Analyses of Denver basin Cretaceous "D" and "J" sandstone fields reveal trends in field size and areal extent that should be helpful in predicting results of new exploration in areas with similar stratigraphy and geologic history. Within the 11-county "fairway" (excluding Arapahoe and Elbert Counties, Colorado), during the 1949-1969 period, there were 9,512 exploratory wells (11% oil, 2% gas) and 8,650 development wells (52% oil, 2% gas).

A log-log plot showing areal extent (in acres) versus ultimate oil recovery of 557 fields (218 abandoned, 339 extrapolated from production-decline curves) is a straight line and may be used in estimating the ultimate recovery of fields that have areal definition but insufficient history for extrapolatable decline curves. As an example, the 4,640-acre Peoria field (limit of present development) should have an ultimate production of 25 million bbl, if it is an average Denver basin field.

"D" and "J" production to January 1, 1970, was 560 million bbl (Colorado, 304 million bbl; Nebraska, 256 million bbl). Estimated reserves are 75 million bbl (Colorado, 37 million bbl; Nebraska, 38 million bbl). The area analyzed contains approximately 17,000 sq mi; 1,700 sq mi (10%) has oil or gas production (more than 400,000 bbl/productive sq mi). Approximately 200 of the 1,700 sq mi has gas production, but during 1971 an additional 1,000 sq mi has been added in the spaced area of Wattenberg gas field.

Oil fields were divided into 16 size classes (ultimate production), each class twice the size of the next smaller class, and the number of fields was plotted versus the size on semi-log paper. The resulting plots show a log-normal size distribution for both Colorado and Nebraska. It would have been possible, given a projected number of wildcat wells, to predict the approximate number and sizes of fields found in Nebraska from Colorado data, or vice versa.

Extension of the "fairway" into Arapahoe County and the northern townships of Elbert County should result in a predictable number of fields, and their size distribution should follow the pattern developed by past exploration. At least 60 million bbls should be added to the basin oil reserves (including Peoria), if there is sufficient exploratory drilling. Eleven percent of the wildcats should be oil discoveries and 2% of the wildcats should discover fields of one million bbls or more.

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