Thermodynamic Constraints on Anomalous Magnetization in Shallow and Deep Hydrocarbon Seepage Environments
Published:January 01, 1993
Elizabeth A. Burton, Hans G. Machel, Jun Qi, 1993. "Thermodynamic Constraints on Anomalous Magnetization in Shallow and Deep Hydrocarbon Seepage Environments", Applications of Paleomagnetism to Sedimentary Geology, Djafar M. Aïssaoui, Donald F. McNeill, Neil F. Hurley
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Thermodynamic stability diagrams have been constructed for selected iron minerals common to diagenetic settings ranging from near surface to about 6,000 m of depth. These diagrams cover geochemical conditions from 25°C and I bar to 200°C and 600 bars, aqueous sulfur species activities (HS, H2S, or SO42) from 10−2 to 10−14 bicarbonate activities of 10−1 to 10−4, pH values from 5 to 9, Eh values from -0.1 to −1.5 V, and divalent iron activities of 10−4 to 10−6.
The diagrams presented allow predictions of the geochemical conditions favoring generation of anomalous magnetization in hydrocarbon seepage environments. According to these predictions, there are several striking trends in magnetic mineral assemblages and resultant magnetic properties of the rocks: (1) increasing temperature and pressure (i.e., increasing burial) will enhance formation and preservation of magnetic minerals; (2) decreases in magnetic susceptibility will be dominant in surface and shallow subsurface environments, whereas increases in susceptibility will become more common with increasing depth; (3) magnetic susceptibilities in cores taken close to hydrocarbon traps will generally decrease, but at greater distances from traps are more likely to increase.
These predictions may significantly enhance success rates in hydrocarbon exploration because trends in magnetic susceptibilities and/or remanence can indicate the proximity as well as direction of hydrocarbon traps. These trends also suggest that magnetic surveys are more likely to be successful when applied to subsurface cores than when used in surface exploration. In areas targeted for exploration, the utility of predicted trends in magnetic properties is maximized through coupling magnetic surveys with other geological studies, particularly determination of the relative quantities, types and origins of iron- and sulfur-bearing minerals, and at least limited analyses of formation water chemistry.
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Applications of Paleomagnetism to Sedimentary Geology
Applications of Paleomagnetism to Sedimentary Geology - Based on the 1991 SEPM Research Symposium, the results are directed towards bringing the disciplines of Paleomagnetism and sedimentary geology closer together. Advances in the field of sedimentary geology will likely result from continued development of new ideas, questioning of old dogma, and, most importantly, providing means for testing these new hypotheses. It is hoped that the union of these two disciplines will help address many fundamental geological questions, such as the perennial problems of precise age-dating, stratigraphic correlation and geometries, understanding the timing and nature of post-depositional diagenetic fabrics, and the intriguing relationship between hydrocarbons and magnetization. The reader will find an unusual diversity of research topics presented in this volume. This diversity serves as a testimony to the potential applications awaiting the sedimentary geologist willing to explore these new paleomagnetic tools.