Postulated and Known Petroleum Systems

A classification of Australian petroleum supersystems has been established by AGSO based on a continent-wide coverage of data for palaeogeography, petroleum systems and oil geochemistry (Bradshaw, 1993; 1994). These supersystems are a framework for understanding hydrocarbon occurrence in Australia that can be used predictively in frontier areas. A petroleum system can be defined as a mature source rock and all its generated hydrocarbon accumulations (Magoon & Dow, 1991). The system includes all the geological factors necessary for the oil and gas fields to exist - that is the source, reservoir, seal, trap, overburden required for maturation and migration pathways. The petroleum system concept stresses the processes involved from source to trap and then the preservation of that accumulation. For the system to operate successfully and hydrocarbon to be accumulated it is essential to have all the crucial elements and have them occur in the correct sequence.

The petroleum system concept has been extended in Australia to establish a broad classification of supersystems (see: Figure1 and Table 1 [PDF 20kB]). These link individual petroleum systems that share the same age and facies of source rock across basin boundaries. Source rock environment is a strong control on hydrocarbon chemistry and maturation behaviour. Hence the supersystem framework is mirrored and verified in the structure of Australian oil families as demonstrated in Edwards and others (1997,1999) and AGSO & GeoMark Research Inc  (1996).  Six Phanerozoic petroleum supersystems are recognised and hydrocarbon occurrences in Proterozoic sequences are indicative of another three supersystems in these older rocks. (Table 1)

Figure 1

The character of the supersystems is imparted by the changing global patterns of climate, tectonics, palaeogeography and biological evolution. Thus, during the lower Palaeozoic Australia was located in tropical palaeo-latitudes and sea levels were high across the continent; and the characteristic facies of the Larapintine Supersystem are carbonates, evaporites and shallow marine clastics. By the Late Carboniferous Australia had slipped into the south polar regions and was in the grip of the Gondwana-wide glaciation. Coarse glacially influenced clastics and terrestrial source rocks typify the corresponding Gondwanan Supersystem. The Westralian Supersystem is related to the breakup of the northern and western margins of the continent in the Jurassic; and the Austral Supersystem is the product of the breakup of the southern margin in the Cretaceous. Some supersystems have been further subdivided to discriminate specific source rock intervals or environments within a general climatic and palaeogeographic setting for example Larapintine 1, 2, 3 and 4 relating to marine source rocks of Cambrian, Ordovician, Late Devonian and Early Carboniferous age. These subdivision also extend across basin boundaries and are at a higher level in the hierarchy than the individual petroleum system related to a single source pod.

References

AGSO and Geomark Research Inc., 1996. The Oils of Western Australia, Proprietary Report, Canberra and Houston, unpublished.

Bradshaw, M.T., 1993. Australian Petroleum Systems. PESA Journal 21, 43-53.

Bradshaw, M.T., Bradshaw, J., Murray, A., Needham, D.J., Spencer, L., Summons, R., Wilmot, J. & Winn, S., 1994 - Petroleum systems in west Australian Basins. In: Purcell, P.G. and R.R (eds), The Sedimentary Basins of Western Australia. Proceedings of the Petroleum Exploration Society of Australia Symposium, Perth, 1994,93-118.

Edwards, D.S., Bradshaw, J., Bradshaw, M., Foster, C.B., Kennard, J.M., Nicoll, R.S., Summons, R.E. & Zumbunge, J.E., 1997. Geochemical characteristics of Palaeozoic petroleum systems in northwestern Australia. The APPEA Journal, 37 (1), 249-277.

Edwards, D.S., Struckmeyer, H.I.M, Bradshaw, M.T. & Skinner, J.E., 1999. Geochemical characteristics of Australia's southern margin petroleum systems. The APPEA Journal, 39 (1), 297-321.

Magoon, L.B., & Dow, W.G., 1991. The petroleum system - from source to trap. AAPG Bulletin 75 (3), 627pp.