Origin and Migration of Petroleum in the Gidgealpa Ridge Area, Cooper /Eromanga Basins, South Australia

Hallmann, Christian Olivier Eduard (2004). Origin and Migration of Petroleum in the Gidgealpa Ridge Area, Cooper /Eromanga Basins, South Australia. Masters thesis, Universität zu Köln. Open Access

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Abstract

The oil play of the Permo-Carboniferous Cooper and Jurassic-Cretaceous Eromanga Basins in South Australia is complicated to unravel due to the presence of multiple source rocks that bear similar geochemical characteristics and has consequently been subject to intense debate. The underlying Warburton Basin, although initially regarded an exploration target, subsequently has never been seriously considered to having participated in the oil play. The Cooper/Eromanga couplet is Australia's largest onshore petroleum province and still bears potential for undiscovered commodities, making a profound understanding of origin and migration of petroleum a prerequisite for efficient future exploration. Being a typical example of structurally trapped oils in stacked fluvial sandstone reservoirs, the Gidgealpa Ridge was chosen as subject for this study. A suite of 13 sandstone cores, 11 source rocks and 24 drill stem test (DST) oils were processed at the University of Cologne by extraction, sequential extraction of sandstone cores to yield residual oils, preparative liquid chromatography and solid phase extraction to yield fractions of saturated hydrocarbons, aromatic hydrocarbons, maltene resins and a carbazole enriched fractions. While biomarker analyses on saturates and aromatics were performed by Dr. Khaled Arouri at the University of Adelaide, molecular characterisation of carbazoles and bulk geochemical characterisation was also done at the University of Cologne. Sequentially extracted residual oils are particularly useful because they allow a temporal resolution of petroleum reservoir filling histories. While hydrocarbon compositions are quickly equilibrated over a few tens of metres in oil columns, there is substantial evidence that the same is not true for polar species such as e.g. carbazoles and for lateral variations in hydrocarbon composition. Carbazoles were initially regarded as molecular markers indicating relative migration distances. However, counterevidence soon appeared, showing the dependency of carbazole distributions on other factors. The results of this study show that, while the distribution of benzocarbazoles seems to depend only on fractionation processes during primary and secondary migration, alkylated carbazole distribution patterns are governed by source rock facies. This new finding allowed the recognition of a Warburton Basin contribution in certain residual oil fractions, yielding the first conclusive evidence that Warburton Basin rocks bear a source potential and have actively participated in the oil play of the Cooper and Eromanga Basins. The application of carbazole facies parameters, benzocarbazole migration parameters and aromatic maturity data on oils and residual oils from the Gidgealpa Field has allowed the reconstruction of its charging history. Generally, Cooper Basin oils exhibit large maturity variations but a uniformly short migration distance, indicating their origin from a nearby deep trough (eastern source kitchen). Eromanga Basin oils are characterized by similarly low maturity values but highly varying migration distances, suggesting that they were sourced by an extensive and shallow source kitchen. More precise consideration of the carbazole distribution in Eromanga Basin oils shows that the Gidgealpa Field has received two discrete Birkhead charges: a Late Cretaceous pulse from an eastern source kitchen, which migrated a short distance, and became locally displaced by a Tertiary charge from a western source kitchen, which had to migrate further before reaching the Gidgealpa Field. Residual oil fractions from the sandy intervals of the Toolachee and Patchawarra Formations indicate that while the former is a source for Cooper Basin Family 2 oils, the latter can unambiguously be tied to the production of Cooper Basin Family 1 oils. After petroleum generation in the Patchawarra Formation, this petroleum was probably retained by the coaly Patchawarra source rock, and underwent further maturation before it was expelled as condensate-like light oil in gaseous solution. The analysis of sequentially extracted residual oils promises to become a powerful tool in reservoir geochemical studies by adding a temporal resolution to otherwise time-integrated samples, thereby allowing a relatively precise reconstruction of an oil field's charging history. The finding of facies dependent markers in the polar fraction of oils is a constructive motivation for further research into the facies and maturity dependency of NSO compounds. As many residual oils are characterized by minimum extracts yields and a high percentage of NSO compounds, conventional biomarker analyses are complicated and polar markers are necessary for a more precise evaluation.

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Thesis (Masters thesis)

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