Preliminary Assessment of Hydrocarbon Gas Sources from the Mt . Elbert No . 1 Gas Hydrate Test Well , Milne Pt . Alaska
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چکیده
Hydrocarbon gases were collected from well cuttings and core at the MtElbert-01 gas hydrate stratigraphic test well, drilled within the Milne Point field on the Alaska North Slope. 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) containing 14 meters of gas hydrate-bearing sands and a lower zone (“C” Unit), containing 16 meters of gas hydrate-bearing sands with log-interpreted gas hydrate saturations of 60 to 75 percent. The hydrocarbon gases from well cuttings from 604 to 914 meters are composed of methane with less than 1 ppm ethane. The isotopic composition of the methane ranges from -50.1 to -47.2 percent, decreasing with depth. Hydrocarbon gases collected by Modular Dynamics Testing (MDT) sampling in hydrate-bearing units C and D were similarly composed of mainly methane, with up to 270 ppm ethane. The isotopic composition of the methane ranged from -48.2 to -48.0 percent in the C sand and from 48.4 to -46.6 percent in the D sand. These results are consistent with the concept that the Eileen gas hydrates contain a mixture of deep-source thermogenic gas and shallow, microbial gas, comparing to regional methane carbon isotopic composition ranges from -54 to -46 percent in the gas hydrate zones. Thermal gases likely source from existing oil and gas accumulations that have migrated up-dip and/or up-fault and formed gas hydrate. The shallow microbial gas, likely having a biodegraded oil gas source contribution was either directly converted to gas hydrate or was first concentrated as free gas in existing conventional traps and later converted to gas hydrate in response to climate cooling or changes in surface conditions.
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