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AAPG Bulletin, V.
Water washing of Proterozoic oil in the Midcontinent rift system
1Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, 48109; current address: Geology Department, University of Auckland, Private Bag 92019, Auckland, New Zealand; email: [email protected]
2U.S. Geological Survey, MS 954, Reston, Virginia, 22092; email: [email protected]
Jeffrey L. Mauk holds a B.S. degree from the University of North Carolina, an M.S. degree from the University of Montana, and a Ph.D. from the University of Michigan. He worked for seven years as an exploration, mine, and research geologist. In 1993, he joined the University of Auckland, where he is a senior lecturer; his research interests include relationships between organic matter and ore deposits.
Robert C. Burruss received a B.S. degree in chemistry from Juniata College, Huntingdon, Pennsylvania, in 1971 and a Ph.D. in geology from Princeton University in 1977. From 1977 to 1984 he worked for Gulf Oil and then moved to the U.S. Geological Survey, where he currently works on petroleum migration in the North Slope of Alaska and in the Appalachian basin and on carbon sequestration.
We thank the Copper Range Company for allowing access to the White Pine mine. Glen Hieshima, Eileen Ho, Scott Imbus, Philip Meyers, and Lisa Pratt have provided helpful discussions about the organic matter in the Nonesuch Formation. We thank William Orem, Dan Hayba, A.-Y. Huc, and Robert Goldstein for helpful reviews. Acknowledgement is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research.
Whole oil gas chromatography shows that petroleum inclusions trapped in 1.05 Ga veins in the Midcontinent rift experienced variable water washing but no significant biodegradation or phase separation. By comparing our results with published experimental data, we estimate that water:petroleum ratios varied from 200:1 to much greater than 9000:1. Extreme water washing with water:petroleum ratios very much greater than 9000:1 likely produced pyrobitumen (solid hydrocarbon) that was locally trapped as primary inclusions in vein calcite. Hydrothermal ore deposits, with their extremely high water:rock ratios, are one of the best places to search for end-member water-washed petroleum. Sedimentary basins, however, with their variable water:rock ratios, also have a wide range of water:petroleum ratios, which significantly affect the composition of crude oils along the flow path between source rock and trap. Water:petroleum ratios may be a useful concept to help evaluate crude oil compositions in a variety of settings.
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