Publication Name: PESA's Eastern Australasian Basin Symposium II
Authors: S.C. George, H. Volk, M. Ahmed, H. Middleton, T. Allan and D. Holland
Date Published: December 2004
Number of Pages: 31
Reference Type: Book Section
Abstract:
Virtually all current oil production in Papua New Guinea is from the Fold belt in the western Papuan Basin, wheremainly Jurassic-sourced oils are reservoired. Jurassic-sourced oils contain low abundances of methylhopanes (2a and 3B isomers), and are typified by high relative abundances ofC19 tricyclic terpane, C24 tetracyclic terpane and rearranged hopanes. These characteristics are typical of terrestrially-influenced marine source rocks containing significant amounts of terrestrial organic matter, deposited under oxic conditions. Puri-1 oil and Family A solid bitumens from the Subu wells (Aure Scarp) correlate with these oils, and indicate that Jurassic source rocks have also generated and expelled oil in the eastern Papuan Basin. Other solid bitumens (Family B) from the Subu wells and also Subu-1 fluid inclusion (FI) oil contain high relative amounts of2a-methylhopanes. These biomarkers are often associated with cyanobacterial input in shallow-water carbonate-rich facies. The high contents of 30-norhopanes, 28,30-bisnorhopane and extended hopanes supports a calcareous-source. The nearby Bwata-1 condensate has similar biomarker distributions, indicating that a calcareous source rock may be regionally significant in the eastern Papuan Basin. In the western Papuan Basin, FI oils from the Iagifu-7X and P'nyang-2X wells (western Fold belt), and in Bujon-1 from the Foreland, have moderate amounts of 2a-methylhopanes, Cz3 and Czs-3o tricyclic terpanes and oleanane (? lupane), indicative of sourcing from a
Late Cretaceous or younger marine source rock containing only minor terrestrial organic matter. Biodegraded oil and pristine FI oil from Koko-1 in the Foreland have high relative abundances of 3~-methylhopanes, which together with high Cz61Czs tricyclic terpane ratios and the presence of significant amounts of gammacerane and B-carotane indicate an effective lacustrine source rock. The high amounts of 3~-methylhopanes are probably due to inputs of specific methanothrophic bacteria. Oil stains in Bujon-1 are also likely derived from this source, the age of which is unconstrained, but which must have expelled later than the Late Cretaceous or younger marine source rock which generated the FI oil in Bujon-1. These geochemical data support the notion that future petroleum plays in PNG are not restricted to the Jurassic source-mid Cretaceous reservoir paradigm. Additional effective source rocks are clearly indicated by the presented data, and by previous work on oil seeps. Furthermore, these source rocks may have been deposited in lacustrine and shallow water marine nvironments, with little terrestrial organic matter input, and some are likely to have a wide geographical distribution.