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Geochemical Evidence for Gas Hydrate Occurrences in the ConocoPhillips Gas Hydrate Production Test Well, North Slope, Alaska

Lorenson, Thomas D.*1; Collett, Timothy S.2; Schoderbek, David 3
(1) U.S. Geological Survey, Menlo Park, CA.
(2) U.S. Geological Survey, Denver, CO.
(3) CononcPhillips, Anchorage, AK.

Hydrocarbon gases were collected from well cuttings and mud gas at the Ignik Sikumi #1 gas hydrate production test well (TD, 791.6 m), drilled on the Alaska North Slope. The well was drilled to test the feasibility of producing methane by carbon dioxide injection that replaces methane in the solid gas hydrate. The well has been drilled, cased, and suspended awaiting carbon dioxide injection testing.

Ignik Sikumi #1 penetrated a seismically-defined, fault bounded prospect within the Eileen gas hydrate accumulation. Regionally, the Eileen gas hydrate accumulation overlies the more deeply buried Prudhoe Bay, Milne Point, and Kuparuk River oil fields and is restricted to the up-dip portion of a series of nearshore deltaic sandstone reservoirs in the Sagavanirktok Formation. Hydrate-bearing sandstones penetrated by Ignik Sikumi #1 occur in three primary horizons; an upper zone, (“E” sand, 579.7 - 597.4 m) containing 17.7 meters of gas hydrate-bearing sands, a middle zone (“D” sand, 628.2 - 648.6 m) with 20.4 m of gas hydrate-bearing sands and a lower zone (“C” sand, 678.8 - 710.8 m), containing 32 m of gas hydrate-bearing sands with neutron porosity log-interpreted average gas hydrate saturations of 58, 76 and 81% respectively. Carbon dioxide injection is currently planned for the upper part of the “C” sand.

The hydrocarbon gases in cuttings samples from 0 to ~400 meters are composed of microbial with traces of higher molecular weight hydrocarbons. From 400 m to 791.6 m (TD) a mix of microbial and biodegraded (secondary) thermogenic gas is present. Hydrocarbon gas samples collected in hydrate-bearing sands “E” and “D” were similarly composed mainly of methane and traces of ethane, with large concentration increases (up to 25-fold) in the Structure II forming gases propane and isobutane, thus Structure II gas hydrate is likely present in minor amounts. The “C” sand does not contain Structure II forming gases. Propane and isobutane carbon isotopic compositions are more 13C enriched indicating these gases are derived from primary thermogenic sources and are not from secondary sources. The “C” sand is in contact with liquid water below about 680 m, here methane and ethane isotopic compositions become more 13C-enriched while hydrocarbon concentrations decline rapidly, suggesting that is an important geochemical boundary. These results show that the Eileen gas hydrate deposits contain a mixture of deep-sourced thermogenic gas and shallow, microbial gas.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California