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Estimating Total Organic Carbon Content in the Cretaceous Mancos Shale

Ryan Hillier, Lisa Stright, and Robert Ressetar

The Late Cretaceous Mancos Shale is a prospective shale-gas reservoir located in the Uinta Basin in eastern Utah. To assess the hydrocarbon potential of the Mancos, the quantity of organic matter, measured by percent of total organic carbon (%TOC) within the shale matrix and pore space, must be determined. The %TOC can be estimated through a petrophysical approach called the ?logR method. The ?logR method uses wireline logs, geochemical data, and thermal-maturity information to calculate an in-situ %TOC. Using the ?logR method, calculated %TOC values are often underpredicted in shale-gas systems that have reached thermal maturation values within the dry-gas window. However, the exact degree of thermal maturity is often uncertain due to subjective visual characterization in vitrinite reflectance analysis. To compensate for the uncertainty associated with thermal maturity, petrophysicists have incorporated adjustment parameters into the ?logR equations to achieve an optimal match between observed and calculated %TOC values. These parameters create an improved match between calculated and observed %TOC values, but they do not address the underlying geologic relationship between thermal maturity and %TOC. Vitrinite reflectance values in the Uinta Basin indicate portions of the Mancos Shale are within the dry-gas window. We present an improved ?logR workflow that incorporates results from one-dimensional burial history modeling in order to more accurately characterize thermal maturity. This method optimizes input parameters, including the aforementioned adjustment parameters and thermal maturity model results, to obtain an improved fit between calculated and measured %TOC values. This optimization indicates that of all of the parameters, thermal maturity is the most dominant in accurately predicting %TOC. Adding thermal-maturity models into a modified ?logR workflow can achieve a more robust method for calculating %TOC in shale-gas systems that are within the dry-gas maturation window.

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