An isotopic biogeochemical study of the Green River oil shale

Thirty-five different samples from three different sulfur cycles were examined in this stratigraphically oriented study of the Shell 22x-1 well (U.S.G.S. C177 core) in the Piceance Basin, Colorado. Carbon isotopic compositions of constituents of Green River bitumens indicate mixing of three main components: products of primary photoautotrophs and their immediate consumers (σ ∼ -30‰ vs PDB), products of methanotrophic bacteria (σ ∼ -85‰), and products of unknown bacteria (σ ∼ -40‰). For individual compounds synthesized by primary producers, σ-values ranged from -28 to -32‰. ¹³C contents of individual primary products (β-carotene, steranes, acyclic isoprenoids, tricyclic triterpenoids) were not closely correlated, suggesting diverse origins for these materials. ¹³C contents of numerous hopanoids were inversely related to sulfur abundance, indicating that they derived both from methanotrophs and from other bacteria, with abundances of methanotrophs depressed when sulfur was plentiful in the paleoenvironment. γ-Cerane coeluted with 3β(CH₃),17α(H),21β(H)-hopane, but α-values could be determined after deconvolution. γ-Cerane (σ ≈ -25‰) probably derives from a eukaryotic heterotroph grazing on primary materials, the latter compound (σ ≈ -90‰) must derive from methanotrophic organisms. ¹³C contents of n-alkanes in bitumen differed markedly from those of paraffins generated pyrolytically. Isotopic and quantitative relationships suggest that alkanes released by pyrolysis derived from a resistant biopolymer of eukaryotic origin and that this was a dominant constituent of total organic carbon.