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

COMPARISON WITH PREVIOUS ASSESSMENTS

One cannot make a meaningful comparison with previous assessments without knowledge of differences in assessment methodology, assumptions, and data. That information is not always available for the previous assessments of the ANWR 1002 area. Among previous assessments of ANWR 1002 area petroleum resources, only the 1987 USGS assessment of in-place resources is directly comparable. The technically and economically recoverable petroleum resource estimates cannot be compared directly because different methods were used in preparing those parts of the 1987 Report to Congress. The current assessment shows an overall increase in estimated in-place oil resource when compared to the 1987 assessment. Ranges are 11.6 to 31.5 BBO versus 4.8 to 29.4 BBO, (95- and 5-percent probabilities) and mean values are 20.7 BBO versus 13.8 BBO (current assessment compared to 1987 assessment). The increase results in large part from improved resolution of reprocessed seismic data, which allowed the identification of many more potential petroleum accumulations in parts of the area, and analog information provided by recent nearby oil discoveries.

Another significant change is in the geographic distribution of resources. In the 1987 assessment, about 75 percent of the mean estimated in-place oil was in the southeastern or deformed area and only 25 percent was in the northwestern or undeformed area (fig. 2). In the current assessment, nearly 85 percent of the in-place oil is in the undeformed area and only about 15 percent is within the deformed area. Again, the reason is largely related to improved resolution of the seismic data, especially in the undeformed area where, in various plays, it allowed the identification of many more potential petroleum accumulations than were previously thought to exist. The southeastern area-with only a single well offshore and complex geology onshore-carries great uncertainty. Further, part of that area considered oil prospective in 1987 is now considered prospective only for gas because of new understanding of the thermal history of the rocks.