Evidence for Two Major Cycles of Petroleum Generation in Mississippian Chainman Shale of East-Central Nevada: ABSTRACT

Abstract

Marine Chainman Shale from drill cores in Railroad Valley and from outcrops in several mountain ranges of east-central Nevada contains a mixture of marine-sapropel and detrital terrestrial-plant organic matter. The organic matter ranges from immature to supermature in thermochemical evolution as indicated by organic geochemical data on kerogen and on hydrocarbon extracts and oil, by vitrinite reflectance, and by alteration colors of palynomorphs and conodonts. Hydrocarbon contents (< 30 to 2,000 ppm) and organic-carbon contents (< 0.1 to 7 weight %) vary widely. A discontinuity in thermochemical maturity has been identified between Paleozoic and Paleogene rocks in uplifted terranes, whereas a more continuous kerogen-maturation profile exists across the Paleozoic and Paleogene boundary where deeply buried beneath Neogene rocks. A two-cycle model of petroleum generation is proposed to account for these variations. The first cycle of petroleum generation began probably in early Mesozoic time when the Chainman was buried beneath upper Paleozoic and lower Mesozoic rocks. Depth of burial and hence the degree of thermochemical maturation varied with late Paleozoic and Mesozoic folding, faulting, and erosion. Organic matter in the Chainman is supermature in many localities where higher paleotemperatures were related to subcrustal hotspots or deeper burial. Analyses of surface and subsurface data indicate that although Chainman rocks were subjected to thermochemical degradation and generated some hydrocarbons in Mesozoic time--as evidenced by oil of varying viscosities trapped in fractures, voids, and invertebrate-fossil cavities in dense limestone concretions and beds--organic matter was not completely transformed into petroleum where buried at moderate depths. Uplift of buried Chainman Shale prior to the late Mesozoic arrested the first cycle of petroleum generation in thermochemically immature and mature rocks. Many organically immature and mature Chainman rocks in this region are now undergoing a second cycle of thermochemical degradation and renewed oil generation in Neogene basins where adequate fill and temperature increase have occurred. Rocks of the Chainman Shale probably are a major source of petroleum in Railroad Valley where oil has accumulated in fractured, welded ash-flow tuffs of Oligocene age. Oil accumulations are inferred to occur in other valleys in eastern Nevada where similar geologic conditions exist. End_of_Article - Last_Page 838------------