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Geologic estimates of undiscovered recoverable oil and gas resources in the United States were published in U.S. Geological Survey Circular 725 in mid-1975, based on data through 1974. In the 1975 study, 47 onshore provinces included within 11 regions, were evaluated. Three of these geologic provinces, (1) West Texas-Eastern New Mexico, (2) North Slope, Alaska, and (3) the Overthrust belt of Idaho-Utah-Wyoming, were selected as case studies to show the relation between the 1975 resource assessments and subsequent exploration results.
U.S. Geological Survey resource estimates for the maturely explored West Texas-Eastern New Mexico province range from 4 to 14.4 billion bbl of oil and 35 to 100 Tcf of gas, based on the 95 and 5% probability percentiles. Exploration wildcat drilling from 1974 through 1977 resulted in more than 840 oil and gas discoveries, mainly of small field or pool size. Exploration results in this province, based on finding rates and the amounts of oil and gas discovered, do not appear to have met the resource predictions.
Resource estimates for the immaturely drilled North Slope province range from 3 to 10 billion bbl of oil and 7 to 25 Tcf of gas. About 40 wildcats were drilled from 1974 through 1977, resulting in 15 successful oil wells and 1 gas well. Two of the oil discoveries appear to be in the > 50 million-bbl field size ("A" class). Resource predictions appear to have been met, or exceeded for the North Slope, based on this recent exploration performance.
Estimates of resources for the Overthrust belt, the most recently successful of the three provinces, range form 0 to 0.2 billion bbl of oil and 0 to 1.1 Tcf of gas. Three gas and five oil fields were discovered out of 89 total wildcats drilled from 1974 through 1977, at least four of which are estimated to be of "A" field-size class or greater. Exploratory performance to date has exceeded the earlier resource estimates in the Overthrust belt.
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