Authors / CoAuthors
Boreham, C.J.
Abstract
Oil sourced from terrestrial organic matter accounts for over half of Australia?s oil reserves. The majority of this lies with the 4 billion barrels of recoverable oil in Late Cretaceous?Early Tertiary reservoirs of the Gippsland Basin. However, the role of oil expulsion from coal still raises considerable debate, both in the regional and global context. This question was addressed by a detailed gas-oil-source correlation study in the Bass Basin with complementary geochemistry on gas, oil and coal from the adjacent Gippsland Basin. Both basins shared a similar extensional tectonic and depositional setting throughout the Cretaceous to Tertiary leading to the breakup and isolation of continental Australia. Potential oil-prone source rocks in the Bass Basin are the early Tertiary coals. These coals have hydrogen indices (HI) up to 500 mg HC/gTOC and H/C ratio of 0.8 to 1.0. Associated disseminated terrestrial organic matter in claystones is mainly gas prone. Maturity is sufficient for oil and gas generation with vitrinite reflectance up to 1.8 % attained solely through burial. The key events in the process of petroleum generation and migration from the effective coaly source rocks in the Bass Basin are: (i) the onset of oil generation at a vitrinite reflectance (VR) of 0.65 %; (ii) the onset of expulsion (primary migration) at a VR of 0.75 %; (iii) the main oil window between VR of 0.75 % and 0.95 %; and, (iv) the main gas window at VR >1.2 %. Sub-economic oil accumulations in the Bass Basin form a single oil population, based on carbon isotopes and biomarker, and are distinct from the oils from the Gippsland Basin. Natural gases are generated over a broader maturity range than the oils but nonetheless are associated with the same source rocks. Oil-to-source correlation in the Bass Basin based on biomarkers shows that the latest Paleocene?Early Eocene coals are the effective source rocks in the Bass Basin and provides the strongest geochemical evidence yet that coal has sourced petroleum in Australia. Carbon isotopes provide the main discrimination for coals, and their derived gases and oils, with the Early Eocene coals being the most depleted in 13C compared to older Late Cretaceous and Early Tertiary coals. It is likely that the carbon isotopes reflect both secular changes in the isotopic composition of atmospheric CO2 and floral influences, with the Early Eocene isotopically-light, angiosperm-dominated coals and the Late Cretaceous?Paleocene isotopically heavier, gymnosperm-dominated coals being the sources for the oil in the Bass and Gippsland basins, respectively.
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nonGeographicDataset
eCat Id
60127
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Cnr Jerrabomberra Ave and Hindmarsh Dr GPO Box 378
Canberra
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Australia
Keywords
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- External Publication
- ( Theme )
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- organic geochemistry
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
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2003-01-01T00:00:00
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