Development of an improved methodology to assess potential unconventional gas resources in North America

Since the 1970s, various private and governmental agencies have conducted studies toassess potential unconventional gas resources, particularly those resources contained intight sands, fractured shales, and coal beds. The US Geological Survey (USGS) hasassessed the amount of unconventional gas resources in North America, and its estimatesare used by other government agencies as the basis for their resource estimates. Whilethe USGS employs a probabilistic methodology, it is apparent from the resulting narrowranges that the methodology underestimates the uncertainty of these undiscovered,untested, potential resources, which in turn limits the reliability and usefulness of theassessments.The objective of this research is to develop an improved methodology to assess potentialunconventional gas resources that better accounts for the uncertainty in these resources.This study investigates the causes of the narrow ranges generated by the USGS analyticprobabilisticmethodology used to prepare the 1995 national oil and gas assessment andthe 2000 NOGA series, and presents an improved methodology to assess potentialunconventional gas resources. The new model improves upon the USGS method byusing a stochastic approach, which includes correlation between the input variables andMonte Carlo simulation, representing a more versatile and robust methodology than theUSGS analytic-probabilistic methodology. The improved methodology is applied to the assessment of potential unconventional gasresources in the Uinta-Piceance province of Utah and Colorado, and compared to resultsof the evaluation performed by the USGS in 2002. Comparison of the results validatesthe means and standard deviations produced by the USGS methodology, but shows thatthe probability distributions generated are rather different and, that the USGSdistributions are not skewed to right, as expected for a natural resource. This studyindicates that the unrealistic shape and width of the resulting USGS probabilitydistributions are not caused by the analytic equations or lack of correlation betweeninput parameters, but rather the use of narrow triangular probability distributions as inputvariables.Adoption of the improved methodology, along with a careful examination and revisionof input probability distributions, will allow a more realistic assessment of theuncertainty surrounding potential unconventional gas resources.