Gas Hydrates of Arctic Alaska

T. S. Collett, K. J. Bird, K. A. Kvenvolden, L. B. Magoon

Gas hydrates are crystalline substances composed of water and gas in which the solid-water latice accommodates the gas molecules in a cage-like structure, or clathrate. Significant quantities of naturally occurring gas hydrates have been detected in many regions of the Arctic including Siberia, the Mackenzie River delta, and the North Slope of Alaska. Gas hydrates are generally regarded as a potential (unconventional) source of natural gas. The Soviets, however, have demonstrated that hydrates are an immediate, producible source of natural gas; their Messoyakh gas field in the West Siberia basin produced about 100 billion ft³ of gas from hydrates over an eight-year period (1971-1978).

The primary purpose of the U.S. Geological Survey Gas Hydrate Project (funded by the U.S. Department of Energy) is to identify gas hydrates and to evaluate the geologic properties controlling their distribution on the North Slope of Alaska. Our studies suggest that the methane-hydrate stability zone is areally extensive beneath most of the coastal plain province and has thicknesses greater than 3,000 ft in the Prudhoe Bay area.

Gas hydrates have been identified in 34 exploratory and production wells using well-log responses calibrated to the response of an interval in one well where gas hydrates were recovered in a core by ARCO Alaska and Exxon. Most gas hydrates we identified occur in six laterally continuous Upper Cretaceous and lower Tertiary sandstone and conglomerate units; all these hydrates are geographically restricted to the area overlying the eastern part of the Kuparuk River oil field and the western part of the Prudhoe Bay oil field. Our calculations suggest that the volume of gas within these gas hydrates in approximately 8-10 trillion ft³, or about one-third the volume of conventional gas in the Prudhoe Bay field.

Geochemical analyses of well samples in the Kuparuk area suggest that the gas hydrates consist of a mixture of biogenic and thermogenic gas. We suggest that the identified hydrates originated from a mixture of deep-source thermogenic gas and shallow biogenic gas that was either directly converted to gas hydrate or first concentrated in existing traps and later converted to gas hydrate. We postulate that the thermogenic gas migrated from deeper reservoirs along the same faults thought to be migration pathways for the large volumes of shallow, heavy oil that occur in this area.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.