Developing a Predictive Geologic Model of the Green River Oil Shale, Uinta Basin
Lauren Birgenheier, Michael Vanden Berg
- Assess geologic heterogeneity of the oil-shale bearing Green River Formation, Uinta Basin, Utah.
- Build a predictive, regional-scale sequence stratigraphic model of oil shale deposits that can be used to predict away from available datasets in the Uinta Basin and addresses geologic drivers of deposition and resulting heterogeneity.
Department of Energy, National Energy Technology Laboratory
Michael Vanden Berg, Utah Geological Survey
The Green River Formation is the record of an Eocene, continental interior, terminal lake basin system that covered a significant area across northeastern Utah (Uinta Basin), western Colorado (Piceance Basin), and southwestern Wyoming (Greater Green River Basin). It is one of the most well-cited examples of an ancient lacustrine system. In Utah, the Green River Formation hosts a vast oil shale resource in the Uinta Basin, estimated at 1.32 trillion barrels in-place (Oil Shale Resources of the Uinta Basin, Utah and Colorado, United States Geological Survey, August 2010). Nevertheless, a solid geologic framework for the Green River Formation in the Uinta Basin is less developed compared to the neighboring Piceance and Greater Green River Basins, and a predictive sequence stratigraphic framework is lacking. In particular, there has been relatively little effort focused on the facies and stacking patterns in the mudstone-dominated basin depocenter as compared to the alluvial and shallow lacustrine facies on the basin margin.
The first steps toward building a predictive, regional-scale sequence stratigraphic model have been achieved with the completion of a systematic, detailed, sedimentologic, stratigraphic, and geochemical study performed on four cores (P-4, Coyote Wash 1, Utah State 1, and EX-1) ranging in length from 960 to 1640 ft, along an east-west transect through the basin's paleo-depocenter (Figure 1). Key features noted in each core include grain size, lamination style, sedimentary structures, mineralogy, bioturbation, biotically influenced features, body fossils, and plant fossils. Nondestructive qualitative X-ray fluorescence (XRF) analysis was performed on whole-rock samples according to key lithologic changes at roughly 10-foot intervals to help determine inorganic mineralogy. The dominant inorganic mineralogy of the mudstones was defined based on XRF criteria. Siltstones and sandstones were identified based on visual inspection. Next, a detailed core log was constructed to graphically represent the data. An east-west cross section was drafted with the core logs plotted next to geophysical log curves and Fischer assay oil yield data (Figure 2). Correlations were made between similar oil shale zones, highlighting how these zones change across the basin.
Figure 1: Map showing locations of four cores examined to construct an east-west cross section. Shades of blue indicate thickness of a continuous interval of oil shale averaging 25 gallons per ton, with color shading darkening with increased thickness.
From this work, it has been determined that Lake Uinta evolved in three phases: (1) a freshwater rising lake phase below the Mahogany zone, (2) an anoxic deep lake phase above the base of the Mahogany zone and (3) a hypersaline lake phase within the middle and upper R-8. This long-term lake evolution was driven by tectonic basin development and the balance of sediment and water fill with the neighboring basins. Early Eocene abrupt global-warming events may have had significant control on deposition through the amount of sediment production and deposition rates, such that lean zones below the Mahogany zone record hyperthermal events and rich zones record periods between hyperthermals. This type of climatic control on short-term and long-term lake evolution and deposition has been previously overlooked.
Figure 2: East-west cross section that highlights unit composition, stratigraphy and lake evolution.
This geologic history contains key points relevant to oil shale development and engineering design including:
- Stratigraphic changes in oil shale quality and composition are systematic and can be related to spatial and temporal changes in the depositional environment and basin dynamics.
- The inorganic mineral matrix of oil shale units changes significantly from clay mineral/dolomite dominated to calcite above the base of the Mahogany zone. This variation may affect pyrolysis products and geomechanical properties relevant to development and, if so, should be incorporated into engineering experiments.
- This study includes a region in the Uinta Basin that would be highly prospective for application of in-situ production techniques. Stratigraphic targets for in-situ recovery techniques should extend above and below the Mahogany zone and include the upper R-6 and lower R-8.