VGP Technical Report 35 - Regional 3D geological framework model, Otway Basin, Victoria.

Category: Victorian Gas Program	Product Code: MP-R-161893
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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:
A new regional 3D geological framework for the onshore and offshore Otway Basin has been built using existing seismic and well data held by the Geological Survey of Victoria (GSV). This also included a more detailed 3D model of the Port Campbell Embayment and Shipwreck Trough. This study supersedes previous regional interpretations in scope and detail, integrating seismic, well and potential field interpretations; developing a new, regionally continuous (onshore to offshore) seismic interpretation framework and velocity model.

As part of the Victorian Gas Program (VGP) the Geological Survey of Victoria (GSV) is studying the Otway Basin’s petroleum systems components (reservoir, seal and source), to assess its petroleum prospectivity and to estimate the potential for further conventional gas discoveries onshore and offshore within Victoria’s jurisdiction. This new regional geological framework model will ensure that fit-for-purpose products inform subsequent petroleum systems modelling and a prospectivity assessment and resource estimate.

Geological framework models are constructed using specialist software from existing data, including seismic surveys and well data that were obtained via exploration for underground resources (e.g. oil and gas, groundwater or minerals). Rock layers are interpreted from the datasets and the data points are joined together to create surfaces in three dimensions. These 3D surfaces represent geological units.

This new regional model expands an existing regional-scale onshore model to incorporate the offshore Victorian Otway Basin sedimentary sequence. The sequence includes the top of the Palaeozoic basement to the bathymetric surface/present day land surface and all mappable sedimentary units in the basin including the Late Jurassic to Early Cretaceous Crayfish Subgroup, the Early Cretaceous Eumeralla Formation, the Late Cretaceous Sherbrook Group plus the Cenozoic Wangerrip, Nirranda and Heytesbury groups.

Frogtech Pty Ltd was engaged by GSV to construct the new regional 3D geological framework model. The study comprises two elements – a regional 3D model of the Victorian Otway Basin – onshore and offshore (Element 1) and a detailed 3D model of the Port Campbell Embayment and Shipwreck Trough (Element 2). Element 2 covers the area containing past and existing gas discoveries and production and has the greatest concentration of seismic data.

The seismic interpretation was undertaken using two-dimensional (2D) and 3D seismic data that has been acquired by industry over the past 50 years and is publicly available in the Geological Survey of Victoria’s archives (earthresources.efirst.com.au). Nine regional horizons (including the seafloor and topographic surface) were mapped across approximately 15,000 kilometres of 2D seismic lines and 4,000 square kilometres of 3D seismic data. The nine regional horizons were selected based on the geological properties of potential reservoirs and key structural surfaces, to identify the timing of different stages of basin formation. Ten additional horizons covering the onshore or offshore only were mapped where the data and presence of these units allowed.

The horizons mapped from the seismic data were tied to 242 wells using time depth relationships calculated from 96 key wells, distributed across the onshore and offshore Otway Basin. The horizon interpretations derived from the 2D/3D seismic and well data were mapped to produce surfaces representing the extent and geometry of significant rock layers. A velocity model was also constructed, using a combination of well-derived velocity information and seismic stacking velocities. This velocity model was then used to convert the time surfaces to depth.

The final Kingdom™ project, an ArcGIS™ dataset and regional surface maps are available in Attachment A1 - Otway Basin regional 3D geological framework model data package. The regional and high-resolution models will be the primary inputs for further VGP and GSV studies and provide a reference for future analysis.

The current dataset has been curated and used to achieve the objectives of this study. For future analysis, surveys that were not included in this study would require datum, location and/or navigation issues to be rectified, to gain further insights from the data. Observations made during the interpretation phase indicate that more detailed sequence stratigraphic studies within the regional framework are achievable and would be beneficial at a subregional to local scale.

Bibliographic reference:
Romine, K., Vizy, J., Nelson, G., Lee, J.D., Baird, J. & Boyd, M., 2020. Regional 3D geological framework model, Otway Basin, Victoria. Victorian Gas Program Technical Report 35. Geological Survey of Victoria. Department of Jobs, Precincts and Regions. Melbourne, Victoria.

Download:
The downloadable version of this report is supplied in PDF format (PDF 163 MB), Att A1 Kingdom data (ZIP 105 GB), Att A1 Open data (ZIP 32 GB) & Att A1 ESRI Arcmap data (2.4 GB) - Download warning! Very large Att A1 data files - fast internet connection necessary.