Eocene hydrocarbon migration, Green River Formation, Utah
Résumé
The renowned Eocene Green River Formation presents a spectacular field setting to study both source rock and migration of hydrocarbon. From the Uinta basin, we combine field observations with Re-Os data from pristine outcrop and drill core samples to interpret hydrocarbon migration history. To build a meaningful Re-Os data set, we employ our sampling strategy to capture just a few mm of stratigraphic section. Larger bulk samples risk homogenization of real variations in the initial Os ratio and any time occupied by non-depositional or erosional intervals. Re-Os data for the Mahogony Bed and the petroliferous Mahogony Zone in Hells Canyon (Utah) are combined with Re-Os data from drill core from the Parachute Creek and Douglas Creek members to provide a compelling story for hydrocarbon migration. Samples from designated oil-rich zones show a narrow range of Re and Os concentrations and a narrow range in 187Re/188Os, the latter a recipe for misleading Model 1 ages with low MSWDs, but huge age uncertainties. We interpret this as a snapshot of Re-Os systematics depicitng the hydrocarbon migration process with the majority of the oil still sitting in its source rock (unconventional hydrocarbon). Samples from designated oil-lean zones have nearly an order of magnitude lower Re and Os, but only slightly lower 187Re/188Os and 187Os/188Os. Re-Os scatterchrons of 47-49 Ma can be drawn from some data sets. Scatterchrons and analysis of 187Os/188Os ratios suggest considerable hydrocarbon was generated shortly after shale deposition. We suggest that kerogen maturation began in deeper parts of the basin before complete dewatering and consolidation of upper basin sediments, with migration facilitated by seismicity. This is supported by syneresis cracks, folded sand dikes, stratigraphically confined isoclinal folds in some organic-rich mud intervals, tar balls and oil spills. Our Re-Os data, and more importantly the interpretation of those data, bring new understanding to unconventional hydrocarbon systems in lacustrine rocks. Rather than depositional ages, Re-Os results show us how oil is liberated, gathered, and how it migrates. Field relationships directly support this interpretation.