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Log-Interpreted Reservoir Potential from Cretaceous Mancos Shale in the Uinta Basin

Trevor Stoddard, Lauren Birgenheier, Ryan Hillier, Laini Larsen, and John McLennan
University of Utah

Mancos formation log data from several key wells in the Uinta basin have been analyzed and approximate reservoir potential assessed. One of the primary production concerns in the Uinta basin involves high water production rates. A workflow to accurately determine water saturation in shales using normalized resistivity logs has been created and general water saturation trends across the basin have been resolved to give estimates on production across the basin. In addition to water saturation a logging interpretation method to determine rock mechanics properties using triple combo logs have been used to estimate the rock mechanics properties across the basin. Passey's 'DlogR' method was also employed to estimate TOC within the Mancos unit in areas across the basin. All of the methods employed to reduce these various logs were used to create an overall estimate as to the amount of Mancos gas potential across the Uinta basin. With regard to estimated water saturations across the basin it was found that the Mancos B had the highest water saturation, with a low of 50% water saturation. Generally the water saturations in the northern portion of the basin were higher than in the south. All of the water saturation findings agree with high water production rates reported by operators. 'DlogR' analysis has shown that there are areas of TOC within the Mancos unit, on average between 0.5% to 4.0%. Currently the method for estimating shear and compressional slowness from triple combo logs is being used on those wells without sonic data to utilize the 'DlogR' method to determine how well the values estimated from density and neutron porosity overlap with resistivity curves compare with what the estimated DlogR overlap would be. The approximated sonic log was used to generate estimates as to the rock mechanic properties within the Uinta basin which were then extended to stress estimations in stratigraphic units across the basin. The estimated stresses across the basin were confirmed by operator well reports suggesting average fracture gradients of 0.88 throughout the Mancos stratigraphic unit. This research has indicated that within the Mancos in the Uinta basin there are areas of higher TOC, though typically it is accompanied with high water saturation and high fracture gradients. This suggests that additional research on optimal fracturing strategies needs to be completed to maximize production of the log estimated gas potential within the reservoir.

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