Wyoming Geological Association: 1982 Luncheon Meetings Casper, Wyoming: Abstracts of Papers

The addition of much seismic and well data since 1974 in the National Petroleum Reserve in Alaska has aided considerably in understanding the depositional history of Lower Cretaceous rocks in that part of the Colville basin. These rocks range in thickness from more than 7,000 m along the basin axis on the south to about 1,200 m on the Barrow arch along the north coast. Seismic and well data indicate that early Neocomian strata on the north flank of the basin consist of southward prograding marine shelf and slope deposits of shale and minor sandstone units. Uplift, erosion and subsequent transgression on the northernmost flank of the basin resulted in deposition of the pebble shale unit in late Neocomian time and termination of the northern provenance. Following this, the basin was downwarped and little deposition occurred on the north flank until distal deep-water deposits of the Torok Formation onlapped and down-lapped the south-dipping flank in mid or late Albian time.

The depositional history of the south flank of the Colville basin is inferred from outcrop data. Turbidites of the early Neocomian Okpikruak Formation were derived from the ancestral Brooks Range on the south, and subsequently were thrust northward in late Neocomian or Aptian time. The early Albian Fortress Mountain Formation, which consists of as much as 3,000 m of mainly deep-water deposits, unconformably overlies the Okpikruak and older rocks on the southernmost flank of the basin. Filling of the Colville basin occurred in mid to late Albian time as thick prodeltaic and deltaic deposits of the Torok Formation and Nanushuk Group, respectively, prograded across the basin from the south on the south side of the basin, but principally from the west-southwest over most of the basin.

Regional subsidence (or sea-level rise) and subsequent transgression by the Upper Cretaceous Colville Group approximately coincided with the termination of Early Cretaceous deposition.

Reservoir sand development is largely dependent on depositional facies, provenance and mineral diagenesis. Early Neocomian sandstones in the northern part of the area are mineralogically mature, quartzose shelf sandstones derived from a northern (present day) provenance. These sandstones are generally of limited thickness and fine or very fine grained. All other sands are derived from an orogenic source terrane on the south and southwest, and are lithic rich. Deltaic or marine-shelf processes are important in winnowing out unstable labile constituents of these sands, and hence improving their reservoir potential.