Abstract

The petroleum system of the Mahakam delta is investigated using a two-dimensional reconstruction of the history of HC generation and migration along a 80 km long regional section. We find that the classical 'perascensum' model, in which gaseous HC are generated in the deep overpressured, overmature shales, and sweep liquid HC during their vertical migration, needs to be revised. The main conclusion of this study is that the coal-rich, sand-rich and normally pressured delta-plain facies, located in the synclines, need to be considered as the most effective source rock, rather than the deep overpressured marine shales. Also, migration appears to take place mostly parallel to bedding up-dip along structure flanks rather than vertically across bedding. Our model explains the distribution of gaseous HC at Tunu structure, and of oil at Tambora structure. It is consistent with distribution of present-day subsurface temperatures, shown to be affected by recent meteoric water circulation, with coal maturity and with overpressures distribution.