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Abstract

Lorenson, T. D., T. S. Collett, and M. J. Whiticar, 2009, Hydrocarbon gas composition and origin of gas hydrate from the Alaska North Slope, in T. Collett, A. Johnson, C. Knapp, and R. Boswell, eds., Natural gas hydrates—Energy resource potential and associated geologic hazards: AAPG Memoir 89, p. 584–597.

DOI:10.1306/13201125M893361

Copyright copy2009 by The American Association of Petroleum Geologists.

Hydrocarbon Gas Composition and Origin of Gas Hydrate from the Alaska North Slope

Thomas D. Lorenson,1 Timothy S. Collett,2 Michael J. Whiticar3

1U.S. Geological Survey, Menlo Park, California, U.S.A.
2U.S. Geological Survey, Denver Federal Center, Denver, Colorado, U.S.A.
3School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada

ACKNOWLEDGMENTS

This work was supported in part by the U.S. Geological Survey Gas Hydrate Project, the U.S. Department of Energy, and the Bureau of Land Management. Industry provided access to drill cuttings and well-log data. The chapter benefited from the reviews of Ray Boswell, Dennis Coleman, and one anonymous reviewer. Paul Eby provided the isotopic analyses.

ABSTRACT

Hydrocarbon gas composition and isotopic composition of methane were analyzed from cutting samples obtained from industry oil wells penetrating the Eileen and Tarn gas-hydrate deposits. These gas-hydrate deposits overlie the Prudhoe Bay and Kuparuk River oil fields and are restricted to the updip part of a series of nearshore deltaic sandstone reservoirs in the lower Tertiary (Eocene) Mikkelsen Tongue of the Canning Formation and the Tertiary Staines Tongue of the Sagavanirktok Formation, respectively. The Eileen and Tarn gas hydrates are thought to contain a mixture of deep-source thermogenic gas and shallow, microbial gas (methane carbon isotopic composition ranges from minus54 to minus46permil in the gas-hydrate zone). Thermogenic gases likely come either from existing oil and gas accumulations or from source rocks within the oil- and gas-generating window that have migrated updip and or upfault and formed gas hydrate. The timing of gas source mixing is unknown. The microbial gases likely have a source contribution from biodegraded oil or gas in the underlying oil fields, as evidenced by the carbon isotopic composition of methane, ethane, propane, and carbon dioxide. The distribution of the Eileen and Tarn gas-hydrate accumulations appears to be controlled in part by the presence of large-scale regional faults that may have acted as vertical and lateral gas migration conduits.

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