Potential Tight Gas Resources in a Frontier Province -- Jurassic through Tertiary Strata beneath the Brooks Range Foothills, Arctic Alaska

Philip H. Nelson, Kenneth J. Bird, and David W. Houseknecht
U.S. Geological Survey

Jurassic through Tertiary strata beneath the Brooks Range foothills (Fig. 1) comprise two major overlapping tectonostratigraphic megasequences. The Jurassic - Lower Cretaceous Beaufortian megasequence (Kingak Shale) is an offlapping depositional succession that prograded southward (present coordinates) during rifting and opening of the Arctic Ocean basin to the north. The Lower Cretaceous – Tertiary Brookian megasequence consists of a series of depositional sequences derived from the Brooks Range and deposited to the north in the Colville foreland basin, which was progressively filled as clastic systems prograded eastward and northward. Both the Beaufortian and Brookian megasequences generally display similar vertical successions of facies, including in ascending order (1) a basal condensed section that includes low velocity shales with good to excellent source-rock character, (2) a base-of-slope turbidite system containing varying proportions of sandstone and mudstone, (3) a relatively thick marine slope section consisting of mostly mudstone, and (4) a marine shelf to shore-zone system that includes sandstone and mudstone. In the Brooks Range foothills, these generally coarsening-upward sequences are stacked vertically, with the Beaufortian megasequence as thick as ~4,000 ft and the Brookian megasequence as thick as ~20,000 ft of stratigraphic thickness. Even greater thicknesses will exist southward towards the Brooks Range where structural deformation has produced stacked sections.

Although relatively few wells have penetrated the Beaufortian and Brookian megasequences in the deeper part of the Colville basin, inspection of logs and other well data indicates the widespread presence of undercompacted and/or overpressured intervals in the lower parts of both megasequences. In the Tulugak well (Fig. 2) which penetrates the Brookian section, a transition from high to low sonic transit time and an increase from low to high resistivity with decreasing depth suggest that the top of an overpressured interval lies at a depth of about 10,500 ft. However, higher than normal pressure was also encountered in the sand-shale sequence above this depth, as evidenced by the drill-stem test and increase in mud weight to more than 12 pounds per gallon at 9,000 ft. In this particular well, the three tested intervals at 16,000, 9,000, and 8,000 ft all produced gas to surface on the first test, but subsequent tests at the same depth showed declines in gas rates and pressure, indicating either poor reservoir quality or limited reservoir volumes. Conditions for gas generation in the lower part of the Torok Formation are substantiated by (1) intermittent gas shows, (2) a vitrinite reflectance trend that exceeds 1.0% at 8,000 ft and 2.0% at 14,000 ft, and (3) total organic carbon values of 1.5 weight percent from 11,000 to 14,000 ft depth (Fig. 2). Similar conditions for gas generation probably exist in the Hue Shale, a condensed section and oil-prone source rock interval at the base of the Brookian megasequence.

Potential tight gas reservoirs consist mostly of amalgamated sandstone facies deposited in turbidite fan and channel systems in a base-of-slope setting. Sandstone thicknesses and lateral extent of these facies are variable, although Beaufortian sandstones are poorly known because of limited well penetrations. In contrast, Brookian sandstones are known from both outcrop exposures and well penetrations, and gross thicknesses of sandstone-prone successions typically range from 2,000 to 6,000 ft in the deep basin. Within this gross interval, stacked sandstones comprising hundreds of feet of potential reservoir have been observed (e.g., Fig. 2). Seismic data and sparse well data suggest that coalesced fan systems have lateral dimensions of tens of miles. Data from Brookian sequences in twelve cored intervals in eight wells show that average porosity is around 10 percent, permeability is less than 1.0 millidarcy and commonly less than 0.1 millidarcy, based upon conventional (unconfined) core measurements. Descriptions of samples from these same intervals indicate that sandstones are well consolidated and very fine to fine grained. The gamma-ray log of Fig. 2 is typical of other penetrations of Brookian sandstones and siltstones where gamma-ray levels range from 40 to 80 API units. Reservoirs are likely to consist of low-permeability sandstones because clean, porous sandstones have not been observed in logs or core.

Evidence of undercompaction, gas charge, and overpressure (Fig. 1) in Brookian and Beaufortian rocks extends nearly the entire length and breadth of the Brooks Range foothills region, an area of approximately 40,000 square miles. Although the volumes and locations of tight gas occurences are unknown, some of these undiscovered gas resources may lie in close proximity to a proposed natural gas pipeline, the route of which will likely parallel the Trans-Alaska Pipeline System (Fig.1).

Figure 1. Map of northern Alaska, showing areas in which well data suggest the existence of moderate so strong overpressure in Brookian megasequence rocks.