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Mechanisms of Hydrocarbon Migration in Mahakam Delta, Kalimantan, Indonesia

B. Durand, G. Bessereau, P. H. Ungerer, J. L. Dudin

In the Mahakam delta, hydrocarbons formed from landplant debris, either dispersed in clays or concentrated in coal levels. The hydrocarbon zone is located partly or entirely in overpressured zones.

Hydrocarbon migration is primarily a polyphasic mechanism, i.e., water and hydrocarbons move in separate phases. When hydrocarbon generation occurs in normally pressured zones, hydrocarbons are easily expelled to close carrier beds. Then they migrate toward the top of structures through a network of abundant interconnected sand bodies. However, most hydrocarbons are generated in overpressured zones, in which they move preferentially toward the structural highs. Simultaneously, excess pressure is transmitted to the top of the structures because of the sedimentary load in the synclines, which results in a high pressure gradient at the top. This pressure gradient facilitates hydrocarbon filtration from overpressured zones to normally pressured zones, or it may cause hydraulic fracturing, which provides avenues for migration.

Gas-rich hydrocarbons formed in deep overpressured zones, probably in a single phase owing to high temperature and pressures. The passage from overpressured zones to normally pressured zones resulted in decreased temperature and pressure, which produced several hydrocarbon phases by retrograde condensation. Finally, lighter hydrocarbons pooled above the heaviest ones.

These mechanisms have been similated by a numerical model of basin evolution, including a two-phase migration modulus, and by a numerical model of retrograde condensation.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.