Overview

Location of the Oaklands Basin in NSW

The Oaklands Basin covers approximately 3 800 square kilometres in the Riverina district of southern New South Wales, between the towns of Mulwala, Jerilderie, Griffith, Coleambly, Oaklands and Albury. This Permo-Triassic basin trends north-northwest-south-southeast and is concealed beneath the south-eastern portion of the Murray Basin. The basin overlies the Ovens Graben (Ovens Valley Graben in Victoria) which extends from the Murrumbidgee River west of Darlington Point in New South Wales to Wangaratta in Victoria. It is approximately 25 km wide at Oaklands and approximately 15 km wide at its northern extent. The Oaklands Basin hosts to more than 1000 m thick sediments. The basin was first discovered in 1916 when water drillers discovered coal near Coorabin. Coal has been the main focus for exploration within the basin since this time. Extensive coal exploration drilling has been undertaken, mostly in the Coorabin area in addition two coal drilling programs by the Department, designed to help define the limits of the basin, have resulted in wide-spaced drilling coverage over most of the basin. Petroleum exploration within the basin has been minimal, consisting of a small number of geophysical surveys and two stratigraphic wells. In 2009 four new lines of high quality 2D seismic have been acquired by DTIRIS (formerly the NSW Department of Industry and Investment). This survey has further delineated the boundaries of the basin, established the thickness of the sediment fill and imaged the structures of the Ovens Valley Graben along the margin and central portion of the basin.

Stratigraphy

The Oaklands Basin was named after geographical area under which Permian and Triassic sedimentary rocks were encountered during drilling in the vicinity of Oaklands.

The stratigraphy of the Oaklands Basin is established exclusively from exploration drilling and waterbore intersections since there are no surface outcrops. The stratigraphy of the deepest portions of the basin is unknown and has been postulated based on seismic data interpretation and analogy to the stratigraphy of other basins in the region (Hawley, 2003).

Basement to the Oaklands Basin appears to consist of Ordovician metasediments of the Lachlan Fold Belt, which crop out to the southeast of the basin, and Siluro-Devonian granites, which are exposed in various locations outside the basin. Pre-Permian (Devonian?) sedimentary rocks appear to be thin or absent in the Oaklands Basin (Pitt Research, 1995). In the Ovens Valley Graben in Victoria Late Devonian and Carboniferous sedimentary rocks have been intersected near where the southern fringe of the Murray Basin extends into the Victorian border, subsequently, it was suggested by Earth Satellite Corporation (1981) that a thick sequence of Late Devonian and Carboniferous sedimentary rocks may exist in the deeper portions of the Ovens Graben.

The oldest sedimentary units encountered in the Oaklands Basin at Jerilderie 1 are the Stage 3 Early Permian Urana Formation. However, the Urana Formation outside the Oaklands Basin also consists of Stage 1 and 2 units and Early-Late Carboniferous sequences. The Urana Formation consists of marine sediments deposited in glaciogene environment (O'Brien, 1981). The depositional environment changes to deltaic towards the upper part of the Urana Formation. The top of the formation is weathered and unconformably overlain by the Narrow Plain Formation / Coorabin Coal Measures in the Ovens Graben and to the south-east, and by the Jerilderie Formation in the west. The Coorabin Coal Measures were interpreted as meandering stream deposits represented by cyclic fluvial point bar sequences (Morgan, 1977).

The Triassic Jerilderie Formation unconformably overlies the Coorabin Coal Measures where it is preserved, the Urana Formation in the west, and basement elsewhere. The Jerilderie Formation was deposited in a fluvial channel and floodplain environment. The formation is very poorly consolidated and friable (Yoo, 1997), and lithologically similar to the overlying Tertiary Olney Formation (the oldest unit of the Murray Basin sequence). The units were described as possible interconnected aquifers (O'Brien, 1991).

There is a considerable time gap between the mid-Triassic age of the Jerilderie Formation to the Eocene age of the oldest Murray Basin units in the area. In part because the Oaklands Basin is located on the most eastern margin of the Murray Basin, the Tertiary sequence in the area is non-marine and a number of marine units that occur in the centre or west of the Murray Basin are locally absent in the Oaklands Basin area.

Structure and tectonic settings

East-west cross-section through the Oaklands Basin

The dominant structural feature of the Oaklands Basin is the northwest trending Ovens Graben. In New South Wales the Ovens Graben is approximately 190 km long by 20 km wide. The Oaklands Basin is one of a small number of troughs with similar trends below the Murray Basin in southern New South Wales (Netherby Trough, Numurkah Trough). This structural trend appears to reflect the structural fabric of the underlying Lachlan Fold Belt. Other infra-basins are noted below the Murray Basin further to the north and northwest trending northeast-southwest, overlying the Kanmantoo Fold Belt (Hawley, 2003). Originally the Ovens Graben has been regarded as a simple graben with a continuous western bounding normal fault and an eastern bounding fault that has been offset near Jerilderie (Yoo, 1981). More recent review of available data revealed that the Ovens Graben is more complex than previously perceived. It was suggested by Gunn (2003) that much of the gravity anomaly associated with the Oaklands Basin / Ovens Graben is due to granites within the basement.

