Understanding the hydrocarbon potential of Australia’s sedimentary basins is critical to ensuring the nation’s future energy security. The Pedirka and western Eromanga basins have proven petroleum potential with a sub-commercial oil discovery at Poolowanna 1 in the Poolowanna Trough and several wells drilled over the Colson Shelf and Madigan Trough showing evidence for residual oil zones. However, these basins remain relatively underexplored with only 42 petroleum wells drilled and relatively sparse 2D seismic data coverage. Geoscience Australia’s AFER Project has undertaken a qualitative and quantitative play-based assessment of the Pedirka and western Eromanga basins to enable a better understanding of their undiscovered hydrocarbon resources.
The AFER Project’s assessments are underpinned by new geological insights into the western Eromanga Basin and a supporting upscaled 3D geological model. A play-based common risk segment (CRS) mapping approach has been applied to eleven play intervals to delineate basin areas with relatively high prospectivity based on five geological risk elements: reservoir presence, reservoir effectiveness, top seal, trap presence, and hydrocarbon charge. Results from this qualitative component of the assessment indicate that the highest potential for future hydrocarbon discoveries is likely to be conventional oil resources across the Poolowanna Trough, Colson Shelf and Madigan Trough. The most prospective exploration targets are the Namur-Murta, Poolowanna and Peera Peera play intervals on a geological probability of success basis. The Peera Peera and Poolowanna play intervals have proven hydrocarbon charge from the Poolowanna 1 oil discovery but show poor reservoir quality (porosity <10%) in wells drilled across the Poolowanna Trough. These play intervals likely represent tight conventional oil exploration targets across their main play fairways in the Poolowanna Trough. The Namur-Murta interval has high reservoir qualities across all potentially prospective areas but has lower certainty regarding hydrocarbon charge with the most significant exploration result to date being a residual oil zone in the Madigan Trough. Moderate to high prospectivity for conventional oil is interpreted to occur in the Adori-Westbourne, Birkhead and Hutton play intervals over the eastern flanks of the Poolowanna Trough and western flanks of the Birdsville Track Ridge. The Walkandi, Upper Purni, Lower Purni and Crown Point play intervals are assessed as having moderate prospectivity for conventional oil over the Eringa Trough, Madigan Trough and Colson Shelf.
A quantitative assessment of the ‘Yet to Find’ hydrocarbon volumes has been undertaken to provide a play-level indication of the possible undiscovered conventional oil volumes. The risked volumes include a ‘Base Case’ that reflects the current exploration understanding of the basins, and a ‘High Case’ that reflects the potential impact of a new working petroleum system being discovered in the basins. The mean risked recoverable oil volume for the Base Case scenario total 22.2 MMbbl for the four plays evaluated (Namur-Murta, Poolowanna, Peera Peera and Lower Purni). About 70% of the risked mean volumes occur in the Poolowanna and Namur-Murta play intervals. Results from the High Case model highlight the significantly greater YTF potential across the basins if the geological requirement for a new working petroleum system eventuates from further exploration, with a total mean risked volume of 234.8 MMbbl for the three play intervals evaluated (Namur-Murta, Poolowanna and Lower Purni). Risked volumes are relatively evenly distributed across the three play intervals.
Unconventional hydrocarbons are evaluated as being less prospective than conventional hydrocarbons in the western Eromanga basin. Shale oil plays have not previously been explored but may be present within organic-rich shales from the Poolowanna and Peera Peera play intervals. These shale oil plays are evaluated as being moderately prospectivity due to their thin and heterogeneous character. Coal seam gas (CSG) wells drilled into the Upper Purni and Lower Purni play intervals have to date only demonstrated the presence of gas-undersaturated coal seams over the Andado Shelf. However, CSG is the most likely hydrocarbon resource type to produce hydrocarbons from the Pedirka Basin if future exploration can identify sweet spots where different geological conditions occur that are conducive to preserving high gas saturations.
