ABSTRACT: Overpressuring and Hydrocarbon Expulsion

WAPLES, DOUGLAS W., and AKIHIKO OKUI, Japan National Oil Corp., Chiba, Japan

We have applied a computer model, which simulates burial, compaction, heating, hydrocarbon generation, and fluid expulsion from source rocks, to study the expulsion of hydrocarbons from source rocks that become overpressured. The onset and end of the conditions that produced overpressuring are specified by the user, as are both the rate of fluid expulsion during overpressuring and the rate of return of the rock to hydrostatic pressure once the conditions that created overpressuring disappear.

Not surprisingly, overpressuring results in a decrease in expulsion efficiency of oil and greater amounts of cracking of oil to gas as long as the overpressure-inducing conditions are in effect. However, during the return to normal pressure, oil expulsion is highly efficient. Therefore, if overpressuring ends before cracking becomes extensive, its only effect is to increase slightly the depth at which oil expulsion occurs. However, since overpressure-inducing conditions often include high sedimentation rates, oil expulsion could actually occur earlier than in the absence of overpressuring.

In contrast, where source rocks remain overpressured longer, oil expulsion efficiency is lowered as oil is cracked to gas. As long as fluid is leaking from the overpressured rocks, gas will be expelled preferentially over oil or water. Therefore, overpressured source rocks at high maturity levels will have high expulsion efficiencies for gas.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)