Overview

Location of the Murray Basin in NSW

The Murray Basin is located in the southwest of New South Wales extending west and south into South Australia and Victoria, respectively. Strictly speaking the Murray Basin refers to a widespread thin sequence of Cenozoic consolidated and unconsolidated sediments. However, geographically the basin overlies a number of basement depressions that contain Palaeozoic and Mesozoic sediments. It is these sequences that are likely to have the most potential for petroleum within the Murray Basin area.

Approximately 20 wells have been drilled in the New South Wales portion of the Murray Basin. Many of the earlier wells were drilled as stratigraphic tests or were drilled without seismic control. The later wells were restricted to the Tarrara-Menindee Trough, in the western part of the basin. Recent reviews confirm that they did not test valid structures. Despite this there have been gas shows in 8 of the wells drilled in the basin.

Geology

Murray Basin

Within New South Wales the Murray Basin unconformably overlies and onlaps the Proterozoic sequences of Willyama and Broken Hill Blocks in the northwest; unconformably overlies metasediments of the Kanmantoo Orogen in the west; overlies a strongly folded and partially metamorphosed Ordovician to Devonian Lachlan Orogen sequence in the east; and onlaps slightly deformed Cambrian to Lower Carboniferous Lachlan Fold Belt sediments in the south. To the north the Cenozoic Murray Basin sequence overlies the Palaeozoic Darling Basin and its infrabasins. The Murray Basin sequence forms a thin but extensive platform cover succession with its main depocentre located in the western part of the basin.

For many years, a number of 'troughs' have been recognised, based on Bouguer gravity lows, lying beneath the Murray Basin. The existence of these structures has been confirmed in many cases by drilling. The troughs in the west and north of the basin in New South Wales generally contain Devonian to Early Carboniferous sequences resembling those of the Darling Basin to the north. The troughs in the east generally contain Late Carboniferous to Triassic sediments, most notably the Ovens Graben which contains a thick Permian Coorabin Coal Measures sequence which has not been intersected elsewhere outside of this graben. This sequence resulted in the naming of the coal prospective portion of the Ovens Graben as the Oaklands (coal) Basin. The 'troughs' lying beneath the Murray Basin appear to be mainly a mixture of deep grabens and half grabens, and shallower gentle basement downwarps.

During the 1998 Western Division Seismic Survey conducted by the Department, several seismic traverses were recorded across portions of the Murray Basin considered to be strategic because of their coincidence with regional gravity trends. In particular line DMR98-06 confirmed the presence of a thick sequence of probably Devonian sediments within a depocentre at the northeastern termination of the Balranald Trough, locally referred to as the Booligal Trough.

Recent drilling by the Department in the Booligal Trough (Mossgeil 1) further confirmed the presence of the trough. The sedimentary sequence intersected, below the Murray Basin sequence, appears similar to Devonian strata in the Darling Basin. Sampling and analyses associated with the well are currently underway.

Petroleum potential

Murray Basin

The infrabasins underlying the Murray Basin contain up to 10 000 metres of sedimentary section. The deeper parts of these infrabasins have not been tested; however, they are expected to contain stratigraphic equivalents to the Darling Basin directly to the north. By analogy with the Adavale Basin in the adjacent state of Queensland, which hosts the productive Gilmore gas field, this is now thought to have good petroleum potential. This sequence, in the few places where it has been intersected, near the margins of infrabasins within the Murray Basin has poor source rock characteristics and appears to be gas prone and over mature. Although the sequence lacks source rocks at the margins, it is possible that more favourable facies occur toward the centres of the infrabasins. Reservoir quality sediments are lacking in the intersections to date; however, recent drilling in the Darling Basin has shown that sandstones with good reservoir character exist within the sequence. Drilling within the Murray Basin indicates there is significant facies variability within the sequence.

Better preserved infrabasins probably contain Late Carboniferous to Triassic sequences which are generally immature to marginally mature. Although not comprehensive, limited available data suggests that source rock potential is low. Reservoir characteristics are variable; however, a number of excellent reservoir quality intersections have been recorded. The sequence contains shales and mudstones which may form a seal, although insufficient data prevent recognition of regional reservoir-seal pairs.

As expected the thin Cainozoic cover itself is immature and a poor source. The sequence has variable but commonly very good reservoir quality. The Pyap Member in the Monash Formation, in particular, has excellent reservoir characteristics and is overlain by Merreti and Coombal Members which may provide a regional seal.

Exploration data for the Murray Basin

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

For further information
Coal Resource Assessment and Advice
Phone:
+61 (0)2 4063 6543
coal.geology@geoscience.nsw.gov.au
Postal:
Coal Assessment & Advice, Geological Survey of New South Wales, PO Box 344, Hunter Region Mail Centre NSW 2310
Office:
516 High Street Maitland NSW 2320 Map