Abstract

We have performed a series of simulations varying the geological/geochemical input data for our computer model, which calculates burial, compaction, heating, hydrocarbon generation, and fluid expulsion as a three-phase bulk flow process dependent on relative permeabilities to each fluid. Because relative-permeability curves for source rocks (shales) are difficult to measure in the laboratory, we estimated them based on systematic changes in petrographical features observed in reservoir sandstones (Okui and Waples, in press). Our model also considers adsorption of oil on kerogen.

Results of simulations revealed that oil expulsion is minimal until oil saturation increases to a point where the relative permeability of the source rock to oil is much higher than to water. In general, low amounts of generation (in case of low TOC value or poor kerogen quality) or high porosity in source rock (for example, in case of overpressuring) prevent oil saturation from reaching the necessary level for expulsion. However, our simulations also reveal that overpressured rocks must be very tight and that the overpressure must develop at shallow depths in order for oil expulsion to be seriously inhibited.

We believe these observations may help explain the distribution of oil and gas fields in Tertiary deltaic sedimentary basin in the southeast Asia, such as the Mahakam Delta of Indonesia and the Baram Delta of Malaysia. In these area, oil fields are located onshore or near shore in older sediments, whereas gas fields are found offshore in younger sediments. Because of rapid sedimentation in the later stage of deltaic evolution, the overpressured zone developed at a shallower depth in the offshore areas. There it inhibited oil expulsion and caused oil cracking to gas in the source rock prior to expulsion.