Aromatic Methylphenanthrene Biomarker and Maturity of Oils Keys to Identifying New Active Source Rocks in the Salawati Basin, Indonesia

Awang H. Satyana
Exploration, BPMIGAS, Jakarta, Indonesia

Salawati Basin, Indonesia is an asymmetric foreland basin with a foredeep kitchen bordered by deformed zone of the Sorong Fault. Early-migrated and moderate to heavy oils distribute in the updip area, whereas late-migrated and light oils and gas distribute in downdip area approaching the kitchen. The hydrocarbon sources have been Miocene Kais and Klasafet mudstones and shales.

The maturity of oils from updip to downdip area is shown by equivalent vitrinite reflectance (VRE) of oil derived from formulation of intensity of aromatic biomarker of methylphenanthrene (MP) (m/z 192). The MP index (MPI) and VRE of oils are increasingly higher towards the downdip area showing that oils are increasingly mature (existing MPI data range from 1.1 to 1.6 or VRE from 0.9 to 1.4 %). However, at the Sorong deformed zone, there are oil seeps which do not follow the characteristic pattern. They show MPI of 0.52 or VRE of 0.7 %. This is peculiar since these seeps, if they were sourced by Kais/Klasafet, should have the highest maturity in the basin because the source rocks in this area are maximum buried and they have entered overmature window as proved by the occurrence of high CO2 gas nearby due to thermal degradation of Kais carbonate.

The presence of low maturity oil seeps in the Salawati foredeep kitchen shows that the oils should had been sourced by younger rocks than Kais/Klasafet rocks and should be in the upper oil window. The expected source is early Pliocene Lower Klasaman shales which have significant terrestrial input than those of Kais/Klasafet. Therefore, based on oil geochemistry, we have identified Lower Klasaman shales as additional source in the basin’s petroleum system.

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