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.