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Background Previous HitFrequencyNext Hit of Large Field Occurrence and Its Application to Geologic Risk in Assessment of Frontier Areas

Ronald R. Charpentier
U.S. Geological Survey, Denver, CO

What is the Previous HitfrequencyNext Hit of occurrence of at least one oil or gas field of size X or larger among sedimentary areas at the play to province scale? Estimates of those background frequencies are critical to assessing undiscovered resources in poorly explored (frontier) areas, as is the case for the present assessment of basins north of the Arctic Circle by the U.S. Geological Survey (USGS). Those frequencies, and their Previous HitdependenceTop on field size and assessment area scale, are an important starting point from which the geologic risk can be raised or lowered based on additional geologic knowledge.

The (understandable) concentration of exploration in areas of success biases any frequencies directly calculated from exploration history. For this study of geologic risk, some of that bias was reduced by (1) use of estimates of what might be the largest undiscovered field, and (2) an analysis of the onshore United States as a large, geologically diverse, relatively maturely explored (for conventional fields) area.

The probability of the existence of a field of at least size X should decrease with increasing size X. It is also to be expected that, in general, such probabilities of existence should increase as the assessment area increases. However, within the range of scales used in USGS resource assessments, size of assessment area was determined to be of secondary importance.

The current USGS Arctic assessments are based on a minimum field size of 50 million barrels of oil equivalent (MMBOE), recoverable. At the scale used-several assessment units per basin-the background probability for the existence of at least one field of 50 MMBOE or larger is probably in the range of 50 to 60 %.


AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas