The Elko Formation, an Eocene-Oligocene lacustrine deposit cropping out in northeastern Nevada, is composed of two distinctly different organic-rich facies: a lignitic, gas-prone siltstone and an oil shale. A third organic-lean facies is represented by mudstones containing small amounts of fine-grained reworked kerogen. The deposit is thermally immature with respect to oil generation.

Geochemical indicators of the two major organic facies are distribution of steranes and diterpanes, presence or absence of a specific triterpenoid biomarker (gammacerane), kerogen form, and Rock-Eval pyrolysis parameters, S2/S3 and (S1 + S2)/TOC. The siltstones are characterized by: (1) vitrinitic kerogen, (2) pristane/phytane ratios slightly greater than 1.0, (3) relatively less more negative δ13C values for kerogen and C15+ hydrocarbons, (4) a predominance of C29 steranes and small amounts of rearranged steranes, (5) primarily C19 and C20 tricyclic diterpanes, and (6) a predominance of 17β(H)-22, 29, 30-trisnorhopane with respect to 17α(H) hopanes and 17δ(H) more moretanes. The oil shales are characterized by: (1) algal kerogen, (2) pristane/phytane ratios less than 0.5, (3) relatively negative δ13C values for kerogen and C15+ hydrocarbons, (4) a mixture of C27, C28, and C29 steranes and 4-methyl steranes, (5) a mixture of C19 to C26 diterpanes, and (6) a predominance of hopane and moretane and the presence of gammacerane. Hydrous pyrolysis of solvent-extracted oil shale produced a waxy oil-like bitumen whose more “mature” biomarkers and stable carbon isotopic composition resembled the unreacted oil shale.

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