--> Correlation of Niobrara-Equivalent Strata in the Eastern Uinta Basin, Utah

2014 Rocky Mountain Section AAPG Annual Meeting

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Correlation of Niobrara-Equivalent Strata in the Eastern Uinta Basin, Utah


Examination of geophysical logs from about 150 oil and gas wells allowed us to correlate the Late Cretaceous Niobrara Formation in Colorado to time-equivalent strata within the Mancos Shale of the eastern Uinta Basin, Utah. A preliminary structure map shows the top of the Niobrara equivalent (NE) deepens from near sea level at the Utah-Colorado line to more than 9000 feet below sea level about 40 miles to the northwest. Along the same path, the NE thickens from 1150 to more than 1450 feet. Most of the thickening occurs in the upper part of the NE, which we interpret as a highstand sequence set. The lower and middle parts of the NE, represented by lowstand and transgressive sequence sets, respectively, maintain relatively constant thicknesses. Based on log character and limited core data from the NE, we suggest that the unit contains and is immediately adjacent to some of the more hydrocarbon-prospective parts of the Mancos Shale. Much of the upper NE consists of a sediment-starved shelf facies association (a primary target) and the base of the NE overlies a heterolithic organic-rich facies association (a secondary target). Given the thickness of the NE and variations in burial depth, organic maturation levels of the unit should also vary significantly. Applying our maturation model of the Uinta Basin, we predict vitrinite reflectance values for the top of the NE to range from 0.55 in westernmost Colorado to 1.45 northwest of the Natural Buttes oil field; predicted values at the base of the NE are 0.63 and 1.65 at the same locations. Further exploration should determine whether the NE in the Uinta Basin has petrophysical characteristics similar to those that make the typical Niobrara Formation so productive to the east.