In formulating strategy for mature areas, management has to consider chance factor for success, as well as the chance of finding sizeable reserves. As a case history, we describe an investigation of the Northwest Shelf of Delaware basin carried out in 1979 to locate areas having the best potential of finding new reserves. In 1983, we tabulated the activity since 1980 to compare our predictions against the drilling results.

The 1979 study had shown that out of a total of almost 200 wildcats drilled during 1974-79, the largest number (63) had been drilled for San Andres (Permian) objective with a success ratio of 33%. Forty-six wells had been drilled for various Pennsylvanian objectives with a success ratio of 40%, and 25 wells had been drilled for Siluro-Devonian targets with a success ratio of 16%. However, based on wildcat success ratios and cumulative-frequency plots of field sizes, the probability of a wildcat discovering reserves larger than 1 million bbl was only 6% in San Andres, 19% in Pennsylvanian, and 7% in Siluro-Devonian.

The 1979 study allowed us to "high-grade" three blocks representing 5% of the total 5.25 million acres. These blocks were considered to have the best potential for San Andres and Siluro-Devonian objectives. Subsequent analysis of 1980-83 drilling results shows that these blocks contain 55% of the 20 new successful San Andres tests and the only new discovery in the Siluro-Devonian. However, as predicted, all of these have been small discoveries. Such studies allow management to develop a course of action for mature areas.

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