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The AAPG/Datapages Combined Publications Database

AAPG Special Volumes


Pub. Id: A031 (1985)

First Page: 459

Last Page: 478

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

Article/Chapter: Evaluation of Selected Sediments and Petroleums, National Petroleum Reserve, Alaska: SOURCE ROCK EVALUATION INCLUDING ISOTOPES AND BIOMARKERS

Subject Group: Geochemistry, Generation, Migration

Spec. Pub. Type: Studies in Geology

Pub. Year: 1985

Author(s): K. F. M. Thompson, L. L. Lundell


Samples of 6 oils, a condensate, 15 sediments from NPRA, and one Prudhoe Bay oil (Put River D-3) were evaluated to distinguish oil types, determine sources of the oils, and characterize the samples geochemically. Carbon isotope ratios were determined for all samples and fractions. Two distinctive groups of oils were recognized, with evident mixing between them. The Prudhoe Bay oil was distinguishable from these and was assigned to a third group. The D-3 oil showed the same unique features as other Prudhoe oils. The Dalton, Fish Creek, and South Barrow No. 19 oils comprise the most homogeneous group among the NPRA oils. This group is characterized by clustering of all carbon isotopic values (range 0.4 per mil, PDB), an average of -29.7 for aromatics, and of -29.5 for satur tes. Aromatics that are isotopically lighter than saturates represent a rare occurrence in petroleums. The second group is represented by two oils from the Cape Simpson area, with average delta values of -28.1 for aromatics and -29.0 for saturates. The overall range of values increases to 1.2 per mil, with aromatics isotopically heavier than saturates. Published data suggest that the Cape Simpson oils are not an end-member group but are a mixture of the Fish Creek and an "Umiat" oil, which occurs in southeastern NPRA. The South Barrow No. 20 oil is also interpreted as a mixture of the two types.

Source rock evaluation consisted of rock-Eval, vitrinite reflectance, pyrolysis/light hydrocarbon gas chromatography (PGC), Soxhlet extraction, liquid chromatography, and carbon isotope ratio determinations on kerogen and extract fractions. The pebble shale had the best hydrocarbon generation potential based on Rock-Eval, but the hydrogen index values were low (less than 200), considering maturation level. This suggests that the pebble shale is not a prolific oil source in the area sampled. However, PGC data indicate that the Kingak i a major source rock.

The high maturity of the Shubik, Echooka, and two of three Kingak samples, shown by vitrinite reflectance (R0) values greater than 1.44%, prevents adequate assessment by Rock-Eval, Pyrolysis-GC indicated that the Shublik and Kingak originally contained oil-prone kerogen, whereas the Torok and Echooka did not. The pebble shale probably contains a mixture of kerogen types. Vitrinite reflectance indicates that the Torok Formation and pebble shale are approaching optimum maturity for oil generation in most locations, as iss Kingak in one well.

Carbon isotope ratios of kerogens and rock extracts suggest a Shublik source for Fish Creek group oils. Kerogen from two Shublik samples has a mean delta value of -29.1, similar to the oil. Kerogens from all other formations range -23.8 to -25.5, insufficiently similar to any of the oils to represent feasible sources. Thus, the actual source of the Cape Simpson oils is not represented among the sediments samples.

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