By Mike Fahey, Marden Wark and John Davidson with Phil Cooney, NSW Trade and Investment.
LOCATED in the Riverina District of southern New South Wales, the Oaklands basin contains Triassic and Permian sediments concealed by Cainozoic sediments.
Seismic reflection data was acquired by NSW Trade & Investment, Mineral Resources in 2006 and 2009 to enhance the knowledge of petroleum systems in the basin.
While the interpretation of these and other relevant geological data is set to be presented in a quarterly note, the data suggest that the components of a conventional petroleum system may be in place, with potential for future discoveries of petroleum resources.
The basin architecture is dominated by the Ovens Valley Graben. This NW-SE trending structure can be traced from Wangaratta in Victoria to the Murrumbidgee River, west of Darlington Point. Basin stratigraphy falls into three main units: the Early Permian Urana Formation, the Late Permian Coorabin Coal Measures and the Triassic Jerilderie Formation.
Late Permian and Triassic strata appear to have blanketed the present extent of the basin and regions beyond, however uplift and erosion have resulted in removal of much of the Late Permian cover prior to deposition of the Tertiary Murray basin sediments.
Triassic and Late Permian sediments thin southwards, resulting in Tertiary sediments resting directly on the Early Permian. Early Permian sediments are largely confined within the graben which shallows to the south.
Gravity and aeromagnetic data over the Oaklands basin indicate that the development of the graben within the basin was related to the reactivation of faults in the Ordovician basement, which consists of either Pre-Devonian crystalline metamorphics or Silurian granitic intrusives.
There are indications that another 11km wide graben exists about 50 kilometres west of the Ovens Valley Graben and it has been suggested that this feature could also be a Permo-Triassic depocentre containing Permian coal measures.
Further, there are indications that significant Permo-Triassic sediment thicknesses could occur to the northeast of the Oaklands basin.
Past exploration of the region, including seismic surveys, coal exploration boreholes and petroleum wells, provides evidence that the basin has hydrocarbon potential and may contain a shallow petroleum system in the north east region of the basin.
Two petroleum wells drilled in the centre of the basin reported minor gas shows, and a third, drilled north-east of the Oaklands basin, reported both oil and gas.
In 2006, two 25 kilometre reflection seismic lines were acquired to the west of Griffith. The survey was sited to investigate a graben postulated as a possible northern extension of the Oaklands basin.
The data indicates a variety of trapping structures, including faulted traps and anticlines. A 2009 survey consisted of four lines totalling over 250 km to define the structure of the Ovens Valley Graben, with a view to identifying possible petroleum systems.
HYDROCARBON SYSTEMS AND PROSPECTIVITY
Hydrocarbon generation has occurred in the Oaklands basin and conventional and/or unconventional accumulations may exist. Several petroleum play concepts fit the available data.
Devonian and Early Permian age strata are postulated to have entered the petroleum generation window and may contain sufficient dispersed organic matter to constitute a viable source. Devonian strata may be the source for the oil shows encountered in the Lake Cullivel 1 well. Early Permian-sourced hydrocarbons in the Early Permian strata and hydrocarbons flushed by migrating groundwater may have accumulated in Permian or Tertiary strata.
Murray basin strata and the Late Permian age black coal sequences in the region generally appear to have not achieved the rank needed for petroleum generation.
However, there are numerous occurrences of lignite, sand aquifers and reservoirs bounded by clay beds which constitute effective seals.
The shallow depth of the lignite and aquifers supports biogenic generation of methane.
Methane has been encountered during water bore drilling in other parts of the Murray basin and numerous reservoirs of pocketed methane may exist within the Murray basin strata in the Oaklands region. The peat and lignite of the Murray basin may also be viable for stimulated or enhanced biogenic generation of hydrocarbons.
Oil and gas shows in several exploration wells confirm that source rocks exist in the region and that petroleum generation and migration has occurred. The Oaklands basin is relatively unexplored and has potential for undiscovered hydrocarbon systems.