DYMAN, T. S., J. W. SCHMOKER, and D. B. RIGGIN, U. S. Geological Survey, Denver, CO; and K. W. PORTER and D. A. LOPEZ, Montana Bureau of Mines and Geology, Butte, MT

Abstract: Dealing With Uncertainty: Assessment of Gas Plays in the Rocky Mountains and Northern Great Plains Region

The U.S. Geological Survey (USGS) assesses undiscovered conventional and continuous-type (unconventional) oil and gas resources in onshore and State offshore regions of the U.S. In their 1995 National Assessment, the USGS evaluated 50 gas plays in the Rocky Mountains and Northern Great Plains Region (RM-NGP). Dealing with uncertainty is an important part of geology-based assessments such as these.

Assessment uncertainty is associated with interpreting geologic data, defining and risking plays, estimating sizes and numbers of undiscovered accumulations, and aggregating play estimates to regional levels.

In order to quantitatively express assessment at the play level, we introduce a dimensionless uncertainty coefficient (UC). UC=(F₅-F₉₅)/MEAN, where F₅ and F₉₅ are the 5th and 95th fractiles of a probability distribution representing the estimate of undiscovered hydrocarbons (in this case gas) for each play, and MEAN represents the mean value of that estimate. Use of the UC is based on the assumption that the fractile range of the estimated undiscovered resource (F₅-F₉₅) incorporates all areas of uncertainty in the assessment process. Dividing by the mean normalizes the uncertainty range relative to the magnitude of the undiscovered resource. A ranking of UCs can be used to establish priorities for play reevaluation and assessment planning.

The 50 gas plays in the RM-NGP were analyzed by ranking their UCs. UCs of this group range from 1.23 to 6.03. The 25 most uncertain gas plays (UCs > 1.99) are widely distributed but are most common in the SW Wyoming, Wind River Basin, Big Horn Basin, and Montana Thrust Belt Provinces. Eight of 10 hypothetical conventional gas plays are in the most uncertain half of the 50 gas plays.

Ten of the 50 gas plays are continuous-type plays and have an average UC of 1.84, which is lower than that for the conventional gas plays (UC=2.39). This difference is due to differences in assessment methodology and in the manner in which the play types were defined. Conventional plays are assessed on the hypothesis that undiscovered accumulations may exist. In contrast, for continuous plays in the RM-NGP, accumulations are known to exist; their assessment is a forecast of development potential.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana