Generation and Migration of Petroleum in the Mahakam Delta, Indonesia

B. Durand, G. Bessereau, B. Doligez, J. L. Oudin

The Mahakam delta, located east of Kalimantan, Indonesia, contains sediments of Miocene-Pliocene age. Their thickness may reach more than 8,000 m in places, and sections which have been drilled are generally overpressured below 3,000 to 4,000 m. Petroleum was formed essentially from a kerogen originating in terrestrial land plants grown in an equatorial climate. The kerogen may occur either dispersed in clays or concentrated in humic coal beds. Petroleum potential (oil and gas) of this kerogen at the beginning of catagenesis is 200-250 mg HC/g organic carbon on the average (as measured by Rock-Eval pyrolysis) but is highly variable around this mean value (100-400 mg HC/g organic carbon approximately). Although the kerogen is of terrestrial origin, the kerogen-containing s diments have the capacity to produce and expel oil at depth, as shown by large quantities of oil pooled in sandy reservoirs together with gas in a relatively small area. Depth of top oil "kitchen" varies from 2,500 to 4,000 m and is greater in synclines than on top of structures. Present isotherms follow more or less the same pattern. Migration is very recent and is still at work. Light hydrocarbons have migrated farther from their source than heavy ones did. Thus condensate in pooled gas and the density of pooled oil tend to increase with depth. These variations of oil and gas compositions along secondary and tertiary migration routes are likely to be provoked by evaporative fractionation processes.

Basin modeling indicates that coal beds in deltaic plain and delta front facies may be responsible for most of the oil supply to reservoirs. Present temperature patterns are likely to be influenced by hydro-dynamism in shallow sediments.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.