A series of petroleum super systems have been identified in Australia ranging in age from Proterozoic to Cenozoic. Each super system has been divided into a number of systems based on an understanding of the distribution of organic-rich-rocks that appear to have generated hydrocarbons. The discovered hydrocarbons in the sedimentary basins of the North West Shelf of Australia appear to be part of four super systems, the Cambrian to Devonian Larapintine, Carboniferous Transitional, Permian Gondwanan, and Mesozoic Westralian. However, because most of the information on the super systems and their component systems is derived from trapped hydrocarbons and on-structure penetrations of organic-rich rocks the picture is very incomplete.
How many systems are present on the North West Shelf? How many should we expect to find? To answer these questions five approaches are being pursued in the basins of the North West Shelf. The first involves determining whether sediments were deposited in basins in the North West Shelf during periods of global or more widespread organic-rich-rock deposition. Secondly, a tectonostratigraphic framework for the North West Shelf is under construction to help predict the most likely periods of organic-rich-rock deposition. Organic-richrocks are most likely to be preserved during periods when the rate of generation of sediment accommodation space is at a maximum, often at the beginning of a new basin phase. In addition, a more detailed analysis of the structural/stratigraphic framework aimed at defining the distribution and character of organic-rich rocks and the drainage divides between them within each basin phase in specific sub-basins constitutes the third approach.
At an even finer scale studies of trap integrity and charge history of a number of the fields and traps are underway. The fourth approach involves multivariate statistical analysis of geochemical data derived from liquid hydrocarbons obtained from wells in most areas of the North West Shelf. Up to twenty-three attributes have been correlated into a series of ‘oil families’. The oil families have been mapped geographically to gain insights into the location of the likely sources for the liquids. Lastly, an information system to access petroleum systems, plays and play elements within the tectonostratigraphic framework is under construction. We hope to undertake a regional gas maturity study within the next two years.
The combination of petroleum systems analysis with basin analysis techniques is paying off. A number of new systems have been defined. Recent work in the Browse Basin suggests that a number of new systems may have been active.