VGP Technical Report 56 - Fault and top seal integrity across the Port Campbell Embayment, Otway Basin, Victoria.

Category: Victorian Gas Program	Product Code: MP-R-162125
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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) is studying petroleum systems components (reservoir, seal and source), to assess the hydrocarbon prospectivity of the Victorian Otway Basin and to estimate the potential for further onshore and offshore discoveries within Victoria’s jurisdiction.

This report is part of a series of studies undertaken by CSIRO as part of a collaborative research program with the GSV. This report investigates the impact of faults on hydrocarbon prospectivity in the Port Campbell Embayment in the onshore Otway Basin. The behaviour of faults over geological time influences the distribution of hydrocarbons in a petroleum system through migration, entrapment and leakage.

A 3D structural model was constructed to provide structural insights into fault evolution, reactivation and interaction with regional top seal intervals. Fault analysis was performed to highlight the fault architecture and evolution in the region, while fault seal potential was investigated for drilled structures to generate a relative containment potential map. The map reflects the potential for faults to provide cross- and up-fault seal in the Port Campbell Embayment, demonstrating confidence in structural trap integrity in a structurally complex area.

The modelling suggested fault seal integrity and top seal bypass represent an exploration risk in the Port Campbell Embayment. Comprehensive fault analysis indicated peak tectonic activity occurred during the deposition of the Waarre Formation, which then decreased during the deposition of the Belfast Mudstone and Nullawarre Greensand before reaching a low point during the deposition of the Skull Creek Mudstone. The Waarre Formation is the primary reservoir in the Port Campbell Embayment and the Belfast Mudstone is the regional top seal. Faulting during the Turonian-Campanian controlled the distribution of the top seal interval and its structural architecture. Deformation during the Campanian-Maastrichtian controlled fault reactivation, which could influence up-fault flow and top seal integrity. Tectonic extension and fault reactivation were renewed during the early Cenozoic.

The assessment of juxtaposition and membrane seal capacity suggests that structural closures are expected to rely primarily on juxtaposition seal, where the sandstone rich Waarre Formation is juxtaposed against the Belfast or Skull Creek mudstones. The low permeability of these units is expected to impede lateral flow. This is demonstrated in drillholes McIntee-1, Lavers-1 and Skull Creek-1, which represent gas discoveries with juxtaposition seal. The sealing potential of a fault juxtaposing the Waarre Formation against the sandstone rich Nullawarre Greensand is controlled by the seal capacity of the fault membrane. This represents low structural trap integrity and is a key containment uncertainty in the region. The fault-seal potential of this scenario is heavily reliant on clay smear from the Belfast Mudstone, as demonstrated by either dry prospects (for example Narringal-1, Melba-1 and Vogel-1) or hydrocarbon discoveries (for example Skull Creek-1, Skull Creek West-1, Wallaby Creek-1 and Seamer-1).

A relative containment potential map, generated from fault seal analysis, reflects the potential for faults to provide cross- and up-fault seal in the Port Campbell Embayment. The map was derived from key controlling parameters such as the distribution and thickness of the Belfast Mudstone and Nullawarre Greensand, and fault plane geometry and strain distribution. The map illustrates a higher probability of structural trap integrity in the central Port Campbell Embayment due to the distribution of the Belfast Mudstone, the thinning of the Nullawarre Greensand and restricted Neogene fault reactivation. The probability of structural trap integrity is relatively lower on the northern margin of the Port Campbell Embayment due to a combination of a thin Belfast Mudstone and a thick Nullawarre Greensand.

When incorporated with untested seismic anomalies, the relative containment potential map highlighted potential undiscovered fields with a higher probability of structural trap integrity. This suggests exploration opportunities exists in the Port Campbell Embayment with potentially adequate containment to the east around the North Paaratte structure and to the north-west near the greater McIntee structure.

Bibliographic reference:
Swierczek, E., Langhi, L. & Strand, J., 2021. Fault and top seal integrity across the Port Campbell Embayment, Otway Basin, Victoria. VGP Technical Report 56. Geological Survey of Victoria, Department of Jobs, Precincts and Regions, Melbourne, Victoria.

Download:
The downloadable version of this report is supplied in PDF format (16.8 MB), Att A1 (ZIP 271 MB) & Att A2 (XLSX 25 kb).