Recent progress in organic geochemical and other fields of earth sciences has made feasible the development of methods for evaluating the likeliness of hydrocarbon occurrence in an undrilled trap. A fundamental step in hydrocarbon charge prediction is to determine whether an undrilled trap has had access to hydrocarbon migration from mature source rocks.

Areas underlain by mature source rocks are called "petroleum generative depressions" or "hydrocarbon kitchens." A "generative basin" is defined as a sedimentary basin that contains one or more petroleum generative depressions. Mapping generative depressions is achieved by integrating geochemical data relevant to maturation and organic facies with structural and stratigraphic information derived from seismic surveys and deep wells.

Locales of high success ratios in finding petroleum are called "areas of high potential," "plays," or "petroleum zones." A rapid worldwide review of 12 sedimentary basins, described in order of geotectonic style, reveals the following regularities:

1. The zones of concentrated petroleum occurrence "areas of high potential" and high success ratios are genetically related to oil generative depressions or basins. These depressions are mappable by integrated methods (geology, geophysics, and geochemistry).

2. The largest petroleum accumulations tend to be located close to the center of the generative basins or on structurally high trends neighboring deep generative depressions.

3. Migration distances commonly range in tens rather than hundreds of miles and are limited by the drainage areas of individual structures. Thus the outlines of generative depressions commonly include most of the producible hydrocarbon accumulations and the largest fields. Unusual cases of long distance migration are documented on certain foreland basin plates where stratigraphy and structure permitted uninterrupted updip movement of oil.

These three regularities provide powerful analogs for forecasting areas of high petroleum potential in undrilled or sparsely drilled basins.

It is important to keep in mind, however, that the predictive accuracy of geochemical mapping systems is dictated by the available data base. Hence, at early stages of exploration, geologists should guard from overtaxing geological and geochemical knowledge, and remain alert to the potentialities of the unknown.