Oil and gas are not at rest in the sedimentary mantle of the earth. They want to leave whether they are finely dispersed or whether they are concentrated in a trap in a reservoir rock. A wide variety of possible mechanisms exists by which they can escape. These include diffusion, continuous single phase flow, solution of oil in gas or gas in oil, and solution in water derived from compaction, clay diagenesis or meteoric sources. The problem is to quantify the possible mechanisms and to rank their relative importance under a given set of physical, chemical, and geologic conditions. The quantitative importance of the various proposed mechanisms can vary by orders of magnitude depending upon the physical, chemical, and geologic conditions.
During the past decade oil-to-source correlations have become reliable and the timing of peak generation and concomitant migration have been sufficiently quantified to permit the geologist-geochemist to make estimates of when and how much petroleum moved from one location to another. Combined with a knowledge of the physical, chemical, and geologic conditions at the time of migration, such quantitative descriptions of subsurface petroleum transfer may permit an empirical test of the applicability of the various proposed migration mechanisms. The application of this technique to selected areas suggests that (1) meteoric water extracted the heavy oil in the Lower Cretaceous sandstones of Alberta from the source rocks, (2) continuous phase flow is a requirement for the petroleum leaving the source rock in the Los Angeles basin and (3) the petroleum in most of the major accumulations in the world did not leave its source rock dissolved in water derived from compaction or clay diagenesis.