A new geochemical-sequence stratigraphic model for the Mahakam delta and Makassar slope, Kalimantan, Indonesia

The generally accepted geochemical-stratigraphic model for the Mahakam-Maka ssar area downgrades the potential for commercial deep-water hydrocarbon ac cumulations on the outer shelf, yet fails to explain recent discoveries in this area. According to this model, effective middle Miocene coaly source r ocks 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 o uter shelf and directly influenced recent oil and gas condensate discoverie s in this area. The middle Miocene source rock interval is not buried as de eply as was previously believed and is now within the oil window based on o ur regional seismic reinterpretation and source-specific kerogen kinetics. Genetically distinct petroleum accumulations are charged from local kitchen s between anticlinal structural trends aligned parallel to the coastline. H igh-resolution geochemistry confirms that crude oil samples from these diff erent 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 terrig enous type III organic matter are recognized. The highstand, lowstand 1, lo wstand 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 fie lds represented by these samples. These fields account for about 13 of the 16 BBOE (billion bbl of oil equivalent) estimated ultimate recoverable rese rves in the entire Kutei basin. (1) Waxy highstand oils (e.g., Handil, Nilam) occur mainly onshore in middl e Miocene-Pliocene reservoirs. These oils originated from middle-upper Mioc ene coal and shale source rocks deposited in coastal-plain highstand kitche ns 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 m ostly 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 ge nerally are more mature than lowstand 1 oils and originated from lower-midd le 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 maturit y from middle Miocene suboxic marine shales deposited near maximum flooding surfaces. Our three-dimensional geochemical-stratigraphic models for highstand and lo wstand source rocks indicate that less fractional conversion of the kerogen occurred than had been predicted by the generally accepted stratigraphic m odel and classic type III kerogen kinetics; furthermore, two-dimensional fl uid 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 dee pwater 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.