Bampo-Peutu(!) Petroleum System, North Sumatra, Indonesia: Chapter 38: Part VI. Case Studies--Eastern Hemisphere

The Bampo-Peutu(!) petroleum system occurs near the northern end of the island of Sumatra, Indonesia. This system, which is within the larger North Sumatra basin, includes 10 gas and gas-condensate discoveries and has produced more than 9.5 tcf of gas and 500 MMbbl of condensate and natural gas liquids (C2-C4+) as of May 1991. Original discovered reserves are estimated at 15 tcf of gas and 1.0 Bbbl of condensate and natural gas liquids. Source rock analytical data and carbon isotopic analyses indicate that the Oligocene Bampo Formation is the principle source of hydrocarbons in the system, with shales of the Miocene Peutu Formation a potential secondary source. These strata are lean in organic matter (TOC < 1 wt. %) and contain primarily type III erogen. However, this lean source rock is offset by the substantial volume of mature to overmature, fine-grained rock in the deeps. Miocene reefal carbonates of the Peutu Formation constitute the primary reservoir strata, while overpressured shales of the Baong, Peutu, and Bampo formations provide excellent seals. Most accumulations occur in structural-stratigraphic traps. Reefs developed preferentially on paleotopographic highs and were later encased in impermeable shales. Multiple migration pathways, including basal coarse clastics, several unconformity surfaces, and localized faults, all provide conduits for hydrocarbon migration from the mature source rocks to the adjacent reefs. The timing of peak migration is estimated at 12-4 Ma. A trapping efficiency of 3.6% is calculated for the entire petroleum system by comparing volumetric estimates of hydrocarbons generated to the amount trapped in known discoveries. A much higher trapping efficiency (40-70% range) characterizes the Arun gas field. Ideal geologic conditions, including centripetal migration from several flanking depocenters, explain this high migration efficiency.