The generally accepted geochemical-stratigraphic model for the Mahakam-Makassar area downgrades the potential for commercial deep-water hydrocarbon accumulations on the outer shelf, yet fails to explain recent discoveries in this area. According to this model, effective middle Miocene coaly source rocks are restricted to updip shelfal areas, whereas age-equivalent rocks on the outer shelf are thermally postmature for generating oil and buried too deeply to preserve good reservoir quality.
Our revised geochemical-stratigraphic model upgrades the potential of the outer shelf and directly influenced recent oil and gas condensate discoveries in this area. The middle Miocene source rock interval is not buried as deeply as was previously believed and is now within the oil window based on our regional seismic reinterpretation and source-specific kerogen kinetics. Genetically distinct petroleum accumulations are charged from local kitchens between anticlinal structural trends aligned parallel to the coastline. High-resolution geochemistry confirms that crude oil samples from these different trends (e.g., Handil-Nilam-Badak, Bekapai-Tunu-Attaka, Perintis-Sisi-Ragat) are genetically distinct.
Two major (1, 2) and two minor (3, 4) petroleum systems dominated by terrigenous type III organic matter are recognized. The highstand, lowstand 1, lowstand 2, and transgressive systems tract oils account for about 46, 31, 15, and 8%, respectively, of the 61 oil samples and about 45, 32, 11, and 12%, respectively, of the estimated ultimate recoverable reserves from the fields represented by these samples. These fields account for about 13 of the 16 BBOE (billion bbl of oil equivalent) estimated ultimate recoverable reserves in the entire Kutei basin.
(1) Waxy highstand oils (e.g., Handil, Nilam) occur mainly onshore in middle Miocene-Pliocene reservoirs. These oils originated from middle-upper Miocene coal and shale source rocks deposited in coastal-plain highstand kitchens now near the peak of the oil window.
(2) Less waxy lowstand 1 oils (e.g., Perintis, Sisi, Ragat) occur offshore in middle-upper Miocene reservoirs. These oils originated from middle-upper Miocene coaly source rocks deposited in deep-water lowstand kitchens now mostly in the early oil window.
(3) Lowstand 2 oils (e.g., Semberah 037) are similar to the lowstand 1 oils but occur mainly onshore in lower-middle Miocene reservoirs. These oils generally are more mature than lowstand 1 oils and originated from lower-middle Miocene coaly source rocks.
(4) Nonwaxy transgressive oils (e.g., Badak) occur mainly onshore in middle-upper Miocene reservoirs. These oils were generated at low thermal maturity from middle Miocene suboxic marine shales deposited near maximum flooding surfaces.
Our three-dimensional geochemical-stratigraphic models for highstand and lowstand source rocks indicate that less fractional conversion of the kerogen occurred than had been predicted by the generally accepted stratigraphic model and classic type III kerogen kinetics; furthermore, two-dimensional fluid flow modeling supports independent geochemical evidence for commingling of oils in the Tunu field from highstand and lowstand kitchens to west and east, respectively. Finally, our model successfully predicted that oil and gas, rather than gas only, would be discovered at the recently drilled deep-water Merah Besar and West Seno fields. Geochemical analyses of oils from the Merah Besar field confirm that they belong in the lowstand 1 oil group.