Topic 1 Introduction
The job of a petroleum explorationist is to find oil and gas. One measure of his ability is the wildcat success ratio or the number of dry holes drilled for each one that produces commerical petroleum. Wagner and Iglehart (1974) reported that 25,562 new field wildcat wells drilled in the United States between 1969 and 1973 found 572 significant new fields — those containing more than 1 million barrels of oil or 1 bcf gas (AAPG categories A through E). In other words, only one wildcat in 45 led to significant production in this five-year period.
At present, explorationists rely almost entirely on seismic methods of geophysics to locate subsurface features and define traps. Although “bright-spot” geophysical procedures offer hope of identifying the presence of gas in some types of reservoirs, there is no generally applicable method for deciding whether any particular structure will contain oil, or be empty.
Let us assume for a moment that geophysics will be developed to the point where every subsurface feature can be defined unambiguously, including lithologic changes which control stratigraphic traps. What will be the chance of finding oil if a structure is drilled? Statistically it will be the same as the ratio of the number of traps containing oil to the number that do not. Because many wildcat wells test structures, the wildcat success ratio gives a first approximation to the ratio of productive to dry structures of 1 to 45. However, some wildcat wells fail to penetrate structures. If we assume