Petroleum Geochemistry of North Slope Alaska: Source Rocks, Crude Oil Properties, and Migration of Hydrocarbons
Published:January 01, 1985
J. Connan, D. A. DesAutels, A. K. Aldridge, 1985. "Petroleum Geochemistry of North Slope Alaska: Source Rocks, Crude Oil Properties, and Migration of Hydrocarbons", Alaska North Slope Oil-Rock Correlation Study: Analysis of North Slope Crude, Leslie B. Magoon, George E. Claypool
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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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Alaska North Slope Oil-Rock Correlation Study: Analysis of North Slope Crude
The Alaska North Slope oil-rock correlation study was organized because several oil companies requested oil and rock samples for geochemical analyses that were recovered during the exploration drilling in the National Petroleum Reserve in Alaska (NPRA). Samples acquired with public funds could not be given to private organizations unless some guarantees could be provided that the information acquired from these samples could be made available to the public. For this reason, in August 1981, we sent out over 40 invitations to research laboratories in industry, government, and academia.
Requirements to participate in this study included: (1) participation in an AAPG-sponsored research conference, (2) presentation of the data interpretations at the 1983 Annual AAPG Meeting in Dallas Texas, and (3) contribution of a manuscript, to include all acquired data and interpretations, that would be included in a symposium volume. If a research group wished to participate, they were to write a letter of intent that included their proposed analytical program and a statement indicating that the requirements would be adhered to by their group. Even with these stringent requirements, 30 research groups wished to participate. A balanced cross section of research groups are participating and are as follows: 15 from oil companies, 7 from commercial laboratories, 7 from government laboratories, and 1 university laboratory. These groups are listed in Table 1.
In January 1982, each research group was sent 8 oils and 15 rocks recovered from NPRA drilling and 1 oil from the Prudhoe Bay field. Each group then proceeded to analyze these samples as they indicated in their letter of intent.