VGP Technical Report 71 - Hydrocarbon charge history and petroleum accumulation, Otway Basin, Victoria.
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Product description:About the Victorian Gas Program: The Victorian Gas Program (VGP) is a comprehensive science-led program, incorporating geoscientific and environmental research to assess the risks, benefits and impacts of potential onshore conventional gas exploration and production. The program is also investigating the potential for further discoveries of onshore conventional and offshore gas in the Otway and Gippsland geological basins and assessing the feasibility of additional onshore underground gas storage in depleted reservoirs around the Port Campbell area. The VGP includes an extensive, proactive and phased community and stakeholder engagement program, through which the results of the scientific studies are being communicated. Executive summary: As part of the Victorian Gas Program (VGP), the Geological Survey of Victoria (GSV) and Commonwealth Scientific and Industrial Research Organisation (CSIRO) entered into a collaborative research agreement with the objective of developing a more comprehensive understanding of the distribution and characteristics of potential hydrocarbon source rocks, their thermal history, and the generation and migration of petroleum in the Otway Basin. The numerous oil and gas shows and discovered petroleum resources are empirical proof of hydrocarbon migration and accumulation in the Otway Basin and key objectives of this investigation were evidence-based assessments of the timing of these key processes, and the geochemical relationship between observed hydrocarbons and likely source rocks. Migration and accumulation were investigated by detection and analysis of hydrocarbon-bearing fluid inclusions that preserve the fluids in the formation at the time the inclusion became sealed and can be used to evaluate charge history. Fluid inclusion petrography may also reveal hydrocarbons that were undetected by standard formation evaluation techniques. More than 300 samples from 77 wells were screened for the presence hydrocarbon-filled inclusions (expressed as Grains with Oil Inclusions, or GOI; and Frequency of Oil Inclusions, or FOI). The sampling program covered the geographic extent of the Otway Basin and predominately focussed on potential reservoirs in the Waarre, Eumeralla and Pretty Hill formations. GOI or FOI anomalies of greater than 2% were detected in the Eumeralla Formation at Fergusons Hill-1 and Tirrengowa-1; in the Pretty Hill Formation at Digby-1, Ross Creek-1, Tirrengowa-1, and Windermere-2; and in the Laira Formation and Casterton Formation at Glenaire-1 ST1 and Digby-1 respectively. The role of natural fractures in hydrocarbon migration is highlighted by FOI anomalies in calcite-filled veins the Eumeralla Formation at Fergusons Hill-1 and quartz veins in Palaeozoic basement at Gordon-1 and Sawpit-1 (South Australia). The absence of significant GOI anomalies in gas-bearing Waarre Formation sandstones in the Port Campbell Embayment and Shipwreck Trough is convincing evidence that these traps did not receive an early oil charge that was subsequently displaced by a later influx of gas, possibly because the Austral 2 source rocks in the area are predominantly gas-prone. The molecular geochemistry of fluids extracted from oil-filled inclusions suggests that the hydrocarbons were generated from terrestrial, higher plant organic matter with some bacterial material that was deposited in well oxygenated fluvial, or lacustrine environments. A statistical approach to characterising the relationship of the fluid inclusion (FI) oils to other oils and condensates from the Otway Basin recognised four groups. Group 1 mainly contains condensates from the Waarre Formation in the Shipwreck Trough but includes the FI oil from the Pretty Hill Formation in Windermere-2. Group 2 comprises oils and condensates from the Penola Trough and includes FI oils from the Pretty Hill Formation in Digby-1 and Haselgrove-1 (South Australia), and from the Laira Formation in Glenaire-1 ST1. Group 3 is geographically and stratigraphically more diverse and, in addition to including FI oils from the Casterton Formation in Digby-1 and from the Pretty Hill Formation in Tirrengowa-1, contains conventional oils from the Pretty Hill Formation in the Penola Trough, and from the Eumeralla Formation in the Port Campbell Embayment. The fourth group contains an FI oil from Tirrengowa-1 and a conventional oil from Windermere-1, both of which are from the Eumeralla Formation. Cluster analysis was also used to examine the relationship between the FI oils and mature potential source rocks. While the Windermere-2 FI oil shows strong affinity with Group A source rocks from the Eumeralla Formation (Austral 2) in the eastern Otway Basin, and Group 2 FI oils can be correlated with Austral 1 source rocks in the Penola Trough (source rock Group B), the other source rock groups defined by the cluster analysis contain a mix of Austral 1 and Austral 2 source rocks. The continuity in gross depositional environment and climate throughout the Early Cretaceous is likely to have resulted in relatively minor variations in organic facies and the associated biomarker abundances in the oils and source rocks, which may contribute to the apparent lack of geochemical differentiation between the Austral 1 and Austral 2 petroleum systems. The timing of oil charge was assessed by integrating Pressure Volume Temperature analysis (PVT), and compositional analysis (x) of the fluid inclusion oils with modelled burial history at five locations across the Otway Basin. Oil emplacement at the five locations is estimated to have occurred during the Late Albian (110 - 100 Ma) and shows remarkable consistency across the basin. This broad synchroneity, combined with the strong GOI anomalies in Ross Creek-1 and Tirrengowa-1, point to the Austral 1 petroleum system being significantly more widespread, extending as far east as Tirrengowa-1. The Austral 1 system has previously been interpreted as mainly confined to the western parts of the Otway Basin. This inference in turn, strongly suggests that mature Austral 1 source rocks are present in the lower part of the Pretty Hill Formation in the eastern part of the basin and, more speculatively, that the underlying Casterton Formation may extend further east than currently mapped. Bibliographic reference: Kempton, R.H., Gong, S., Wang, H., Russell-Cargill, L., Vergara, T., Northover, S., Pickard, A., Trefry, C., Ross, A.S. & Whittam, D.B., 2022. Hydrocarbon charge history and petroleum accumulation, Otway Basin, Victoria. VGP Technical Report 71. Geological Survey of Victoria, Department of Jobs, Precincts and Regions, Melbourne, Victoria, 94 pp. Download: The downloadable version of this report is supplied in PDF format (27 MB), Att A1 (PDF 95 MB) & Att A2 (XLSX 152 KB). | ||||||
