Occurrences of Natural Gas Hydrates beneath the Permafrost Zone in Mackenzie Delta: Visual and X-ray CT Imagery

Abstract: The JAPEX/JNOC/GSC Mallik 2L-38 research well was drilled to a depth of 1,150 m beneath the permafrost zone in the Mackenzie Delta, N.W.T., Canada, early in 1998. A large amount of natural gas hydrates were successfully retrieved from a variety of sandy and gravel sediments. Over 110 m of gas hydrate-bearing sediments were found to be distributed between 897 m and 1,100 m deep. Approximately 37 meters of core were recovered in this interval with most of the recovered gas hydrates being less than 2 mm in size occurring mainly in intergranular porosity of silty to clean massive sand and conglomerate (granule to pebble). Typically, hydrate-bearing strata were between 10 cm and more than one meter thick with an estimated porosity of 25 to 35%. The largest form of hydrate was about 2 cm in diameter, occurring as clasts and intergranular porosity within granular sands. Occurrences of natural gas hydrate have been observed visually at the drill site and in core samples preserved in pressurized storage vessels utilizing an X-ray CT scanner technique. Quantitative assessments of gas hydrate concentrations in core samples have been made based on pressure response of dissociation vessels and direct volumetric measurements. Six types of gas hydrate have been recognized: (1) pore-space hydrate, (2) platy hydrate, (3) layered/massive hydrate, (4) disseminated hydrate, (5) nodule hydrate, and (6) vein/dyke hydrate. The X-ray CT images proved useful for characterizing macroscopic forms of gas hydrate. Finer grained occurrences were more difficult to study, however the distribution of gas hydrates and granular grains can be recognized. The occurrences of natural gas hydrates in the Mallik well are compared to the previous natural gas hydrate core samples obtained from ODP/DSDP programs and other field studies.