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Assessing under Extreme Uncertainty--USGS Methodology for the Circum-Arctic Resource Appraisal

Charpentier, Ronald R.1
1 US Geological Survey, Denver, CO.

Resource appraisals of frontier areas such as those assessed by the U.S. Geological Survey (USGS) in its 2008 Circum-Arctic Resource Appraisal (CARA) are difficult because the petroleum system models are not established or are poorly understood. Such appraisals must therefore take into account multiple hypothetical petroleum system models and the uncertainties associated with them. The primary effort in assessing frontier areas such as those in the Arctic was the development of hypotheses about possible petroleum systems.

Relatively few of the areas assessed in CARA had sufficient exploration maturity to allow use of traditional assessment methodologies such as extrapolation of exploration histories. Therefore, a new assessment methodology was developed that was more appropriate to frontier areas, many of which had no wells and little or no seismic data. The new methodology was based on an analog database of oil and gas assessment units and the probability of occurrence of large fields.

Two hundred and forty-six assessment units in the USGS 2000 World Petroleum Assessment were used to construct an analog database. These 246 assessment units account for over 95 percent of the world’s discovered oil and gas resources, exclusive of the United States. The database was used to relate numbers and sizes of accumulations with geologic factors such as basin type, structural style, trapping mechanisms, source and reservoir rock characteristics, and fluid properties. Sets of analogs, most commonly based on tectonic styles and hypothetical trapping mechanisms, were developed for comparison to the areas being assessed. The analog sets provided realistic ranges of uncertainty for accumulation sizes and numbers of accumulations per unit area.

It was critical to consider the possibility that an assessed area had no fields larger than the minimum size assessed in CARA, which was 50 million barrels of oil equivalent (MMBOE). A study of the worldwide occurrence of large fields, based on several data sets at various scales, suggested that the base probability for the occurrence of at least one accumulation of 50 MMBOE or larger in assessment units of the scale used in the CARA is approximately 50 percent. The probability of at least one field of 50 MMBOE was estimated by modifying that base probability upward or downward based on the geologic favorability of the area being assessed.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009