Estimates of Undiscovered Natural Gas Volumes, Onshore North Slope: A Retrospective

By

K.J. Bird (U.S. Geological Survey)

Increasing demand for natural gas and renewed interest in pipeline construction to the North Slope bring new relevance to estimates of undiscovered natural gas resources, a North Slope resource previously considered non-economic for at least 20 years. The most recent North Slope-wide assessment (onshore and State offshore) of undiscovered natural gas resources was part of the U.S. Geological Survey’s 1995 National Assessment of U.S. Oil and Gas Resources. In that assessment, a mean-value of 64 trillion cubic feet (TCF) of technically recoverable gas was hypothesized to occur in nine petroleum plays. Slightly more than 90-percent of the gas was estimated to occur separate from oil as non-associated gas accumulations. Interestingly, this is the opposite of the volumes of gas already discovered, i.e., most gas is associated with oil accumulations.

Estimates of non-associated gas show considerable uncertainty with values of 95th- and 5th-percentile estimates ranging from about 5 to 140 TCF. Approximately 70-percent of the gas (mean-value) was estimated to occur in structural traps in the Brooks Range fold and thrust-belt. Within this belt, about one-third of this resource was postulated to occur in Brookian reservoirs and two-thirds in Ellesmerian reservoirs.

In the 1995 assessment, the proportion of oil vs. gas was significantly influenced by a newly available analysis of the region’s thermal history. In the fold- and thrust-belt, this resulted in a decrease in oil and an increase in gas estimates relative to an earlier assessment. Apatite fission-track analyses since 1995 show that structural traps formed 10–40 m.y. after peak hydrocarbon generation. This information promises to have a significant influence on the evaluation of fold- and thrust-belt resources in the new regional assessment currently being conducted. 

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.