CARBON ISOTOPIC COMPOSITION OF INDIVIDUAL BIOMARKERS IN GILSONITES (UTAH)

Carbon isotopic compositions of saturated hydrocarbon, aromatic hydrocarbon, NSO (resin), and asphaltene fractions of vein-forming gilsonites (Cowboy, Bonanza, Independence, Wagonhound and Harrison veins) in the NE Uinta Basin, Utah, are all similar and isotopically resemble Mahogany Zone shale extracts of the Green River Formation (deltaC-13 approximately -29 to -31 parts per thousand). Individual molecular fossils in the gilsonites show a wider variability in compositions indicative of their paleoecological origins within the Eocene Uinta/Greater Green River paleolake system. Carbon isotopic compositions of C28 and C29 steranes (deltaC-13 = -25 to -32 parts per thousand), pristane and phytane (-33 to -34 parts per thousand) and perhydro-beta-carotene (carotane) (deltaC-13 = -33.2 parts per thousand) suggest that organisms producing these compounds grew near the lake surface. The C33 carotenoid, lexane, is isotopically identical to the C40 carotenoid, carotane, supporting a common C40 carotenoid precursor. Two groups of hopanes can be differentiated in the gilsonites based on carbon isotopic compositions: C29, C31 and C32 hopanes and moretanes (-40.9 to -44.3 parts per thousand), consistent with a source from midwater bacteria and C30 hopanes and moretanes (-51.9 and -60.5 parts per thousand) for which the strong C-13 depletion suggests at least partial derivation from methylotrophs. The isotopic signatures of these molecular fossils suggest the source beds of the gilsonites were deposited in a stratified paleolake system with a depositional environment similar to that which formed the Mahogany Zone shales of the Piceance Creek Basin. Algal biomarkers in the gilsonites are isotopically uniform throughout the area of the gilsonite vein occurrence (approximately 1500 km2) and resemble those in the Mahogany Zone shale in the Piceance Creek Basin over 50 km to the East. This remarkable lateral uniformity suggests that very similar photic zone water conditions existed across the entire Uinta/Piceance Creek paleolake system at the time of deposition of the gilsonite source bed (Mahogany Zone shale). Sterane isotopic compositions are effective for the genetic correlation of the solid bitumens/immature oils. In contrast, the carbon isotopic compositions of the bacterial biomarkers in the gilsonites vary significantly, suggesting that conditions in deeper waters of the paleolake system were more variable. Carbon isotopic signatures of individual hopane biomarkers, in the Green River formation, appear to be valuable for identifying different source facies characteristics within the same paleolake depositional system.