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Robert C. Burruss1, Timothy S. Collett2
(1) U.S. Geological Survey, Reston, VA
(2) U.S. Geological Survey, Denver

Abstract: Stable isotope geochemistry of natural gas, North Slope, Alaska: Evidence for multiple sources, mixing, and alteration

During the current U.S. Geological Survey (USGS) evaluation of the petroleum resource potential of the National Petroleum Reserve in Alaska (NPRA), we recently sampled gases from three exploration wells drilled by the U.S. Navy between 1945 and 1952. Analyses of new samples included molecular compositions and isotopic analyses (^dgr 13C of C1, C2, C3, i-C4, n-C4, CO2 and ^dgr D of C1.) These data, combined with gas analyses from the U.S. Navy/USGS exploration wells drilled in 1977 to 1982 and publicly released industry data, provide information on the origin and migration of natural gas on the North Slope. Gases from the Umiat and Wolf Creek anticlines of the Brooks Range foothills are isotopically identical to gas from the Walakpa field on the Barrow Peninsula. These gases have relatively heavy isotopic compositions (^dgr 13C3=-27 ‰ PDB) and are rich in methane (C1/C2+ > 40). This is consistent with generation from source rocks rich in Type III organic matter of probable Cretaceous age. Gases from the greater Prudhoe Bay/Kuparuk field area are richer in higher hydrocarbons (C1/C2+ < 20) and have isotopic compositions ranging from isotopically light (^dgr 13C3 < -32 ‰ PDB), consistent with generation from a source rich in type I-II organic matter (for example, Shublik Formation), to isotopically heavy gases consistent with Cretaceous sources. This suggests that gases within the Prudhoe Bay field area are mixtures of at least two sources. The isotopically heavy gases within the greater Prudhoe Bay area have unusual compositions with ^dgr 13C2 > ^dgr 13C3 > ^dgr n13C4. The origin of this isotopic composition is not well understood, but may be related to biodegradation processes or possibly to mixing with a third source of isotopically heavy hydrocarbon gases.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana