GEOLOGY AND GEOTHERMAL ORIGIN OF GRANT-CANYON AND BACON-FLAT OIL-FIELDS, RAILROAD-VALLEY, NEVADA

Eastern Nevada's Grant Canyon and Bacon Flat oil fields show strong ev idence of formation in a still-active, moderate-temperature geothermal system. Modern manifestations of this system include unusually elevat ed oil-reservoir temperature at shallow depth, 116-122 degrees C at 1. 1-1.6 km, and dilute Na-HCO3-Cl thermal waters directly associated wit h hot oil. Hydrogen and oxygen isotopic compositions indicate that the se thermal waters are meteoric in origin, but were probably recharged prior to the Holocene (before 10 ka). The waters apparently ascended t o oil-reservoir elevations after deep heating in response to the norma l regional thermal gradient; there is no evidence for a modern magmati c heat source. The beginning of oil-reservoir evolution at both fields is recorded by late-stage, fracture-filling quartz in the vuggy, brec ciated, Paleozoic dolostone reservoir rocks. Oil and aqueous solutions were trapped as fluid inclusions in the quartz at temperatures com pa rable to those now prevailing in the reservoirs. Apparent salinities o f the aqueous inclusions closely match the actual concentrations of cu rrent oil-field waters, and the quartz has the oxygen isotopic composi tion predicted for its crystallization from these waters at contempora ry temperatures. Present-day and fluid-inclusion temperatures define e ssentially coincident isothermal profiles through and beneath the oil- reservoir interval, a phenomenon consistent with near-constant convect ive heat transfer since inception of the geothermal system. Textural a nd mineralogic clues indicate that hot waters circulating in this syst em also increased porosity by dissolving carbonate minerals, and helpe d seal reservoir margins by precipitating silica and kaolin. More impo rtantly, the rising thermal waters may have aided oil transport and ac celerated source-rock maturation through an increase in the shallow (< 3 km) local thermal budget. Along with the aforementioned fluid-inclu sion and isotopic evidence, radiometrically dated life spans for numer ous extinct geothermal systems (epithermal ore deposits) in the Basin and Range make it likely that the Grant Canyon-Bacon Flat system and a ssociated oil reservoirs are no older than 2.5 Ma.