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The AAPG/Datapages Combined Publications Database

AAPG Special Volumes


Pub. Id: A031 (1985)

First Page: 3

Last Page: 29

Book Title: SG 20: Alaska North Slope Oil-Rock Correlation Study: Analysis of North Slope Crude

Article/Chapter: The Framework Geology of the North Slope of Alaska as Related to Oil-Source Rock Correlations: INTRODUCTORY PAPERS

Subject Group: Geochemistry, Generation, Migration

Spec. Pub. Type: Studies in Geology

Pub. Year: 1985

Author(s): K. J. Bird


The North Slope petroleum province consists of a Mississippian to earliest Cretaceous continental platform sequence and an overlying Cretaceous to Quaternary successor basin sequence. Both sequences are highly deformed along a foreland fold and thrust belt to the south but are relatively undeformed along a passive margin to the north. Most oil and gas accumulations in this province occur along the east-plunging Barrow arch--a structural axis separating the foreland basin from the passive margin.

Preliminary evaluation of geologic and geochemical information indicated that four rock units, the Shublik Formation, Kingak Shale, pebble shale unit, and Torok Formation, are likely source rocks for the oils considered in this study. Together, these four rock units represent 140 Ma (Middle Triassic to late Early Cretaceous) of continuous marine deposition totaling more than 20,000 ft of rock in the southern part of the province while they are interrupted by one or more unconformities and range from several hundred to several thousand feet in thickness to the north. These rocks consist of an estimated 90% shale and siltstone and 10% sandstone deposited in a variety of sedimentary environments and at various rates of sedimentation. The Shublik Formation and pebble shale unit represent elatively slow rates of sedimentation in a shelf environment that was periodically anoxic. The Torok Formation and Kingak Shale represent relatively high rates of deposition along a shelf-slope-basin depositional profile in water depths calculated to be 1,000 to 3,000 ft or greater. Source rock characteristics may vary along the profile of deposition.

Analysis of basin development suggests that North Slope oil and gas formed between the time of sufficient source rock burial (~100 Ma) and the time of tilting of oil-water contacts in oil fields in the Prudhoe Bay region (~40 Ma). Sediment loading and subsidence associated with northeastward basin fill made the Prudhoe Bay region the focus of migrating hydrocarbons. Later, during Tertiary time, regional eastward tilting changed migration directions from the Prudhoe area to the Barrow area.

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