Biomarker evidence for two sources for solid bitumens in the Subu wells: Implications for the petroleum prospectivity of the East Papuan Basin

The fine-grained samples in the Subu wells are thermally immature (VRE 0.5-0.6%), organically lean, and contain type III organic matter with little hydrocarbon generation potential. Most of the solid bitumens have peak oil window thermal maturities (VRE 0.8-1.0%), but some have lowered maturities that are interpreted to be due to co-extraction of indigenous, detrital, low maturity organic matter. Low molecular weight n-alkanes, alkylcyclohexanes, methylalkylcyclohexanes, and alkylbenzenes in many of the samples are attributed to overprinting of pre-existing, variably biodegraded solid bitumens in the sandstones with pristine hydrocarbon charge with a condensate composition. This condensate charge has not undergone any major biodegradation or water-washing, implying that it is a relatively recent event. Solid bitumen from a vein in a Miocene volcanolithic sandstone from the Ouha anticline was generated from an early mature Palaeogene or late Cretaceous source rock that contained predominantly terrestrial organic matter deposited in an oxic environment. This sample proves the existence of a different oil generative source rock in the East Papuan Basin in addition to the two oil source rocks inferred from the Subu data.