MGPalaeo is excited to announce the release of a new biostratigraphy project undertaken for the Geological Survey of Victoria (GSV). The task was to review and supplement the current biostratigraphic dataset for the onshore Otway Basin in Victoria. This project was the result of a collaborative effort with Stephen Gallagher, Fabian Stanislaus and Vera Korasidis from The University of Melbourne, and Mike Macphail from the Australian National University.
As part of the Victorian Gas Program, GSV is studying petroleum systems components (reservoir, seal and source) to assess the petroleum prospectivity of the Victorian Otway Basin and to understand the risks, benefits and impacts of onshore conventional gas exploration and production.
The new analyses will contribute to GSV’s regional geological knowledge, allowing more confident assessments of the hydrocarbon prospectivity of the Otway Basin by dating rock formations and understanding the environment that the rocks were deposited in.
Existing biostratigraphic data from 249 onshore Otway Basin wells and boreholes were reviewed, resulting in the selection of 1060 rock samples for new analyses from 120 onshore Otway Basin boreholes (Figure 1). The study utilised both palynological (fossilised spores, pollen and organic-walled microplankton) and micropalaeontological (fossilised foraminifera) data. These complementary approaches typically work better in differing rock types (mud/siltstones versus carbonates) and thus provide a more complete understanding of the stratigraphy and higher resolution correlations when combined.
During the review phase of the study, legacy data from vintage reports were updated to the most recent biostratigraphic zonation schemes. Some datasets had not been assessed since the 1960s. Revising the interpretation of such reports on a basin scale permitted the accurate comparison of biostratigraphic data between these older boreholes and more modern wells. This in turn led to revised correlations and the identification of data deficient intervals, as well as areas where the biostratigraphy needed further refining.
The final sampling program included new analysis of 748 palynology samples ranging from the Early Cretaceous to Cenozoic. A further 312 Late Cretaceous to Neogene micropalaeontological samples were selected by Associate Professor Steven Gallagher from the University of Melbourne, who led the micropalaentological aspect of the project. Analysis of such a large volume of samples substantially increased the number of biostratigraphic data points across a broad geographic area of the basin, providing additional correlative constraints on the stratigraphy.
Specifically, analysis of Late Cretaceous palynological samples enabled MGPalaeo to apply its high-resolution zonation for the Otway Basin across multiple wells. This identified the position of the Cretaceous/Palaeogene boundary regionally and tested published hypotheses regarding the diachroneity of lithostratigraphic units above the basin’s producing reservoir, the Waarre Formation.
Following the analysis phase, the new biostratigraphic results were plotted against log data, allowing further refinement of well correlations. This in turn permitted the construction of revised stratigraphic charts for multiple depocenters across the onshore Otway Basin, summarising key concepts and regional knowledge.
The full reports, including individual summaries for every well with biostratigraphic data in the onshore Otway Basin in the state of Victoria, are available following the links below:
MGPalaeo’s Additional Otway Basin datasets
Separate to this, we have also been working on a palynological add on pack to the study, summarising all the biostratigraphic data for the Otway Basin in South Australia and offshore Victoria, updated to the latest zonation scheme. More information will be coming soon, but please contact us on [email protected] to discuss further.