VGP Technical Report 70 - Molecular geochemistry of petroleum source rocks, Otway Basin, Victoria.

Category: Victorian Gas Program	Product Code: MP-R-169377
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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.

A key component of the collaborative research was an extensive program of sampling and geochemical analysis of potential source rocks in the Otway Basin. Although analysis of bulk geochemical properties has been a routine part of evaluating petroleum exploration wells, the application of molecular geochemistry to characterise the depositional environment, composition, and thermal maturity of the organic matter of source rocks in the basin has been very limited.

The project took a staged evaluation approach, with 225 samples screened for total organic carbon (TOC) content, the majority of which were also subjected to Rock-Eval pyrolysis to determine bulk geochemical properties. Organic matter was separated from 125 samples by solvent extraction and was analysed by gas chromatography-mass spectrometry (GC-MS) to identify the types and relative abundances of organic compounds (biomarkers) in the source rocks. Interpretation of the biomarkers provides more detailed insights into the origin and environment of deposition of the organic matter, which affect the likely type and amount of hydrocarbons that might be generated from the source rock. Biomarkers, especially those derived from aromatic organic compounds, also provide an indication of the thermal maturity of source rocks. Genetic relationships between the source rocks were investigated by cluster analysis of selected biomarkers.

Bulk geochemical analysis indicates that gas-prone Type III kerogen, commonly derived from terrestrial higher-plant material, is the predominant labile organic matter in all the analysed source rock units with minor mixed Type II/III kerogen which is likely to generate condensate. However, 40% of the samples are interpreted to comprise inert Type IV organic matter with no potential to generate hydrocarbons.

Molecular geochemical analysis (GC-MS) reveals low ratios of steranes to hopanes, a predominance of C₂₉ steranes in the C₂₇-C₂₉ homologues, and high abundances of C₂₄ tetracyclic terpanes which are all indicative of terrestrial organic matter input. There is evidence from aromatic biomarkers of significant input of organic material from Araucariacean conifers in several samples from the Casterton, Pretty Hill, and Eumeralla formations. Environmental indicators are consistent with the organic matter having been deposited in predominantly clay-rich, oxic, fluvial or deltaic environments.

Five source rock groups were identified in the Otway Basin by hierarchical cluster analysis of selected biomarker parameters from thermally mature source rocks and oil samples extracted from fluid inclusions. Source rock groups A and B mainly contain Eumeralla Formation samples from the eastern part of the Otway Basin, and Casterton and Pretty Hill formation samples from the Penola Trough respectively. Groups C and E are more heterogenous, containing Eumeralla Formation source rock samples from wells in the eastern Otway Basin, and samples from the Pretty Hill and Casterton formations in the western areas of the basin. Source Rock Group D consists of three samples that are geographically and stratigraphically dispersed across the basin. Continuity in gross depositional environment and climate throughout the Early Cretaceous is interpreted to have resulted in relatively minor variations in organic facies and hence the biomarker abundances, contributing to an apparent lack of geochemical differentiation between potential source rock units.

Bibliographic reference:
Gong, S., Schinteie, R., Vergara, T., Northover, S., Trefry, C., Ross, A.S. & Whittam, D.B., 2022. Molecular geochemistry of petroleum source rocks, Otway Basin, Victoria. VGP Technical Report 70. Geological Survey of Victoria, Department of Jobs, Precincts and Regions, Melbourne, Victoria, 60 pp.

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The downloadable version of this report is supplied in PDF format (10 MB) & Att A1 (XLSX 300 KB).