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The AAPG/Datapages Combined Publications Database

AAPG Special Volumes


Pub. Id: A031 (1985)

First Page: 139

Last Page: 161

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

Article/Chapter: Biodegradation and Other Complications in the Organic Geochemistry of Oils and Rocks from the NPRA-USGS Interlaboratory Study: BASIC SOURCE ROCK EVALUATION AND CARBON ISOTOPE

Subject Group: Geochemistry, Generation, Migration

Spec. Pub. Type: Studies in Geology

Pub. Year: 1985

Author(s): C. C. Walters (1), R. C. Burruss (2), T. C. Ashbaugh (3)


Nine oils and 16 core samples were analyzed as part of the Alaskan North Slope Oil-Rock Correlation Study organized by the U.S. Geological Survey (USGS). Carbon isotopic measurements and gas chromatographic analyses were performed on whole oils, oil fractions, and rock extracts. Preliminary screening and more detailed analyses were made on the kerogens to determine their quality as sources of petroleum, maturity, and isotopic composition.

Oil analyses suggest that there are three different genetic oil types. The Barrow-Prudhoe oils were defined by Magoon and Claypool (1981). They are characterized by their relatively high sulfur content, low API gravity and pristane/phytane ratio, and light carbon isotopic composition. The Simpson-Umiat oil type is identified by its low sulfur content, higher API gravity and pristane/phytane ratio, heavier carbon isotopic composition, and anomalously high concentrations of cyclic and aromatic light hydrocarbons. The third type is not an oil but represents compounds that oils, primarily Simpson-Umiat type, have extracted from immature sediments during migration. This third component was called Torok-Nanushuk and is noted by an anomalous light hydrocarbon distribution and relatively heav carbon isotopic composition.

It is possible to describe each of the oil samples in terms of mixing these three oil types in varying proportions. The Put River D-3 oil is typical of the petroleum found at the Prudhoe Bay field and contains mainly Barrow-Prudhoe type mixed with a small Simpson-Umiat component. The South Barrow No. 19 is comprised of pure Barrow-Prudhoe oil. It has lost a significant proportion of its light hydrocarbons, and the influence of a Simpson-Umiat component may have obscured. The Fish Creek No. 1 oil is severely biodegraded but can be classified as Barrow-Prudhoe from its isotopic composition. Maturity parameters indicate that the Dalton No. 1 oil is an immature equivalent of the Barrow-Prudhoe oils.

The oil from the Simpson Core Test is considered to best represent the Simpson-Umiat genetic type although it too lost all light hydrocarbons. The severely degraded Cape Simpson oil is believed to be a mixture of Simpson-Umiat and Barrow-Prudhoe. Umiat No. 4 oil is representative of the Simpson-Umiat type with a large component extracted from immature, Cretaceous reservoir rocks. Various parameters suggest that the Seabee condensate is of similar maturity to the Umiat oils and probably is the result of phase separation. It too has a substantial quantity of the extracted Torok-Nanushuk component. Finally, the South Barrow No. 20 oil is a slightly biodegraded mixture of all three oil types, but it is not possible to define the proportions clearly.

From the samples used in this study, the pebble shale samples are considered to have the highest potential for oil generation because they are comprised predominantly of amorphous or herbaceous organic matter. The Torok samples contain mainly coaly fragments and are considered gas prone. One sample of Kingak contains an abundance of herbaceous organic material. Other examples of Kingak, Shublik, and Fortress Mountain units are overmature and contaminated with nonindigenous hydrocarbons.

Because most of the extracts were either contaminated or generated from immature samples, rock/oil correlations were based on the carbon isotopic composition of kerogen isolates, oils, and oil fractions. From the suite of samples used in this study, only the Shublik could serve as a source for any of the oils. Evidence suggests, but does not

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conclusively prove, that the Shublik is indeed a major source of the Barrow-Prudhoe oils. There are sufficient data to show extensive lateral and vertical variation in organic facies in the pebble shale and Kingak unit and that these could also be major source contributors to producing oil fields. The pebble shale unit is believed to be the source of the Simpson-Umiat oils.
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