Authors / CoAuthors
Boreham, C.J. | Hope, J.M. | Jackson, P. | Earl, K.L. | Davenport, R. | Krassay, A.A. | Edwards, D.S.
Abstract
Genetic relationships, identified using a combination of molecular and isotopic (carbon and hydrogen) compositions, have been found between natural gases, oils, oil stains, bitumens and potential source rocks in the onshore and offshore Otway Basin. The gas-gas, gas-oil and oil-source correlations herein challenge the validity of some previously accepted oil families and re-enforces the strong compartmentalisation of petroleum systems in the Otway Basin. Previous geochemical studies in the Otway Basin, mainly focussed on the oils and oil stains, have established that the Otway Basin hosts the most diverse array of petroleum systems within Australia. Up to five different oil families have previously been identified. These oils are sourced from a wide range of depositional environments from fresh to saline lacustrine, fluvio-lacustrine to peat swamp and marine, with suspected effective source rock ages from Late Jurassic to Late Cretaceous. Such depositional settings are consistent with the progressive development of source rocks facies intimately linked to basin development from initial rifting to thermal sag. It is now concluded that there is no indigenous representation of the saline lacustrine oil population in the Otway Basin. The geochemical signal is attributed to downhole contamination from gilsonite; a solid bitumen from the Eocene Green River Formation, USA. Oils stains are thought to be a result of primary migration from mature source rocks into juxtaposed sands and are not a strong advocate for secondary oil migration fairways. The natural gases show a strong geochemical association with their respective oils, suggesting that both are generated together from the same source. Also the gases and oils and their effective source rocks have a strong stratigraphic and geographic relationship, indicating mainly short- to medium-range migration distances from source to trap. Gas and oil in the western Otway Basin are sourced from the fluvio-lacustrine Casterton Formation?Crayfish Group sediments while in the eastern Otway Basin the gas and oil from the Shipwreck Trough and its onshore extension are from the coaly Eumeralla Formation sediments. Gas and oil in the central Otway Basin have a mixed source but predominantly are of Eumeralla Formation source Multiple charge histories are also evident with the widespread influx of overmature, dry gas focused in the western Otway Basin and more recently magmatic CO2 influx. Successive natural gas charges have the potential to displace and/or alter the composition of the pre-existing reservoired gas and oil. In-reservoir biodegradation of oil is seen in the shallower reservoirs but this is not a significant risk in the Otway Basin since nearly all reservoired petroleum is below the temperature/depth limits for biologically sustainable life.
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nonGeographicDataset
eCat Id
61142
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Keywords
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- External PublicationArticle
- ( Theme )
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- organic geochemistry
- ( Theme )
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- natural gas
- ( Theme )
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- hydrocarbons
- ( Theme )
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- petroleum geology
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
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2004-01-01T00:00:00
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