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AAPG Bulletin, V.
Dating penecontemporaneous dolomitization in carbonate reservoirs: Paleomagnetic, petrographic, and geochemical constraints
1Earth Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada; email: [email protected]
2Earth Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada; email: [email protected]
3Earth Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada; email: [email protected]
4Vox Terrae International, #40, 820-9a Street N.W., Calgary, Alberta, Canada T2N 1V1; email: [email protected]
Maria Cioppa is assistant professor of Earth Sciences at the University of Windsor (Canada), investigating the paleomagnetism and rock magnetism of hydrocarbon-associated reservoir and source rocks in the Western Canada and Williston basins. She obtained her Ph.D. (1997) in geological sciences from Lehigh University (Pennsylvania), her M.Sc. degree from the University of Victoria (Canada), and her B.Sc. degree from Carleton University (Canada).
Ihsan Al-Aasm is professor of Earth Sciences at the University of Windsor (Canada), investigating chemical, isotopic, and mineralogical aspects of diagenesis of carbonate and clastic rocks. Al-Aasm obtained his Ph.D. from the University of Ottawa and his B.Sc. and M.Sc. degrees from the University of Baghdad.
David Symons is a university professor and professor of Earth Sciences at the University of Windsor (Canada). After a year as a postdoctoral fellow at the University of Newcastle-upon-Tyne, United Kingdom, and three years as a research scientist at the Geological Survey of Canada, he joined the University of Windsor in 1970, where he teaches geophysics and structural geology. Symons holds a B.A.Sc. degree in geological engineering, a Ph.D. from the University of Toronto, and an M.A. degree from Harvard University.
Kevin Gillen is president of Vox Terrae International, a company he formed in 1994 that studies various aspects of fractured reservoirs using log and core information. After completing his M.Sc. degree at the University of Alberta in 1988, he joined Shell Canada Research. Gillen obtained his B.Sc. degree (1985) in geological engineering from the University of Windsor (Canada).
We thank Anderson Oil and Gas Ltd. and Texaco Ltd. for permitting paleomagnetic and geochemical sampling of their core. We also thank Jeff Packard for providing considerable information on the Dunvegan field. Stephanie Van Hoekelen, Julie Clarke, and Jeff Lonnee aided in petrologic and geochemical sampling and analysis. Reviews of an earlier version of this article by Bruce Fouke, David Budd, and Jon Humphrey have greatly benefited the article. Funding was provided by a Natural Sciences and Engineering Research Council grant to D. T. A. Symons and I. S. Al-Aasm.
The predominant dolomitization in the Mississippian Debolt Formation, Western Canada sedimentary basin was hypothesized to be early in three gas fields, based on stable oxygen and carbon isotopic values and on strontium isotope ratios that are similar to postulated Mississippian dolomite values and seawater ratios, respectively. As the absolute age could not be determined by this method, paleomagnetism was used to place constraints on the age of the dolomitization. The magnetic analyses on Debolt specimens from the same wells revealed three magnetization components that could be tied to geologic/diagenetic events: (1) a low-temperature (<180 degreesC), low-coercivity (<20 mT) A component found in all lithologies that is a combination of modern viscous and drilling-induced magnetizations; (2) a dominant B component removed between 180 and 350 degreesC or 20 and 80 mT in all lithologies that is a Cretaceous chemical remanent magnetization; and (3) a residual high temperature (>350 degreesC) and coercivity (>80 mT) C component of primary or early diagenetic origin that is found mostly in the fine-grained limestone and dolomitic muds. The preservation of a primary or early diagenetic magnetization, combined with the preservation of primary isotopic values, indicates that little or no extrabasinal fluid flow is likely to have occurred. Thus, in this particular area, orogenically induced fluid flow cannot explain the presence of the Cretaceous B magnetization. Consideration of the potential methods for forming this B component suggests that it is likely an in-situ chemical remanence resulting from either hydrocarbon migration or dissolution and reprecipitation of Fe-rich minerals in the original pore fluids.
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