Arctic National Wildlife Refuge, 1002 Area, Petroleum Assessment, 1998

ASSESSMENT METHODOLOGY

The methodology used in this assessment is slightly modified from that used in the 1987 assessment of this area when the USGS estimated in-place petroleum resources for the DOI Report to Congress; the methodology is also slightly modified from that used in the USGS assessment of the National Petroleum Reserve--Alaska (NPRA) (1979-1980). Ten petroleum plays were defined as the initial step of the assessment (fig. 3). A play is a volume of rock that contains similar geological parameters (such as petroleum charge, reservoir, and trap) that determine petroleum potential. In keeping with the USGS responsibility for assessing the petroleum potential of all onshore and State water areas of the United States, the total play area considered was extended to the 3-mile boundary between State and Federal jurisdiction. Thus, in addition to the Federal lands of the ANWR 1002 area, this assessment includes resources associated with State waters and Native lands (fig. 2).

For each play, distributions of the number and size of potential petroleum accumulations were estimated based on a probabilistic range of values for certain geological attributes, such as reservoir thickness and porosity. These distributions were restricted to potential accumulations larger than 50 million barrels of oil (MMBO) in-place so that the assessment would not be influenced by smaller accumulations that are non-economic in most cases on the North Slope.

The resulting distributions were subjected to a geologic risking procedure designed to weigh the likelihood that petroleum charge, reservoir, and trap conditions were sufficient to generate a 50-MMBO in-place accumulation. In turn, a probabilistic estimate of in-place petroleum resources was calculated based on the risked distributions of size and number of potential petroleum accumulations in each play. A recovery factor appropriate to each play was applied to the estimates of in-place petroleum resources to calculate technically recoverable petroleum resources (fig.4).

Estimates for each play were aggregated to calculate total technically recoverable petroleum resources for the entire assessment area, the 1002 area, and the undeformed and deformed parts of the 1002 area (table 1). Costs associated with discovering and recovering petroleum resources, including the costs of constructing pipelines to transport the petroleum, were then applied to estimate economically recoverable petroleum resources (fig. 4).

The assessment methodology yields results that include probabilistic expressions of uncertainty, as illustrated schematically in figure 4. To stress the importance of this uncertainty, results reported here include 95- and 5-percent probabilities, in addition to mean value. The 95- and 5-percent probabilities are considered reasonable minimum and maximum values, and the mean is the average or expected value.