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Hydrocarbon Gas Sources in the Mt. Elbert No. 1 Gas Hydrate Test Well, Milne Pt. Alaska

Lorenson, Thomas D.1; Collett, Timothy S.2; Hunter, Robert B.3
1 USGS, Menlo Park, CA.
2 USGS, Denver, CO.
3 ASRC Energy Services, Anchorage, AK.

Hydrocarbon gases were collected from well cuttings, core, and formation tests at the Mt. Elbert-01 gas hydrate stratigraphic test well (TD, 915m), drilled within the Milne Point Unit on the Alaska North Slope. The well tested a seismically-defined prospect within the Eileen gas hydrate deposit. Regionally, the Eileen gas hydrate deposits overlie the more deeply buried Prudhoe Bay, Milne Point, and Kuparuk River oil fields and are restricted to the up-dip portion of a series of nearshore deltaic sandstone reservoirs in the lower Tertiary (Eocene) Mikkelsen Tongue of the Canning Formation. The tested gas hydrates occur in two primary horizons; an upper zone, (“D” Unit, 614.6 - 627.7m) containing 13.1 meters of gas hydrate-bearing sands and a lower zone (“C” Unit, 650.0 - 661.0m), containing 11.0 meters of gas hydrate-bearing sands with log-interpreted gas hydrate saturations of 60 to 75 percent in both units. The hydrocarbon gases in cuttings samples from 604 to 914 meters are composed of methane with less than 1 ppm ethane. The isotopic composition of the methane carbon ranges from -50.1 to -47.2 permil, decreasing with depth. Methane hydrogen isotopic composition ranged from -260 to -229 permil with no apparent depth trend. Hydrocarbon gas samples collected by the Modular Dynamics formation Testing (MDT) tool in hydrate-bearing units C and D were similarly composed mainly of methane, with up to 270 ppm ethane. The isotopic composition of the methane carbon ranged from -48.2 to -48.0 permil in the C sand and from -48.4 to -46.6 permil in the D sand. Methane hydrogen isotopic composition ranged from -238 to -230 permil, with slightly heavier values in the deeper C sand. These results are consistent with the concept that the Eileen gas hydrates contain a mixture of deep-sourced thermogenic gas and shallow, microbial gas, similar to the known regional methane carbon isotopic composition (-54 to -46 permil) in the gas hydrate zones. Thermal gases are likely sourced from existing oil and gas accumulations that have migrated up-dip and/or up-fault and formed gas hydrate. The shallow microbial gas was likely sourced from deeper biodegraded oil, migrated into conventional traps as free gas, and later converted to gas hydrate in response to climate cooling with permafrost formation.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009