Post Rift Oligocene Marine Source Rock, a New Petroleum System in Greater Bangkanai, Upper Kutai, Indonesia

Abstract

Abstract The long-held view and general understanding on the source rock within the Upper Kutai Basin is that it comes from the fluvial-deltaic facies. This deltaic coals and carbonaceous source rock has been proven generating gas with oil in Western Indonesian tertiary basins such as the Miocene Balikpapan Formation in the Lower Kutai Basin, Tanjung Formation in the Barito Basin and TalangAkar Formation in the South Sumatra Basin. The Oligocene carbonate play in the Upper Kutai Basin is under-explored, with exploration historically focusing on the Miocene deltaic and turbidite plays. These carbonates mainly consist of the UjohBilang or Berai equivalent Formation which outcrops along the southern and western margin of the basin, and is seismically imaged in the subsurface, forming on isolated basement highs and large platform areas. Ophir Energy's Kerendan Gas Field in the Bangkanai PSC is the only Oligocene carbonate gas producer in the Kutai Basin. Development drilling on the Kerendan Field and the West Kerendan-1 exploration well has provided new information which, together with a re-evaluation of the existing carbon isotope and other geochemical data has led to a reinterpretation of the source rocks for Kerendan gas. The gas was previously postulated to be generated from Eocene terrestrial source rocks similar to the source rocks that generated oil and gas in the neighboring Tanjung Field in the Barito Basin, 100 kms to the South. The recent carbon isotope data from the Kerendan wells reveals that the gas in the Oligocene carbonate reservoir in Kerendan was generated from a marine source rock and is not terrestrial in origin. In addition there is also a terrestrial component within the gas found at the younger stratigraphic interval.