Sulfidization and Magnetization Above Hydrocarbon Reservoirs
Published:January 01, 1993
Richard L. Reynolds, Martin B. Goldhaber, Michele L. Tuttle, 1993. "Sulfidization and Magnetization Above Hydrocarbon Reservoirs", Applications of Paleomagnetism to Sedimentary Geology, Djafar M. Aïssaoui, Donald F. McNeill, Neil F. Hurley
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Post-depositional iron-sulfide (Fe-S) minerals that are related to hydrocarbon seepage have changed the original magnetizations at Cement oil field (Anadarko basin, Oklahoma), at Simpson oil field (North Slope basin, Alaska), and above deep Cretaceous oil and gas reservoirs, south Texas coastal plain. At Cement, ferrimagnetic pyrrhotite (Fe7S8) formed with pyrite and marcasite in Permian red beds. The Fe-S minerals contain sulfur from two sources: (1) abiogenic sulfide, which has positive δ34S values, derived from thermochemical reduction of sulfate in deep reservoirs; and (2) biogenic sulfide, which has negative δ34S values, produced by reactions mediated by sulfate-reducing bacteria fed by leaking hydrocarbons. At Simpson, ferrimagnetic greigite (Fe3S4) dominates magnetizations in nonmarine Upper Cretaceous clastic beds that contain epigenetic sulfide (δ34S > +20 per mil) and seeping biodegraded oil. In this setting, the authigenic magnetic sulfide mineral apparently incorporated sulfide produced by bacterial sulfate reduction under limited sulfate conditions. An inferred hydrocarbon food source for the sulfate-reducing bacteria links the hydrocarbon seepage to the greigite. The greigite is perhaps forming today. In middle Tertiary sandstones of southeast Texas, pyrite and marcasite formed when abiogenic H2S (enriched in 34S) migrated upward from deep reservoirs, or when H2S (depleted in 34S) was produced at shallow depths by bacteria that used organic material dissolved in migrating water from depth. The pyrite and marcasite replaced detrital magnetic iron-titanium oxide minerals. The degree of such replacement appears to increase toward faults that connect deep petroleum reservoirs to shallow sandstone. Our results show that abiologic and biologic mechanisms can generate different magnetic sulfide minerals in some sulfidic zones of hydrocarbon seepage. More commonly the magnetizations in such zones would be diminished as a result of the replacement of detrital magnetic minerals by the common nonmagnetic sulfide minerals, or would remain unchanged if such detrital minerals were originally absent.
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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.