Source Potential Index (Spi) As A Hydrocarbon Prospectivity Ranking Factor In Australian Basins.
Spry, Trent B.
Honours Degree, 1993
University of Adelaide
A genetic classification scheme was developed by Demaison and Huizinga (1991) to allow a direct comparison of the petroleum systems associated with prospective basins. Their scheme includes three main factors: a charge factor or source potential index (SPI), a migration factor and an entrapment factor. This thesis classifies a number of Australian basins using a Demaison and Huizinga scheme modified by a new migration factor.
SPI, which combines source rock richness and thickness into a single parameter, was determined for 13 Australian basins (sub-basins). The Rock-Eval pyrolysis numbers, S1 + S2, were combined with average source rock thickness determined from source isopach maps in the hydrocarbon kitchen. Marine shales of Jurassic age were considered as the main oil-prone source in the Barrow, Dampier and Vulcan sub-basins and Browse Basin, a Permian marine shale as the source in the Petrel Sub-basin, middle Jurassic shales and coals in the North and South Perth basins, while a Permian coal measures source was used in the Cooper-Eromanga basins and Cretaceous shales and coal measures were the source considered in the Otway, Bass and Gippsland basins. The quality of each source was assessed with respect to age, palaeolatitude and other modifying parameters. Known reserves and the parameters of the calculations where plotted against SPI. It was shown that SPI generally shows a positive correlation with basinwide reserves. Inherent richness groups and dominant controlling factors in different SPI ranges were also identified.
The recognition of the dominant migration style helps to predict the location of zones of petroleum occurrence in relation to the hydrocarbon kitchens. Migration is considered in terms of lateral drainage, vertical drainage and minimal migration. Entrapment is considered in terms of high or low impedance. Both migration and entrapment are dependent on the structural framework of the basin with the presence and effectiveness of seals as an additional influence on entrapment.
SPI was considered to be the main factor in the classification scheme but the other factors still need to be considered to fully assess basin prospectivity. Source richness (S1 + S2) is the key element of SPI in the low SPI range but thickness becomes equally important as the source becomes thicker. The same relationship does not hold true when the effectiveness of the source is considered. Thin, areally extensive source rocks appear better at charging petroleum systems than thicker, less extensive source rocks of the same quality.
Finally, improvements for the future were suggested. Expulsion efficiency should be accounted for in the charge factor of the classification and modifications need to be made to both the migration and entrapment factors. This thesis introduces discounting of SPI values because of the low expected expulsion efficiencies of the coal-bearing sources and a new migration factor.