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Volcanic rocks and associated lithic sandstones, hitherto thought to be too tight and impermeable, may well form important new reservoirs in the extensive Permian back-arc basins of eastern Australia. Oil and gas production has already been established in these sediments in the Permian Bowen and Surat basins of Queensland with some new discoveries in tuffaceous rocks. Most of these volcanic rocks were derived from andesitic, dacitic, and rhyolitic island-arc complexes that lay to the east or the open paleo-Pacific side of a back-arc depositional area. The sediment in these back-arc areas is composed mainly of sand, conglomerate, and silt, all with low percentages of quartz and derived from the volcanic arc. These sediments are interbedded with 1-5 m thick subbituminous to bituminous coal seams, which have been mined extensively in open-cut and shallow subsurface areas of the Bowen and Sydney basins. Basin analysis, using coal stratigraphy and deep wells in the Sydney basin, has established several potential reservoir fairways controlled by facies distribution of the cleaner sands. The variation in reservoir properties within many new potential fairways can be illustrated for parts of both the Sydney and Surat basins. A combination of better understanding of the reservoir properties of these volcanic sediments combined with better drilling, completion, and stimulation techniques lends credence to the hope that these back-arc provinces may become important new oil and gas basins.
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