UNA VISIÓN GENERAL DE LAS DIVISIONES DE SERVICIOS FINANCIEROS DE LAS COMPAÑÍAS

The Centralian 1 petroleum system is hosted within Neoproterozoic carbonates, evaporates and shales in the Amadeus, Ngalia, Georgina, Officer, Birrindudu and Savory Basins of mainland Australia. These basins were all once part of the Centralian Superbasin, which covered much of Australia although the extent is not fully known (Logan et al., 1999). A map of the Centralian Superbasin, Figure 6.2, shows possible Tasmanian components. Preiss (2000) presents evidence showing connections between the Centralian Superbasin, the Adelaide Geosyncline, Tasmania and the Ross Orogen thus suggesting that a much greater area for the Centralian deposition than just the dismembered basins mentioned above. Oil and gas shows have been discovered within this system but as yet no commercial discoveries have been made. The system has analogs in Siberia and Oman where commercial fields are currently producing from Precambrian source rocks.

Amadeus Basin Georgina Basin Adelaide Geosyncline Officer Basin QLD NSW VIC SA NT WA TAS ACT 0 500 km N

Figure 6.2. Component basins of the Centralian Superbasin are shown in solid green with the outline of the accepted extent of the superbasin shown as a yellow continuous line. Dashed yellow lines show possible extensions to the superbasin, as suggested by Preiss, in northwestern Australia and near the South Australian-Victorian border. Possible extent of the superbasin in Tasmania is shown in solid yellow. Map based on Logan et al., (1999) and Preiss (2000).

The location of Tasmania during the Neoproterozoic is unknown but Calver and Walter (2000) have noted similarities between sediments of this age on King Island and the Adelaide Fold Belt. Li and Powell (2001) have suggested that the Precambrian parts of Tasmania may have been part of a southern extension of the South China Block, which broke away from the rest of Australia during the break-up of Rodinia. Tasmania later broke away from the South China Block and was transported to its present position by at least the Early Ordovician. Recent age determinations from metamorphic rocks on western King Island indicate possible Mesoproterozoic basement in southeastern Australia and this removes a major problem in the correlation of the Adelaide Fold Belt with northwestern Tasmania.

Deformation ages of 1300 Ma for King Island may correlate with similar aged events in the Musgrave Block and also with a metamorphic belt hidden beneath the Transantarctic Mountains (Berry et al., 2005). These correlations would suggest that northwestern Tasmania has remained near its present position since the Mesoproterozoic. There are definite correlations between Neoproterozoic sediments on King Island and some within the Rocky Cape Block of northwestern Tasmania and it is possible these were once all part of the Centralian Superbasin. Correlations have also been made between Rocky Cape Block units and other Precambrian units in Tasmania such as the Weld River Group and the Savage River Dolomite.

In Chapter Four it was shown that the maturity levels of Neoproterozoic potential source rocks in northwestern Tasmania are almost to greenschist facies and so have no capacity to generate hydrocarbons. Potential correlates of Neoproterozoic sequences in northwestern Tasmania may occur beneath the Tasmania Basin but these would also be expected to have at least greenschist facies levels of maturity. The long and turbulent geological history these Neoproterozoic rocks have undergone also reduces the potential for preservation of any hydrocarbons generated during times when potential source rocks may have been mature for generation.

From the results of drilling and geophysical investigations it would appear that sediments of Neoproterozoic age are widespread but due to their high maturity they have no potential as source rocks. The results of this investigation demonstrate that it would be futile to explore for hydrocarbons generated from Neoproterozoic source rocks, onshore Tasmania.

6.3. Hydrocarbon potential of Permian coals and associated siltstones, onshore Tasmania.

As shown in Chapter Three, the Permian coals and associated siltstones investigated during this study have good to excellent source potential. Maturity, as shown in Chapter Four, ranges from immature in northern Tasmania to marginal oil maturity near St Marys and then to oil maturity across the southern Tasmania Basin. In the far southern Tasmania Basin these rocks are within the gas range of maturity as shown in Figure 4.2.

Permian coal and siltstone source rocks have generated and expelled both oil and gas in a western outlier of the Tasmania Basin, near Zeehan, as described in Chapter Five. A likely contributing factor to the generation and migration of oil/gas near Zeehan was the high heat flows in the region during rifting events associated with the separation of Tasmania and Antarctica (Kohn et al., 2002). Deep erosion has also occurred as evidenced by only scattered remnants of Permian-Triassic sediments remaining in the west coast region. Measured maturity of potential Permian petroleum source rocks is further evidence for deep erosion showing that up to two kilometres of overburden is now missing. Fission track studies also indicate that kilometre scale denudation may have occurred in this area during the Late Cretaceous (Kohn et al., 2002). Further evidence of extensive erosion of this area can be deduced from the sedimentary fill in offshore Strahan and Port Davey Sub Basins where sedimentation started in Late Cretaceous (Moore et al., 1992).

The Permian source rocks considered in this study appear to have limited distributions within the Tasmania Basin and occur only as thin discontinuous beds (<700 mm). No evidence was found during this study for petroleum generation within the Tasmania Basin from the coal or siltstone source rocks investigated.