Sulfidization and magnetization above hydrocarbon reservoirs

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.