All Convention Luncheon

Cretaceous Revenant in Arctic Alaska – Giant Oil Accumulations Hidden in Plain Sight

David W. Houseknecht
U.S. Geological Survey, Reston, Virginia, dhouse@usgs.gov

HouseknechtBetween 1944 and 2013, more than 150 exploration wells penetrated the shallow Aptian to Cenomanian Nanushuk Formation, mostly on the way to deeper objectives in some of the most prospective areas of the Alaska North Slope. Although oil was discovered in a few Nanushuk stratigraphic traps and one structural trap, recoverable volumes were insufficient to elevate the formation to “main objective” status. That perspective changed abruptly when an apparent 2013 discovery defined by a 3-D seismic amplitude anomaly on the Colville River delta was followed by two significant tests of the reservoir in 2015 – a vertical well flowed 2,160 barrels of oil per day (BOPD) and a nearby 2,000-ft lateral flowed 4,600 BOPD, both from a Nanushuk stratigraphic trap at approximately 4,100 ft depth. 3-D seismic mapping and subsequent delineation drilling confirm the presence of an oil pool more than 40 miles long and generally less than 3 miles wide containing a 650-ft oil column; recoverable oil estimates for this Pikka-Horseshoe accumulation range up to more than one billion barrels. Production may begin as early as 2022.

Prompted by this success, a 2002 “show well” 30 miles west of Pikka-Horseshoe was followed by discovery and delineation wells in 2016–2019 in the National Petroleum Reserve in Alaska (NPRA). Recoverable oil from the Nanushuk Formation is estimated in the range of 400 to 750 million barrels of oil from these Willow and West Willow accumulations.

The regional setting of these discoveries is well known. A giant Aptian to Cenomanian clinothem, comprising topset facies in the Nanushuk Formation and foreset-bottomset facies in the Torok Formation, covers about 150,000 mi2 of the western Alaska North Slope and adjacent Beaufort and Chukchi shelves. Oil-prone source rocks and the clinothem are draped across the Barrow arch, a structural hinge between the Colville foreland basin and Beaufort Sea rifted margin. Stratigraphic traps lie in a favorable thermal maturity domain along multiple migration pathways across more than 10,000 mi2. Sediment from the Chukotkan orogen (Russia) filled the western Colville basin and spilled over the Beaufort rift shoulder, forming east- and north-facing shelf margins. Progradational shelf margin trajectories in the west change abruptly to “sawtooth” trajectories at midclinothem, the result of reduced sediment influx and greater marine influence. Two end-member stratigraphic trap types are interpreted in Nanushuk basal topsets in the eastern part of the clinothem: (1) lowstand systems tracts, inferred to reflect forced regression, include a narrow, thick progradational stacking pattern perched on a sequence boundary on the upper slope; and (2) highstand progradational systems tracts include a broad, thin wedge of shingled parasequences above a toplap surface. Both include stratigraphically isolated sandstone units sealed by mudstone. Trap geometries in Torok foreset and bottomset facies in the same area include basin-floor fan, slope-apron, and slope-channel deposits that pinch out upslope and are sealed by mudstone. Although Torok oil-saturated sandstone has been penetrated by numerous wells in the region, low permeability and reservoir compartmentalization have impeded successful production thus far.

Amplitude anomalies in both 2-D and 3-D seismic data suggest that dozens of exploration targets remain untested in northeastern NPRA, in both the Nanushuk and Torok Formations. Similar potential likely exists in western NPRA and adjacent offshore areas, although seismic and well control are dated in those areas so greater risk is assumed. Considering that economic viability in Arctic Alaska is directly related to distance from infrastructure and depth, it is likely that Nanushuk targets will drive exploration in a stepwise fashion westward in the foreseeable future.