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The AAPG/Datapages Combined Publications Database

AAPG Special Volumes


Pub. Id: A031 (1985)

First Page: 185

Last Page: 201

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

Article/Chapter: Geochemical Correlation of Oils and Source Rocks, North Slope, Alaska: USGS Interlaboratory Comparison Study: SOURCE ROCK EVALUATION INCLUDING ISOTOPES AND BIOMARKERS

Subject Group: Geochemistry, Generation, Migration

Spec. Pub. Type: Studies in Geology

Pub. Year: 1985

Author(s): Z. Sofer, M. J. Leenheer, S. E. Palmer, J. E. Zumberge


Organic geochemical analyses of 9 crude oils and 15 core samples from the North Slope of Alaska were performed in conjunction with an interlaboratory oil-source rock correlation study initiated by the U.S. Geological Survey (USGS). Correlation parameters employed included data derived from capillary gas chromatography of C15+ aliphatic hydrocarbons; GC-MS-DS analysis for sterane, diterpane, and triterpane biomarkers; and stable carbon isotopic analysis of aliphatic, aromatic, NSO, asphaltene, and kerogen fractions. Of the 9 oils analyzed, the 5 oils from Point Barrow, Dalton, Fish Creek, and Prudhoe Bay areas (the Prudhoe group of oils) represent one distinct family. Two of these oils are biodegraded. The remaining 4 oils from Umiat and Cape Simpson areas form less well-defined family. This latter group of oils has a range in maturity (e.g., biomarker parameters show Cape Simpson oils to be less mature than Umiat) and includes two biodegraded oils and one condensate.

Six of the 15 possible source rocks are thermally overmature with respect to oil generation (R0 > 1.6%) and have low quantities of extractable C15+ hydrocarbons. Thus, these samples did not contain sufficient geochemical information for a reliable source rock-oil correlation study. Samples that can be used for correlation purposes represent the following formations: pebble shale unit (4 samples), Torok (3 samples), Kingak and Shublik (1 sample each). Stable carbon isotope compositions, isoprenoid ratios, and terpane distributions suggest that the extracts from the pebble shale show the highest degree of correlation with the Umiat-Cape Simpson oils. Comparison of the Umiat-Cape Simpson oils with the Torok and Kingak samples shows good correlation when some geoc emical parameters are considered and poor correlation when others are considered. None of the above rock samples correlate well with the Prudhoe group of oils. The Shublik Formation sample is only marginally reliable for correlation because of its high thermal maturity: R0 = 1.6%. However, comparison of several geochemical parameters (in particular carbon isotope ratios, isoprenoid ratios, and diterpane distributions) suggest that this formation may have sourced the Prudhoe group of oils. A better correlation study would be possible if less mature Shublik samples were available because sterane and triterpane biomarkers would have been better preserved. Because of the small number of samples analyzed, positive or negative correlations must be tentatively regarded since organic nhomogeneities within stratigraphic units can result in variations in geochemical parameters. This is especially evident in the isotopic and biomarker variations observed in the pebble shale samples.

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