represents the interface between gas hydrate-bearing sediments and underlying gas-charged sediments. Sites 994, 995, and 997 were drilled on the Blake Ridge to refine our understanding of the in situ characteristics of natural gas hydrate. During Leg 164, a major emphasis was placed on the downhole logging program to obtain critical information about the in situ nature of gas hydrate on the Blake Ridge.
The primary objectives of this report are to document the various downhole log responses to the occurrence of gas hydrate on the Blake Ridge and to use the downhole electrical resistivity log data from Sites 994, 995, and 997 to assess the concentration (saturation) and volume of gas hydrate and associated free gas on the Blake Ridge. This report begins with a technical overview of previous gas hydrate downhole log studies and a review of the known responses of downhole logs to the presence of gas hydrate. The main body of the report deals with the development and utilization of quantitative downhole log evaluation techniques used to calculate the degree of gas-hydrate saturation within the known and logged gas hydrate occurrences on the Blake Ridge. This report concludes with an estimate of the potential volume of gas associated with the downhole log-inferred gas hydrate occurrences on the Blake Ridge.