RELATIVE UNCERTAINTY OF U.S. GEOLOGICAL SURVEY PETROLEUM ASSESSMENTS IN GULF COAST REGION: 1995-2004

The U.S. Geological Survey (USGS) has completed four National petroleum assessments in the last 25 years. We have analyzed uncertainty in estimated volumes of undiscovered gas and oil resources for two successive (1995 and 2004) USGS assessments of the Gulf Coast region. Assessment uncertainty is related to (1) compiling and interpreting geologic data, (2) defining and risking assessment units (AUs), (3) estimating sizes and numbers of undiscovered accumulations, (4) using different assessment methodologies, (5) and changing perceptions of AUs by petroleum geologists through time.

To quantitatively express relative uncertainty at the AU level, we introduce a dimensionless uncertainty coefficient (UC). UC=(F5 - F95)/F50, where F5, F50, and F95 are the 5th, 50th, and 95th fractiles of a probability distribution representing the estimate of undiscovered gas or oil for the AU. The UC is based on the assumption that the fractile range of the estimated undiscovered resource (F5-F95) includes all aspects of uncertainty. Dividing by F50 normalizes the uncertainty range relative to the magnitude of the undiscovered resource.

We analyzed 23 gas or oil AUs in the Gulf Coast region by ranking their UC scores, which range from 1.01 to 4.26. Four of the five most certain AUs (UCs < 1.5) are conventional AUs from the 2004 USGS assessment. The fifth and most certain AU (1995 Cotton Valley Blanket Sandstone Gas, UC=1.01) was assessed as continuous. This most certain AU was reassessed as conventional (UC=1.95) in 2004 when additional data led to a change in interpretation of accumulation type. The five most uncertain AUs (UCs > 3.45) were assessed in 1995. The most uncertain AU in 1995 (Hosston Updip Oil, UC=4.26) was reassessed in 2004 (Travis Peak-Hosston Updip Oil AU) with a larger UC (1.69).

An overall increase in relative AU certainty from 1995 to 2004 for Gulf Coast AUs is a result of a better understanding of the petroleum geology and to changes in assessment methodology. Analysis of relative uncertainty in petroleum assessments improves understanding of petroleum provinces so that priorities can be set for AU reevaluation.