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Four key marine petroleum source rock units were identified, characterized, and mapped in the subsurface to better understand the origin and distribution of petroleum on the North Slope of Alaska. These marine source rocks, from oldest to youngest, include four intervals: (1) Middle–Upper Triassic Shublik Formation, (2) basal condensed section in the Jurassic–Lower Cretaceous Kingak Shale, (3) Cretaceous pebble shale unit, and (4) Cretaceous Hue Shale. Well logs for more than 60 wells and total organic carbon (TOC) and Rock-Eval pyrolysis analyses for 1183 samples in 125 well penetrations of the source rocks were used to map the present-day thickness of each source rock and the quantity (TOC), quality (hydrogen index), and thermal maturity ( T max) of the organic matter. Based on assumptions related to carbon mass balance and regional distributions of TOC, the present-day source rock quantity and quality maps were used to determine the extent of fractional conversion of the kerogen to petroleum and to map the original TOC (TOC o) and the original hydrogen index (HI o) prior to thermal maturation. The quantity and quality of oil-prone organic matter in Shublik Formation source rock generally exceeded that of the other units prior to thermal maturation (commonly TOC o > 4 wt.% and HI o > 600 mg hydrocarbon/g TOC), although all are likely sources for at least some petroleum on the North Slope. We used Rock-Eval and hydrous pyrolysis methods to calculate expulsion factors and petroleum charge for each of the four source rocks in the study area. Without attempting to identify the correct methods, we conclude that calculations based on Rock-Eval pyrolysis overestimate expulsion factors and petroleum charge because low pressure and rapid removal of thermally cracked products by the carrier gas retards cross-linking and pyrobitumen formation that is otherwise favored by natural burial maturation. Expulsion factors and petroleum charge based on hydrous pyrolysis may also be high compared to nature for a similar reason.
Ken Peters researches four-dimensional petroleum system models at the U.S. Geological Survey. He spent 15 years with Chevron and 9 years with Mobil and ExxonMobil and taught courses in petroleum geochemistry and thermal modeling at Chevron, Mobil, ExxonMobil, Oil and Gas Consultants International, the University of California at Berkeley, and Stanford University. Ken is the principal author of The Biomarker Guide (2005, Cambridge University Press).Les Magoon is currently emeritus research geologist on the petroleum system. He worked 8 years for Shell Oil Company in exploration and 32 years for the U.S. Geological Survey. Since 1981, he has investigated and popularized the petroleum system through talks, courses, and AAPG Memoir 60, The Petroleum System—From Source to Trap, for which he and his co-editor received the R. H. Dott, Sr. Award in 1996.
Kenneth Bird specializes in the petroleum geology of northern Alaska, where his experience spans more than 40 years. Currently, he is the coleader of the U.S. Geological Survey Alaska Petroleum Studies Project. With interests primarily in stratigraphy and sedimentology, he has been extensively involved in petroleum resource assessments. He holds geology degrees from Oregon State University (B.S. degree) and the University of Wisconsin (M.S. degree and Ph.D.).
Zenon Valin is a geologist with the Western Earth Surface Processes Team of the U.S. Geological Survey. He specializes in geographic information systems and scientific illustrations.
As a research geologist with the U.S. Geological Survey for 26 years, Margaret Keller has worked on silica diagenesis and stratigraphy of the Monterey Formation and sedimentary basin analysis and source rock studies in California, Alaska, and the Gulf Coast. She is particularly interested in millimeter- to centimeter-scale facies variations in mudstones and petroleum source rock potential determined from geophysical logs. She holds a B.A. degree from Occidental College and an M.S. degree from California State University at Los Angeles in geology.