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Abstract

Fifteen rock samples and 9 crude oils from 16 wells of the North Slope of Alaska have been investigated by organic geochemical techniques in order to assess the source rock potential for oil in Cretaceous formations and to find the tentative origin of oils accumulated in reservoirs ranging from Mississippian-Pennsylvanian to Cenomanian-Albian.

Vitrinite reflectance as well as organic geochemical data (chloroform extract, gas analyses, Tmax) show that the older formations analyzed (Kingak, Shublik, and Sadlerochit) are generally too mature to have generated the oils. Among the rock samples examined, only sediments from the pebble shale unit, the Torok, and the Kingak formations have maturities in agreement with oil genesis. In the two main formations, namely pebble shale and Torok, a maturity suite ranging from immature to mature sediments has been characterized.

Quality of the kerogen, assessed by Rock-Eval pyrolysis, has been found to be at least fair in the Torok Formation and pebble shale unit. Surprisingly, hydrogen indexes (HI) often do not exceed 80. The genetic potential, measured by Sx + S2 from Rock-Eval pyrolysis data, peaks at 4 mg/g of rock but is generally within the 0.5 to 2.5 range. Cretaceous formations of North Slope Alaska are at least moderate source rocks (Sx + S2 = 2-6 mg/g rock). Basic organic geochemistry data (Rock-Eval, chloroform extract, and gas yield) demonstrated that the pebble shale unit is a better (fair to good) source rock than the Torok Formation (poor source rock).

Detailed organic geochemistry by gas chromatography and computerized gas chromatography-mass spectrometer have allowed a maturation assessment of kerogen by molecular measurements on steranes and terpanes. Maturities of dispersed organic matter, assessed by molecular parameters, have been compared to maturities based on vitrinite reflectance measurement data. The vitrinite reflectance scale generally matched the organic geochemical classification; however, some discrepancies have been observed in Torok samples within the 0.5 to 0.6% R0 range.

Maturity of oils, assessed by molecular measurements on steranes and terpanes, is variable. Comparison of maturities of oils to maturities of indigenous chloroform extracts provides a tool to approach migration of hydrocarbons. The moderate maturity of Umiat oil and Seabee condensate suggests a migration of limited extent which is, in addition, in good agreement with a Torok origin. The striking discrepancy between the maturity of pebble shale sediments in the Walakpa No. 1 well and the maturity of South Barrow oils indicates that the oils accumulated in pebble shale and Sag River sandstones originate from much deeper source rocks.

Geochemical characteristics of alkanes and aromatics from the Torok and the pebble shale source rocks are closely related. Oil-to-source rock correlations are, in some cases, difficult to establish because crude oil properties obviously reflect molecular changes through migration (amount of alkanes, of tricyclic and tetracyclic terpanes, of ββ steranes, etc.). A combined review of isotopic, molecular, and geological data has, however, allowed the finalizing of a diagnosis regarding the origin of each crude oil including those that have been recognized as biodegraded oils.

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