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Abstract


Pub. Id: A031 (1985)

First Page: 509

Last Page: 556

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

Article/Chapter: NPRA Alaska-North Slope Oil-Source Rock Correlation Study: SOURCE ROCK EVALUATION AND BIOMARKERS

Subject Group: Geochemistry, Generation, Migration

Spec. Pub. Type: Studies in Geology

Pub. Year: 1985

Author(s): P. B. Hall, L. Schou, M. Bjoroy (*)

Abstract:

Nine crude oil/condensate samples and 16 rock samples have been analyzed from a number of formations from the North Slope of Alaska. Medium pressure liquid chromatography (MPLC) was applied to achieve separation of the oils and extracts into saturated and aromatic hydrocarbons and NSO compounds. The aromatic fractions were then separated further into four subfractions by high pressure liquid chromatography (HPLC).

Capillary gas chromatography was performed on the alkane and aromatic fractions to obtain general fingerprints and values such as CPI, pristane/phytane and n-C17pristane.

Major tri- and tetra-aromatic components, including compounds containing organic sulfur, were identified from retention data and by using dual detector gas chromatograph with flame ionization detector (FID) and sulfur sensitive detector (FPD) after gas chromatographic separation of one of the subfractions on a 100-m capillary column.

Capillary gas chromatography-mass spectrometry (GC-MS) was performed on the alkane and aromatic fractions. Triterpane (m/z 191, 205) and regular and rearranged steranes (m/z 217, 218, 259) were monitored in the alkane fractions. Stereochemical ratios such as %20S- and %14ß(H), 17ß(H)-steranes and %22S-hopanes and Tm/Ts (17^agr(H)-trisnorhopane/18^agr(H))-trisnorneohopane) together with the more source specific C29/C30 17^agr(H)-hopane ratio, were calculated from integration of the mass chromatograms. The aromatic fractions were monitored for the mono- and triarom tic steranes (m/z 239, 253, 231, respectively) and the methylated phenanthrenes (m/z 178 n14) and dibenzothiophenes (m/z 184 n14). The whole oils have also, prior to any separation or dilution, been analyzed by capillary gas chromatography, with temperature programming from -50°C to 270°C. In addition, aromatic hydrocarbon fingerprints were also employed as an aid in correlation of oils and source rocks.

All except one of the sediments seem to be quite mature with a vitrinite reflectance within or beyond the oil producing zone (R0 > 0.5) and a CPI for most of the samples from 0.9 to 1.1. The two shallowest samples appeared to be the least mature, both from vitrinite reflectance, alkane/isoprenoid, sterane, and hopane biological marker ratios.

The crude oils appear, from the gas chromatograms of the alkane fraction, to be quite dissimilar. Three of the oils contained hardly any n-alkanes, probably owing to biodegradation, and could not be correlated to the other oils or to the possible source rocks on the basis of alkane/isoprenoid ratios. Of the "non-biodegraded" crude oils, one was a condensate with only a trace of n-alkanes above C17-C18. The sterane/triterpane maturation parameters showed only minor differences between the oil samples. The source-specific parameters could, however, be applied to subdivide the oil samples into two or three different groups. Aromatic hydrocarbon distributions also allowed a subdivision into two or three different groups.

Further correlation between the crude oils and the source rocks was based on the combined use of molecular and bulk parameters together with the geological information and suggests possible mixed source of certain oils. No positive correlation on several different parameters could be made between the oils and any of the source rocks.

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