Relations among hydrocarbon reservoirs, epigenetic sulfidization, and rock magnetization; Examples from the South Texas coastal plain

This paper focuses on the association between concentrations of iron disulfide (FeS ₂ ) minerals in the shallow subsurface and underlying hydrocarbon accumulations. Such FeS ₂ concentrations are the result of migration of either H ₂ S or organic constituents from the underlying hydrocarbons. The H ₂ S from reservoirs is produced inorganically from sulfate in the reservoir rocks at high temperature (>90 degrees C) and migrates to shallower beds to react inorganically with iron to form FeS ₂ . Organic constituents from reservoirs, in contrast, provide nourishment for sulfate reducing bacteria in shallow relatively cool (<90 degrees C) beds. Sandstone in the Ray Point uranium district in Live Oak County, Texas contains abundant FeS ₂ which formed both from deep-seated H ₂ S and from H ₂ S produced in the shallow subsurface by bacteria that utilized organic materials from depth. Deep petroleum reservoirs were physically connected to near-surface (<100 m) beds containing epigenetic FeS ₂ by the Oakville fault. Epigenetic iron sulfide formation occurred in at least four episodes over at least five million years. Evidence from the Ray Point district and elsewhere in Texas illustrates that sulfidization reactions have destroyed magnetic iron-titanium oxide minerals in the vicinity of major growth faults, resulting in a systematic decrease in magnetic susceptibility and magnitude of remanent magnetization in the vicinity of such faults. Growth faults which tap hydrocarbon deposits may be detectable using aeromagnetic methods.