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New Oil in an Old Place – Geologic Framework of a Giant Oil Discovery in Arctic Alaska


The 2015 announcement of an oil discovery on the Colville River delta suggests the presence of a giant accumulation, with announced contingent C1, C2, and C3 reserves of 497, 1,438, and 3,758 million barrels of oil, respectively (Armstrong Oil & Gas, Inc. press release). If this estimated range of reserves is confirmed by development, the unitized Pikka field will rank as the largest onshore conventional oil discovery in North America in more than two decades. Pikka is located on the Colville high, one of the earliest and most extensively explored areas of Arctic Alaska. The Colville high is a large closure on the Barrow Arch, a regional structure that focused oil migration from the Colville Basin to the south and the Canada Basin rifted margin to the north. Fetch areas include high quality, oil-prone source rocks in Triassic, Lower Jurassic, and Lower Cretaceous strata. Oil generation in the fetch areas occurred between the middle Cretaceous and Paleogene. Source rocks on the Colville high range from thermally mature to immature. The main reservoir at Pikka is the middle Cretaceous Nanushuk Formation, the topset facies of a huge foreland-basin clinothem, which ranges in age from Aptian in the western Colville Basin to Cenomanian at the ultimate shelf margin just east of Pikka. Shelf margins in the clinothem generally trend N-S in the Colville Basin and WNW-ESE along the Barrow Arch. Nanushuk shelf-margin trajectories are highly progradational in the west and change to more aggradational about 125 km west of Pikka. This change reflects a reduction in sediment supply and associated increase of marine influence (wave winnowing). Thus, Nanushuk sandstones tend to be cleaner towards the distal parts of the clinothem. Basinwide, diagenesis of Nanushuk sandstones mainly involved compaction of ductile components, and shallow maximum burial along the Barrow Arch favors preservation of better reservoir quality than elsewhere. Trapping likely involves shelf-margin stratigraphic geometry, although small scale normal faults also may be involved. Until now, the Nanushuk has been a minor reservoir in the basin. Thus, the Pikka discovery demonstrates the potential for a significant and virtually unexplored play fairway covering at least 9,000 to 15,000 km2 that includes both onshore and shallow offshore areas. The Torok Formation, a known oil reservoir comprising marine slope and basin-floor facies coeval to the Nanushuk, also is prospective in this fairway.