EVIDENCE FOR OVERPRESSURED TIGHT GAS IN BROOKIAN AND BEAUFORTIAN SEDIMENTS, BROOKS RANGE FOOTHILLS OF ARCTIC ALASKA

NELSON, Philip H., Geology, U.S. Geol Survey, P.O. Box 25046, MS 939, Denver Federal Center, Denver, CO 80225, [email protected], BIRD, Kenneth J., U.S. Geological Survey, 345 Middlefield Road, MS 969, Menlo Park, CA 94025, and HOUSEKNECHT, David W., U.S. Geol Survey, 12201 Sunrise Valley Dr, Reston, VA 20192

Beneath the Brooks Range foothills, siliciclastic rocks of the Beaufortian megasequence range up to ~4,000 ft of stratigraphic thickness and rocks of the overlying Brookian megasequence range up to ~20,000 ft of thickness. Evidence for overpressured gas consists of gas shows over extensive depth intervals, anomalous resistivity- and acoustic-log signatures, and drillstem-test pressures that exceed hydrostatic pressure. Porosity and permeability data indicate that the gas is in "tight" reservoirs with some drill tests indicating high-pressure conditions but limited volumes of gas. Sparsely distributed wells across the foothills belt have encountered a limited number of sandstone intervals within these mudstone-dominated sequences.

Conditions for gas generation in Brookian rocks are substantiated by intermittent gas shows in some wells and nearly continuous shows in other wells. A vitrinite reflectance trend in one key well exceeds 1% within the Brookian sequence and exceeds 2% at the top of the Beaufortian sequence. Total organic carbon values of 1.5 weight percent were observed throughout a 2,000-ft interval within the lowermost part of the Brookian. Underlying Beaufortian rocks, penetrated by even fewer wells, are expected to have somewhat higher average total organic carbon values.

Ambiguities exist in the data set. In some cases, mud weights are increased to control sloughing shales rather than to counter pore pressure. Resistivity and sonic logs record the competing effects of disequilibrium compaction, uplift, paleopressure, and present-day pressure, complicating their interpretation. Present-day temperature gradients are low instead of high as in many overpressured regimes; the low gradients are attributed to groundwater flow. Despite these complications, we are able to outline a broad area in which there is potential for overpressured gas.