ABSTRACT

Eastern Nevada’s Grant Canyon and Bacon Flat oil fields show strong evidence of formation in a still-active, moderate-temperature geothermal system. Modern manifestations of this system include unusually elevated oil-reservoir temperature at shallow depth, 116–122°C at 1.1–1.6 km, and dilute Na-HCO3-Cl thermal waters directly associated with hot oil. Hydrogen and oxygen isotopic compositions indicate that these thermal waters are meteoric in origin, but were probably recharged prior to the Holocene (before 10 ka). The waters apparently ascended to oil-reservoir elevations after deep heating in response to the normal regional thermal gradient; there is no evidence for a modern magmatic heat source.

The beginning of oil-reservoir evolution at both fields is recorded by late-stage, fracture-filling quartz in the vuggy, brecciated, Paleozoic dolostone reservoir rocks. Oil and aqueous solutions were trapped as fluid inclusions in the quartz at temperatures comparable to those now prevailing in the reservoirs. Apparent salinities of the aqueous inclusions closely match the actual concentrations of current oil-field waters, and the quartz has the oxygen isotopic composition predicted for its crystallization from these waters at contemporary temperatures. Present-day and fluid-inclusion temperatures define essentially coincident isothermal profiles through and beneath the oil-reservoir interval, a phenomenon consistent with near-constant convective heat transfer since inception of the geothermal system. Textural and mineralogic clues indicate that hot waters circulating in this system also increased porosity by dissolving carbonate minerals, and helped seal reservoir margins by precipitating silica and kaolin. More importantly, the rising thermal waters may have aided oil transport and accelerated source-rock maturation through an increase in the shallow (<3 km) local thermal budget. Along with the aforementioned fluid-inclusion and isotopic evidence, radiometrically dated life spans for numerous extinct geothermal systems (epithermal ore deposits) in the Basin and Range make it likely that the Grant Canyon–Bacon Flat system and associated oil reservoirs are no older than 2.5 Ma.

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