Geochemical comparison of fluid inclusion and present-day oil accumulations in the Papuan Foreland – evidence for previously unrecognised petroleum source rocks

Abstract

Petroleum production in Papua New Guinea is limited to the Papuan Fold Belt. Present-day oil accumulations in fields such as Iagifu, Hedinia and Gobe are thought to be mainly derived from clay-rich, Jurassic marine source rocks containing terrestrially-derived organic matter that were deposited under oxic conditions. In this study, other generative source rocks in the Papuan Foreland are indicated using fluid inclusion (FI) oil data from three wildcat wells Koko-1, Bujon-1 and Kimu-1, which were drilled on the Bosavi Arch. Detailed organic geochemistry of these FI oils, and associated free oils or oil stains, suggests that none of these were generated from the aforementioned Jurassic source rocks. Instead, at least three different generative source rocks are indicated. Firstly, a marine, probably Late Cretaceous or younger source rock deposited under reducing conditions is indicated by the Bujon-1 FI oil. This source rock had a moderate contribution from higher plant organic matter, including oleanane and/or lupane. The Bujon-1 FI oil has similarities with FI oils from the Iagifu-7x and P’nyang-2X wells in the Papuan Fold Belt. Secondly, an algal-dominated, lacustrine source rock is indicated by the biodegraded Koko-1 RFT crude oil, the unbiodegraded FI oil from Koko-1, and the Bujon-1 oil stains from the Toro and Imburu formations. These lacustrine oils have high abundances of tricyclic terpanes, pregnane, homopregnane and gammacerane, contain β-carotane, and have high C 26/C 25 tricyclic terpane ratios. The source of the lacustrine oils is inferred to contain moderate amounts of terrestrial organic matter, indicated by a dominance of C 29 steranes and abundant aromatic higher plant biomarkers, and could be gypsiferous shales embedded in Oligocene-Miocene carbonates, or Triassic-age sediments deposited on igneous basement during early rifting associated with the break-up of Gondwana. The Kimu-1 FI oil was generated from a mature, marine, suboxic source rock, but has a different geochemical composition to free oil from a similar interval, which was derived from a carbonate-rich source rock. None of the FI oils correlate well with oil families previously described in Papua New Guinea. Accordingly, the oils recovered from FIs in this part of the Papuan Basin support the presence of laterally varying source rock compositions and qualities, and indicate that there are opportunities to develop new charge models for features across the Papuan Foreland. Furthermore, Koko-1, Bujon-1 and Kimu-1 FI oils have different geochemical compositions compared to the associated free oils or oil stains, indicating that each of these localities has a multi-phase charge history. Further work to lower exploration risk in the Papuan Foreland should be directed to understanding the distribution, maturity and expulsion history of these newly recognised potential source rocks.