Abstract: Secondary Gas Migration within the Prudhoe Bay-Kuparuk River Area of Northern Alaska

Timothy S. Collett

Our recent North Slope geochemical studies have focused on the Upper Cretaceous and Tertiary deltaic Sagavanirktok Formation, which constitutes the latest stage of filling of the North Slope foreland basin. The Sagavanirktok Formation contains an estimated 40-60 billion barrels (in place) of low-gravity oil and 37-44 trillion cubic feet of natural gas in hydrate form. Most oil and gas accumulations in the Sagavanirktok Formation occur at shallow depths (<1,000 m) and overlie the giant Prudhoe Bay and Kuparuk River oil and gas accumulations.

Geochemical analyses of drill cuttings from 10 North Slope exploratory and production wells indicate that methane is the principal hydrocarbon gas in the near-surface (0-1,500 m) strata. Stable methane-carbon isotopic analyses of gaseous drill cuttings from several gas-hydrate-bearing units yield carbon isotopic values averaging approximately -49 per mil (relative to PDB), suggesting that the methane within the hydrates is from mixed microbial and thermogenic sources. Vitrinite reflectance (%R_o) values of about 0.4% show that the gas-hydrate-bearing rocks are thermally immature; thus, the thermogenic gas must have migrated from greater depths. Most of the gas hydrates occur either updip from or near the Eileen fault zone, which may have acted as a conduit for gas migration rom the deeper hydrocarbon accumulations in the Prudhoe Bay field.

The gas cap of the Prudhoe Bay field is composed primarily of methane (83-88% by volume) along with small quantities of ethane (5-7%) and propane (1-2%). If the gas within the near-surface sediments migrated from a deeper source, the shallow gas should have geochemical constituents similar to those of the deeper gas; however, no significant amounts of ethane or propane were detected within the gas hydrates. The thermogenic gas component within the gas hydrates of the Sagavanirktok Formation may have been stripped of most of its heavier hydrocarbons while migrating through underlying marine-slope shale deposits of the Canning Formation.

By comparing the methane-carbon isotopic composition of the gas mixture in the Sagavanirktok Formation to the isotopic composition of the Prudhoe Bay gas cap it is possible to calculate the relative volumes of gas from thermogenic and microbial sources within the Sagavanirktok Formation. The methane-carbon isotopic analyses of the Prudhoe Bay gas cap yield an average value of approximately -39 per mil. The methane component from microbial sources likely had an original methane-carbon isotopic composition ranging from -60 to -70 per mil. Because the mixing of two gases results in a linear and proportional change in isotopic composition, about 50 to 70% of the methane within the gas hydrates of the Sagavanirktok Formation is estimated to be from a thermogenic source and to have migrated from the Prudhoe Bay gas cap.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994