A DETERMINISTIC MODEL FOR OUTCROP TO SUBSURFACE WIRELINE LOG CORRELATION, EOCENE GREEN RIVER FORMATION, EASTERN UINTA BASIN, COLORADO AND UTAH (Invited Presentation)

The Eocene Green River Formation of the Uinta basin is a 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, 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.

To solve for four components, clay volume is calculated before RHOmaa and Umaa values. Each well examined is individually zoned by petrophysical properties and a neutron density crossplot is used to understand clay densities. By assigning values to the clay density and clay photoelectric effect, the clay component of the overall RHOmaa and Umaa values is excluded for each given depth. Then, the altered RHOmaa and Umaa value can be cross-plotted and examined in a ternary diagram. This determines the relative percentage of quartz, calcite, and dolomite in relation to the clays.

This petrophysical method is not without limitations. Borehole conditions must be considered. The system can only identify three constituents at a time. Data points will drift on the cross-plot due to diverse mineralogy. Diagenetic minerals also cause data point drift that must be corrected for and here include analcime and sodium-rich feldspars.

The deterministic model allows for more detailed correlations that show vertical lithofacies variability and stratigraphic changes from littoral to profundal lake environments. Thin clean limestone beds are clearly identified. High-feldspathic rocks require a more mobile quartz end point, but result in the identification of thin siliciclastic beds. Lake margin wells have the highest quartz percentages. Carbonate percentages increase basinward. The gamma ray and neutron-density calculated clay volumes are more consistent in the lower Douglas Creek Member than the Parachute Creek Member, indicating an increase in diagenetic minerals in the upper Green River.