Geochemistry of Natural Gas, North Slope, Alaska: Implications for Future Oil and Gas Resources, NPRA

By

R.C. Burruss, P.G. Lillis, and T.S. Collett (U.S. Geological Survey)

As part of USGS evaluations of the future petroleum potential of the North Slope of Alaska, we are examining all publicly available data on natural gas geochemistry from wells on the North Slope. Our compilation includes new samples collected by the USGS from U.S. Navy wells, gases released by hydrous pyrolysis of source rocks during artificial maturation, and data released by industry. The data include molecular composition, and when possible, isotopic analyses. Gases from wells in accumulations at depths greater than 1500 ft. are dominantly thermogenic with the exception of methane in the Alpine Field (Masterson, 2001, Ph.D. dissertation). Shallow gas with biodegraded oil in the Simpson Field is bacterial and similar to Alpine Field methane. Gas compositions in accumulations from the Endicott Field in the east to the Walakpa field in the west span the range of gas compositions released by hydrous pyrolysis of two major sources (Shublik Formation and Hue Shale). Gases within the reservoirs of the greater Prudhoe Bay-Kuparuk River Fields show evidence of mixing from different sources, consistent with evidence that oils in these reservoirs are mixtures from several sources. Some Prudhoe-Kuparuk gases show evidence of microbial alteration. Gases within NPRA correlate well with gases released from a gas-prone facies of the Hue Shale, which we consider representative of gas-prone Cretaceous sources. The potential of a significant volume of biogenic gas in the Alpine Field raises important questions about migration pathways and the possible extent of biodegradation of oil in reservoirs within NPRA.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.