RESERVOIR QUALITY, DIAGENESIS AND SEDIMENTOLOGY OF THE PALE AND SUBU SANDSTONES: RE-VISITING THE EASTERN PAPUAN BASIN, PAPUA NEW GUINEA

Two shallow diamond drill holes (Subu–1 and Subu–2) continuously cored in August and September 2001 by InterOil Australia represent the first sub-surface penetrations of reservoir quality sandstones in the Eastern Papuan Basin of Papua New Guinea. These wells intersected two sedimentologically distinct thick quartz sandstones (>100 m). The upper sandstone unit is Campanian in age and is correlated with the Pale Sandstone, whereas the lower sandstone is of Turonian age and has not been reported previously, and is tentatively named as the Subu Sandstone in this paper.The core has been the subject of detailed reservoir quality and diagenetic study as part of a multi-disciplinary study conducted by CSIRO Petroleum. The results of the reservoir quality portion of this study form the basis of this report and demonstrate the following:There are two distinct depositional systems present with a lower sandy slope apron and basin floor fan system (Subu Sandstone) and a younger upper shoreface-shallow marine depositional system (Pale Sandstone).While the porosity and permeability data for subsurface samples (5 to 16% and 0.1 to 1000mD) are lower than previously reported by Boult and Carman (1990) for surface samples both the sandstone units demonstrate thick, good reservoir quality reservoir capable of holding significant volumes of hydrocarbons.Bitumen is present in the pore space through out the sandstones in both wells. The presence of biodegraded hydrocarbons demonstrates that liquid hydrocarbons have been generated in the basin and have either migrated through the Subu and Pale sandstone or have been reservoired in them.Associated with the bitumen is pyrite precipitated as an in-situ by-product of shallow biodegradation of the parent liquid hydrocarbon as indicated by sulphur isotope analysis.Diagenetic effects include compaction (the dominant control on reservoir quality), minor quartz cementation, minor secondary porosity generation, and in thin zones localised carbonate cementation.Despite their very different depositional settings and age difference the thin section petrology of the Pale and Subu sandstones are very similar. The subtle difference between them is textural (grain size, sorting) and detrital clay content. The Subu Sandstone is typically finer grained, displays a higher degree of sorting and has a higher detrital clay content than the Pale Sandstone.The character of these sandstones may have as much to do with provenance as with depositional environment and may indicate a separate quartz-rich depositional system sourcing sediment from the Australian craton independent of the Fly Platform Toro/Imburu systems.