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

Abstract

Late Cretaceous sandstones from the Subu-1 and -2 wells (Aure Scarp, Papua New Guinea) contain patchily distributed solid bitumens. The solid bitumens vary in reflectance and geochemistry, indicating that they formed by the biodegradation of two different families of crude oils, termed Family A and Family B. Biomarker characteristics, such as the high abundance of rearranged hopanes and steranes, indicate that the Family A solid bitumens were derived originally from a clay-rich, marine source rock low in sulphur, which had significant input of terrestrial organic matter. These correlate with oils from the Iagifu-7X, Gobe-3X, Omati-1 and Puri-1 wells that are believed to be generated from sources of Jurassic age. The Family B solid bitumens and a fluid inclusion oil from Subu-1 have a high abundance of 2α-methylhopanes, bisnorhopanes, and dibenzothiophene and originated from a marine source rock with a calcareous component. This source rock had a high proportion of prokaryotic organic matter input, a greater sulphur content, and a more reducing depositional environment than the Jurassic source rocks. The lack of correlation with other oils in Papua New Guinea suggests the presence of a second, so far unidentified, generative marine source rock in 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.