Rock property characteristics and correlation from outcrop to wireline logs of the Green River Formation, Red Wash field, eastern Uinta Basin

Abstract

The Green River Formation is an Eocene-aged fluvio-lacustrine system that was deposited across the Uinta and Piceance basins between 53 Ma and 46 Ma. Log evaluation and correlation through the Green River Formation is made difficult due to the heterogeneous mineralogy and depositional facies. Lake deposition responds to environmental and tectonic changes that controls both the heterogeneity and mineral complexity. The use of log curves alone has proved misleading when correlating into the basin where control from core is sparse. The collection of outcrop and subsurface data helped tie physical rock properties to well logs. This allowed for a better correlation of facies changes from the littoral facies that are present in the outcrop to the more profundal facies that are present in the subsurface. Even with the large amount of subsurface core data that is available throughout the basin, the lateral variability requires tightly space control points that is not possible with core data alone. Subsurface logs provide the control needed in such an environment. To better understand the changes in lithology moving into the basin, a combined approach of outcrop and subsurface rock properties in conjunction with modeling was used to predict facies across the Wonsits/Red Wash Field study area. The use of a deterministic model developed by Cluff et al. (2015), and modified by Peacock (2017), aided in the creating of a four-mineral model that consists of silicate, calcite, dolomite, and shale. The model relied on electric log curves that are commonly run in wells drilled throughout the Uinta basin making it easily applied across the study area. The model is qualitative as it does not account for borehole conditions and is not corrected for diagenesis. The model is used to identify large general trends in mineral volume changes deposited across the study area. The model was only applied to wells that have data available through the entire zone of investigation and thus spacing between data points is often larger than the lateral extent of the highly fluctuating lithologies. When applied to the lake stages proposed by Tӓnavsuu-Milkeviciene and Sarg (2017), distinct depositional patterns begin to appear. The mapping of mineralogical assemblages is closely related to the proposed lake stages with an increase in mixed clay and decrease in silicate deposition corresponding to overall lake level rise. Dolomite volumes also increase with lake level rise and make up the dominant mineral assemblage in the Mahogany zone, which is widely associated with dolomite-rich oil shales. A fluvial input source in the northeastern portion of the study controlled the deposition of main reservoir within Red Wash Field, which corresponds to the higher silicate deposition seen in both outcrop at the northern end of Raven Ridge, as well as core within the Red Wash/Wonsits oil field, which is directly down dip of North Raven Ridge. The producing facies within this reservoir is made up of deltaic sandstones and shallow lake carbonates which are encased in non-reservoir mudstones and shales that act as seals.