Prediction of Organic Maturation by Vitrinite Reflectance Regression in Units of the Mancos Shale, Uinta Basin, Utah

Robert Ressetar, Jeffrey Quick, and Andrew D. McCauley
Utah Geological Survey

The Upper Cretaceous Mancos Shale in the Uinta Basin of Utah, with a thickness approximating 4000 ft and burial depths ranging to >15,000 ft, presents challenges in indentifying optimal zones for well completion and stimulation. One technique that may aid in ranking potential Mancos target zones is predicting the organic maturation of selected Mancos subunits. To do this, we compiled vitrinite reflectance (%Ro) data for the Mancos and other Uinta Basin strata, and by regression analysis obtained a formula to predict %Ro based on location and structural depth (elevation). The formula, which is subject to change as additional data are added, has the form: %Ro = 14.9X + 33.2Y – 80.1Z + 0.0847X2 – 0.264Y2 + 29.3Z2 – 0.364XY – 2.48YZ – 642.3 where: X = UTM easting/100,000 Y = UTM northing/100,000 Z = 1,000,000/(300,000 + elevation [ft]) As an example of this technique's application, we identified an ~100-ft-thick informal unit within the lower Blue Gate Member of the Mancos as a potential reservoir target, based on its stratigraphic position and log signature. Along a transect from the basin's southeastern margin to its center, burial depths to the top of the target unit range from 3300 to 13,900 ft. Using the above formula, we predict %Ro to vary from 0.84 (peak oil) in the southeast to 1.47 (dry gas) in the central basin near Ouray, Utah. We predict the transition from oil to gas generation (%Ro = 1.2) to occur about 12 miles north of the Flat Rock and Hill Creek North fields. Finally, we have used the formula to predict the depth to a given %Ro value at various locations. These data combined with geologic structure maps illustrate which stratigraphic units are at the same level of maturation across the basin.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013