Deterministic model for outcrop to subsurface wireline log correlation, Eocene Green River Formation, eastern Uinta Basin, Colorado and Utah, A

Abstract

The Eocene Green River Formation of the Uinta basin is a fluvial-lacustrine system comprised of carbonates, siliciclastics, and oil shale. Log evaluation is difficult, due to complex mineralogy and the thin interbedded nature of diverse rock types. Historically, log correlations have used a zoned model, which excludes detail and suggests continuity that is misleading on a bed-by-bed basis. A deterministic model is applied here which utilizes gamma ray, bulk density, neutron porosity, and photoelectric effect logs. A four-mineral solution gives volume percent of quartz, calcite, dolomite, and mixed clay. To obtain these volume percentages, log-based calculations yield an apparent matrix density (RHOmaa) and an apparent photoelectric cross section (Umaa). To calibrate these results, outcrop work was completed to determine mineralogy, and expected facies changes from littoral to profundal environments. The development of this RHOmaa-Umaa methodology has enabled the building of a stratigraphic framework for the eastern Uinta basin that can be extended from outcrop and core into the basin. Through the integration of outcrop mineralogy work with subsurface calculated mineralogy, this research includes an interpretation of basinward stratigraphic and lithology changes. This understanding allows for the prediction of mineralogy and facies changes using commonly available well data. Resulting correlations successfully identify and correlate rich and lean oil shale zones and sequence boundaries showing stratigraphic thickening into the basin center. The clay volume calculations demonstrate that the Douglas Creek member has a lower volume of diagenetically altered minerals than the Parachute Creek member. Organic rich zones have higher volumes of dolomite, suggesting a link between organic matter productivity and the degree of dolomitization. Rich zones also have lower bulk densities and higher neutron porosity values due to high organic matter volumes. Total carbonate volumes increase higher in the stratigraphic section, driven by an increase in dolomite volumes. This petrophysical method is not without limitations. Borehole conditions must be considered. The system can only identify three constituents at a time as data points will drift on the cross-plot due to diverse mineralogy. Diagenetic minerals, including analcime and sodium-rich feldspars, also cause data point drift that must be corrected for.