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1 U.S. Geological Survey, 345 Middlefield Road, MS 969, Menlo Park, California 94025-3561; kpeters{at}usgs.gov
2 ExxonMobil Research and Engineering Co., 1545 Route 22 East, Annadale, New Jersey 08801-0998; clifford.c.walters{at}exxonmobil.com
3 ExxonMobil Exploration Co., 233 Benmar, Houston, Texas 77060; paul.j.mankiewicz{at}exxonmobil.com
Ken Peters researches four-dimensional petroleum system models at the U.S. Geological Survey. He spent 15 years with Chevron and 8 years with Mobil and ExxonMobil and taught courses in petroleum geochemistry and thermal modeling at Chevron, Mobil, ExxonMobil, Oil and Gas Consultants International, University of California at Berkeley, and Stanford University. Ken is the principal author of The Biomarker Guide (2005, Cambridge University Press).Clifford Walters conducts research at ExxonMobil's Corporate Strategic Research in modeling petroleum generation and reservoir processes and in advanced molecular characterization using chromatographic, mass-spectrometric, and solid-state techniques. He has more than 23 years of experience in applying petroleum geochemistry to exploration and production and is a coauthor of the second edition of The Biomarker Guide (2005, Cambridge University Press).
Paul Mankiewicz has more than 25 years of experience in environmental and petroleum geochemistry. After working for Global Geochemistry Inc., Paul joined ExxonMobil, where his research led to numerous successful endeavors, including an improved understanding of oil-spill biodegradation and remediation. He is currently assigned to ExxonMobil Exploration Company, where he is applying organic geochemistry and basin modeling to new plays and prospects.
Oil-prone marine petroleum source rocks contain type I or type II kerogen having Rock-Eval pyrolysis hydrogen indices greater than 600 or 300–600 mg hydrocarbon/g total organic carbon (HI, mg HC/g TOC), respectively. Samples from 29 marine source rocks worldwide that contain mainly type II kerogen (HI = 230–786 mg HC/g TOC) were subjected to open-system programmed pyrolysis to determine the activation energy distributions for petroleum generation. Assuming a burial heating rate of 1°C/m.y. for each measured activation energy distribution, the calculated average temperature for 50% fractional conversion of the kerogen in the samples to petroleum is approximately 136 ± 7°C, but the range spans about 30°C (
121–151°C).
Fifty-two outcrop samples of thermally immature Jurassic Oxford Clay Formation were collected from five locations in the United Kingdom to determine the variations of kinetic response for one source rock unit. The samples contain mainly type I or type II kerogens (HI = 230–774 mg HC/g TOC). At a heating rate of 1°C/m.y., the calculated temperatures for 50% fractional conversion of the Oxford Clay kerogens to petroleum differ by as much as 23°C (127–150°C).
The data indicate that kerogen type, as defined by hydrogen index, is not systematically linked to kinetic response, and that default kinetics for the thermal decomposition of type I or type II kerogen can introduce unacceptable errors into numerical simulations. Furthermore, custom kinetics based on one or a few samples may be inadequate to account for variations in organofacies within a source rock. We propose three methods to evaluate the uncertainty contributed by kerogen kinetics to numerical simulations: (1) use the average kinetic distribution for multiple samples of source rock and the standard deviation for each activation energy in that distribution; (2) use source rock kinetics determined at several locations to describe different parts of the study area; and (3) use a weighted-average method that combines kinetics for samples from different locations in the source rock unit by giving the activation energy distribution for each sample a weight proportional to its Rock-Eval pyrolysis S2 yield (hydrocarbons generated by pyrolytic degradation of organic matter).
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