The Cooper Basin is unconformably overlain by the Mesozic Eromanga Basin, which is composed of sediments of Early Jurassic to Late Cretaceous age. The unconformity between the two basins acts as a migration pathway for hydrocarbons sourced from Permian-aged rich coals and shale source rocks to reach the overlying younger reservoirs of the Eromanga. Since the initial discoveries, explorers have encountered oil and gas accumulations from the Permian through to Cretaceous aged sedimentary rocks.
Permian coal measures and shales are the principal source in the basin, with the Patchawarra Trough containing the bulk of the oil and wet gas reserves. The main gas reservoirs occur primarily within the Patchawarra and Toolachee Formations. Oil is produced principally from low-sinuosity fluvial sands within the Tirrawarra Sandstone and at the basin margin, from the Patchawarra and Merrimelia Formations.
Intraformational shale and coal form local seals in the major reservoir units, while the early Permian Roseneath and Murteree shales form important regional seals.
Anticlinal and faulted anticlinal traps have been proven exploration targets but potential remains high for discoveries in stratigraphic and sub-unconformity traps, especially where the Permian sediments are truncated by the overlying Eromanga Basin succession.
Recent exploration effort has targeted the poorly explored flanks of the Cooper Basin and its unconventional gas plays. The Western Flank has been a source of many recent oil discoveries, with exploration success rates at over 50%. Much of this success is attributable to improvements in drilling technology and the use of modern 3D seismic data.
Unconventional gas exploration is expanding in the Cooper Basin, driven by increased demand for LNG and declining conventional gas production. Improved technology is lowering the cost of unconventional field development, lowering the breakeven costs.