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The AAPG/Datapages Combined Publications Database

AAPG Special Volumes


Pub. Id: A031 (1985)

First Page: 203

Last Page: 231

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

Article/Chapter: Oil Types and Source Rock-Oil Correlation on the North Slope, Alaska--A Cooperative USGS-Industry Study: SOURCE ROCK EVALUATION INCLUDING ISOTOPES AND BIOMARKERS

Subject Group: Geochemistry, Generation, Migration

Spec. Pub. Type: Studies in Geology

Pub. Year: 1985

Author(s): J. A. Curiale


Chemical analyses of 9 oils and organic matter in 15 rocks from the North Slope suggest that the oils comprise at least two chemically distinct types. Type A oils, including oils from Prudhoe Bay to Point Barrow, can be distinguished from Type B oils, including oils from Umiat Field to Point Barrow, by distinctive 5 ^agr, 20R sterane carbon number distributions, sulfur, vanadium, and nickel concentration, V/(V + Ni) ratios, hopane and aromatic hydrocarbon distributions, and carbon isotope ratios.

Of the 9 oils of this study, at least 4 are degraded. Of the 15 rocks provided by the U.S. Geological Survey for this study, 7 are post-mature. These sample problems limited the results of this work. Those samples having R0 values below 1.4% are all Lower Cretaceous except one, which is Jurassic. Although 12 samples have TOC values greater than 1.0%, only three samples (two pebble shale and one Kingak) contain indigenous extract yields above 500 ppm. Of these, only one pebble shale sample and the Kingak sample have over 200 ppm indigenous extractable hydrocarbons.

None of the 15 rocks examined appear to be the single source for any of the 9 oils, although several similarities between the oils and the organic matter in some of the rocks do exist, particularly for Type B oils. Analysis of the molecular composition of the oils suggests (particularly for the Type A oils) that they may have been sourced by more organic facies. If this is the case, then no single source rock may correspond chemically to the oils studied. Based upon the correlations that are possible on this limited number of samples, Lower Cretaceous (particularly pebble shale), Jurassic, and Triassic rocks are all potential sources. In addition, the probability of lateral changes in organic facies requires that other geographically distinct potential sources also be considered as po sible contributors.

In summary, the very limited number of samples available for study from within the oil window does not allow a definitive conclusion as to the source for these oils. However, it is probable that the pebble shale, Kingak, and Shublik formations have all made contributions in various geographic areas.

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