Geological Survey

Surat Basin - Geological Overview

The Surat Basin is part of the Great Australian Basin, a large Jurassic-Cretaceous intra-cratonic basin that covers 1.7 million square kilometres of Eastern Australia. In New South Wales, the Great Australian Basin occupies almost one quarter of the area of the State and includes the southern parts of the Eromanga, Surat and Clarence-Moreton Basins.

The Surat Basin, and the underlying Bowen Basin, host a large number of oil and gas fields in the adjacent state of Queensland. No commercial oil or gas discoveries have been made in the New South Wales part of the Surat Basin, although a potential gas field has been identified in the underlying Gunnedah Basin in the east. The lack of exploration success in New South Wales does not reflect the true potential of the basin since the majority of drilling predates multiple fold seismic surveys and may not have been valid tests of structures. Most exploration effort in the Surat Basin has focused on the northeastern, deeper areas of the basin where it overlays the Bowen Basin. Only 6 wells have been drilled in the western half of the basin in New South Wales.

Geology

The Surat Basin is a broad intra-cratonic sag containing Jurassic to Cretaceous fluvial, lacustrine and marginal marine sediments, and coal-bearing successions. These sediments attain their maximum thickness of 2500 m in Queensland. The maximum thickness of the succession in New South Wales is approximately 1800 m. The Surat Basin extends across an area of 270,000 square kilometres and the southern third of the basin occupies a large part of northern New South Wales.

In the western to central part of the basin, the sedimentary succession of the Surat Basin unconformably overlies Early to Late Palaeozoic rocks of the Lachlan Fold Belt. These basement rocks, which consist of low grade metasediments, granites and volcanics, crop out along the southern and southwestern margins of the basin and in isolated inliers.

The eastern part of the basin is underlain by the Permian to Triassic Sydney-Gunnedah-Bowen Basin system. This basin system is a north-south elongate system with an asymmetric, synclinal geometry. It evolved initially as a rift complex and later, as a foredeep of the New England Fold Belt. The Surat Basin overlies the Gunnedah Basin in the east and the Bowen Basin in the northeast. The Permian-Triassic basin system onlaps deformed and metamorphosed rocks of the Lachlan Fold Belt to the west.

The eastern structural boundary of the Sydney-Bowen Basin system is marked by a series of thrust faults, which, in New South Wales, include the Hunter, Mooki, and Goondiwindi Faults. These faults developed during thrusting of the New England Fold Belt over the Sydney-Bowen Basin. The New England Fold Belt forms basement to the easternmost part of the Surat Basin and consists of acid volcanics, granites and metasediments.

Palaeozoic basement highs mark the western and eastern boundaries of the Surat Basin. The Nebine Ridge delineates the boundary between the Surat Basin and the Eromanga Basin to the west. The Nebine Ridge is well developed in Queensland and gravity data indicate that the southern extension of the ridge into northern New South Wales is terminated by cross faulting. In Queensland the Kumbarilla Ridge divides the Surat Basin from the Clarence-Moreton Basin to the east.

Outcrop of the Surat Basin succession delineates the northern and southern boundaries of the basin. Outcrop of the basin succession is generally poor and the rocks are largely concealed by Recent alluvium. Good exposures of the basal Jurassic units can be found in the southeast in the Coonabarabran-Coolah area, whereas weathered exposures of Cretaceous sediments are located at Lightning Ridge and Collarenebri.

Water Bore Geochemistry

As part of the Departments Discovery 2000 program a number of the deep artesian water bores in the Surat and Eromanga Basins have been geophysically logged and the water sampled. Geophysical logging and geochemical analysis of artesian waters has enabled detailed stratigraphic correlation and estimation of the hydrocarbon potential of the penetrated sections. The bores draw water from the Mooga and Pilliga Sandstones.

Water samples were collected directly from the flumes of flowing bores or from pressure plugs in the headworks of bores. Other samples were obtained from pumped subartesian bores. Several samples of free flowing gas were also collected. CSIRO Petroleum used gas chromatography to analyse quantity and composition of dissolved hydrocarbons and incorporated mass spectrometry to measure C13 isotope composition of methane and carbon dioxide.

Samples collected from the Surat Basin showed that methane is the main gaseous hydrocarbon and is accompanied by varying amounts of carbon dioxide. Most samples record small quantities of ethane and a few bores contain traces of propane and butane. Nearly all the analyses record dry gas, however, gases from one bore (Hollywood) approach wet gas composition. Stable isotope analysis showed C13 composition of methane to be outside the compositional range for thermogenic gas but similar to that of shallow dry microbiologically altered gas or coal seam gas. John Smith from the CSIRO has described a process whereby after catagenesis, methane and ethane may be converted to carbon dioxide, which may be subsequently reduced back to methane in an environment, such as an acquifer, leaving dry gas only. The C13 composition of methane is consequently altered to isotopically light signatures (depleted in C13), mimicing those of biogenic gas. This mechanism can be used to explain the origins of gases observed in bore waters as coal seams may exist in concealed Permian-Triassic troughs.

Petroleum Potential

Despite being only lightly explored for petroleum there are numerous oil and gas shows in the Clarence-Moreton Basin and a sub-commercial gas field (Hogarth). There is ample evidence of the source rock potential and although reservoirs are generally tight there is certainly potential for localised improved reservoir quality and/or reservoir stimulation. The basin is located close to a large regional population base (approx. 300 000 people). The Moomba-Roma-Brisbane gas pipeline and Jackson-Brisbane oil pipeline are located less than 100 km to the north of the New South Wales portion of the Clarence-Moreton Basin. The basin is ideally suited for exploration by a company willing to exploit smaller fields, rather than a large multinational explorer.

Please refer to the following publications for further details.

Structural Diagram

Pre-Permian basement morphology and Surat Basin subdivisions		34 Kb
Schematic North - south cross section through the Surat Basin and underlying Gunnedah-Bowen Basin		19 Kb

Stratigraphic Table

Surat Basin Stratigraphic Table To Download (Image)

46 Kb

Exploration Data (Gunnedah & Surat Basins)

Seismic Shot Points (October 2001) - Data		706 Kb
NSW well locations		44 Kb

For further information, please contact petroleum@dpi.nsw.gov.au