It was suggested that the number of unconformities within the Oaklands Basin stratigraphy indicate that movement on the graben bounding faults has taken place over most of the Oaklands Basins history from the latest Carboniferous to the Early Tertiary (Earth Satellite Corporation, 1981). The poorly consolidated nature of the Late Permian to Tertiary section of Oaklands Basin suggests that the basin has been isolated from major compressive deformational events, and/or the graben bounding faults have taken up almost all of the compressive deformation that the basin was subjected to. Folding within the basin is gentle. Sediments, particularly in the upper portion of the sedimentary pile, are relatively flat-lying.

The Ovens Graben, is bounded by the "western bounding faults", the Felton Wood Fault, "northern east bounding fault", and the Bundure Fault.

Source rock

The best potential for conventional hydrocarbon source is likely to be in the lower Stage 3 Urana Formation and the Stage 1, and Stage 2 (Late Carboniferous to Early Permian) sedimentary rocks that probably exist in the deepest portions of Ovens Graben. Oil has been recovered from equivalent rocks in western Victoria.

Stage 3 Urana Formation encountered in AOG Jerilderie-1 has a potential for conventional hydrocarbon source. The RockEval analyses, however, is limited and suggests only poor source rock characteristics for gas only. TOC values in Urana Formation in Jerilderie-1 well are from 0.15% to 0.64%. The sample from 1004.6 m depth has a S1 value of 0.28mg/g, which is unusually high for a sample with poor source rock quality and is deemed to be due to migrated oil. S1 is the amount of free hydrocarbons (gas and oil) in a sample before analysis.

Coorabin Coal Measures penetrated in Jerilderie-1 have TOC values are as high as 20.4%. The section is however relatively shallow and is not mature for hydrocarbon generation.

Reservoirs

The section of Urana Formation in AOG Jerilderie-1 that is described as "Jerilderie Sandstone" has good reservoir characteristics with measured porosity value of 26% and horizontal permeability of 62 mD. Other sandstones within Urana Formation have good porosities of 20% but very low permeability.

There is evidence of porosity and permeability within Late Permian Coorabin Coal Measures. Geological assessment of the area revealed that large quantities of underground water exist above and below the seam.

The Triassic Jerilderie Formation also has reservoir capabilities in lower part of the sequence.

Tertiary coals, sands and conglomerates are porous and saturated with salt water. The reservoir potential of these sediments is considered low as they are relatively shallow and require long distance vertical migration from mature source.

The porosity and permeability of potential reservoir units may be enhanced due to fracturing in the vicinity of major structures. The bounding fault systems of the Oaklands Basin are likely to be areas of fracture-enhanced permeability.

Traps and seals

Fine grained sedimentary rocks dominate the lower parts of the Permian sequence and serve as good seals for Early Permian and pre-Permian reservoirs. The Late Permian Coorabin Coal Measures also contain fine-grained sedimentary rocks overlying sandstones in a series of fining upwards cycles. Claystone units within the Triassic would provide seal for underlying Triassic and Late Permian sandstones and conglomerates. The Murray Basin sequence also consists of fine grained sediments providing seals for underlying reservoirs. The Tertiary sediments however are shallow and poorly consolidated to provide an adequate seal.

There is some evidence of silicification that may assist fault seal trapping along major faults and provide seal for dipping reservoirs specially these adjacent to such faults.

It was suggested that structural traps formed within tilted fault blocks, and stratigraphic wedge-outs formed by drape over basement structures are the major Early Permian trapping mechanisms. Sandstone wedges and mounds in the middle Urana Formation onlapping the western flank of the Ovens Graben have been mapped. It was suggested that basal Early Permian sedimentary rocks onlapping the western flank of the graben would provide stratigraphic pinch-outs and that up-dip permeability barriers within prograding Early Permian sequences could also be possible.

Petroleum systems

The Oaklands Basin is very lightly explored for petroleum. Despite the lack of exploration there are strong indications that a conventional petroleum system exists within the basin. The elements of such a system appear to be in place with at least one significant potential sandstone reservoir being identified together with numerous potential seal units in existence and the identification of potential stratigraphic and structural traps.

The petroleum systems are:

  • Early Permian Urana Formation (including "Jerilderie Sandstone") reservoir, sourced from organic rich mudstones and siltstones of lower parts of the Urana Formation or/and underlying Late Carboniferous to Early Permian sediments, sealed by fine grained sediments of lower parts of the Permian sequence, the Coorabin Coal Measures or clays and silts of upper part of the Jerilderie Formation, with Early Permian stratigraphic wedge-outs or Late Permian sedimentary rocks draping over the highs as traps.
  • Late Permian reservoir sands within the Coorabin Coal Measures, sourced form sediments of lower parts of the Urana Formation or older, sealed by intra-formational fine-grained deposits of the Coorabin Coal Measures or claystone units within the Jerilderie Formation. Traps are Late Permian sediments draping over basement highs or palaeo-structures. Early Cretaceous regional uplift has generated large basinal domal anticlines and forms valid traps.

Exploration data for the Oaklands Basin

Exploration data related to the Oaklands Basin can be obtained through online services.

For further information
Strategic Resource Assessment and Advice, Geological Survey of NSW
Phone:
+61 (0)2 4063 6726
coal.geology@geoscience.nsw.gov.au
Postal:
Strategic Resource Assessment and Advice, Geological Survey of NSW, PO Box 344, Hunter Region Mail Centre NSW 2310
Office:
516 High Street Maitland NSW 2320 Map