Geological Survey

Gunnedah Basin - Geological overview

The Gunnedah Basin forms the central part of the Sydney-Gunnedah-Bowen Basin system which extends along the eastern margin of Australia. The Gunnedah Basin covers an area of just over 15 000 square kilometres and comprises rocks of Permian and Triassic age. The basin is in part unconformably overlain by the Jurassic-Cretaceous strata of the Surat Basin.

Until recently the Gunnedah Basin has been only lightly explored for petroleum with 19 petroleum wells drilled, only 3 of which are considered valid tests. Two of these flowed gas to the surface and the third, an offset test of the second, intersected an intruded section in the pay zone of one of previous wells. A further dozen wells have been drilled in recent years by Forecnergy Inc within PEL238. There is a lack of modern seismic coverage over most of the basin. There are however, in the order of 120 coal exploration wells drilled in the basin, many of which were drilled to basement.

Geology

The Gunnedah Basin is a foreland basin with sediments unconformably overlying deformed and metamorphosed Ordivician to Devonian Lachlan Fold Belt strata in the west and abutting Devonian to Carboniferous New England Fold Belt strata to the east, along the east dipping Hunter-Mooki Thrust. The boundary between the Gunnedah Basin and the Bowen Basin to the north is largely arbitrary. A high area north of Narrabri where some of the Early Permian and much of the Late Permian has been eroded is taken as the boundary. The boundary between the Gunnedah Basin and the Sydney Basin, to the south, is argued as being either the Mount Coricudgy Anticline or the Liverpool Range. While the Mount Coricudgy Anticline is a structural boundary, sedimentation typical of the northern Sydney Basin appears to continue north of the anticline. Whereas, it is said that the depositional character of the sediments change across the Liverpool Range in the west of the basins. A major structural feature of the basin which affected sedimentation throughout the basins early history is the Boggabri Ridge. The ridge divides the basin into the Maules Creek Sub-basin in the east and the West Gunnedah Sub-basin in the west.

Onset of deposition in the basin is marked by the localised deposition of colluvial and alluvial material in palaeovalleys on the weathered surface of the basement volcanics forming the Early Permian Leard Formation. This is overlain by the lacustrine Goonbri Formation. This was followed by an influx of course volcanolithic sediments from the Boggabri Ridge and possibly other local highlands forming the Maules Creek Formation. In the Maules Creek Sub-basin the unit is quite thick, in the West Gunnedah Sub-basin deposition was much thinner and the unit is particularly thin or absent across the Boggabri Ridge.

The alluvial Maules Creek Formation gave way to progressively marine conditions and the development of a marine shelf. The Porcupine and Watermark Formations were deposited during this period. The upper Watermark Formation is marked by prograding delta development. Delta development continued into the lower Late Permian Black Jack Group. Inundation followed with the return of marine conditions. The Arkarula Sandstone Member, an important marker horizon in the Gunnedah area is a significant component of the shallow marine facies. The transgression was short lived and followed by regression and the deposition of the western bed load fluvial system along the western basin margin, which delivered fluvial sediments to the delta environment in the main portion of the basin. The Hoskisson Coal resulted from widespread peat development. The upper Black Jack Group was deposited in an alluvial system derived from the New England Fold Belt. Compression and local erosion marked the end of Permian sedimentation.

Early Triassic tectonism in the New England Fold Belt resulted in widespread deposition of coarse clastic of the Digby Formation. The Napperby Formation overlies the Digby Formation (in part disconformably) and is considered to be the result of regression from distal lacustrine to deltaic conditions.

Jurassic to Cretaceous sediments of the Surat Basin unconformably overlie the Permo-Triassic sequence in the north and west and thicken rapidly to the northwest.

Petroleum Potential

Source potential in the Permian sequence is considered to range from fair to excellent. The Arkarula Sandstone Member and the Watermark Formation have the best potential. Both are marine and liptinite rich. The source potential of the Triassic Napperby Formation ranges from poor to good, with excellent potential in some thinner discontinuous units. Jurassic Surat Basin shales and coals have good to excellent source potential. Thermal maturity ranges from marginally mature to mature in the lower Permian sequence to marginally mature in the Triassic and immature in the Jurassic sequence.

The Copper and Bowen/Surat Basins are prolific producers of oil and gas from Permo-Triassic reservoirs in adjacent states. In these areas the reservoir quality is variable. In New South Wales the extensive coal exploration has resulted in an good understanding of the geographic distribution of stratigraphic units. The best reservoir units within the Gunnedah Basin are associated with the Late Permian western bed load fluvial system of the upper Black Jack Group, and the Early Triassic quartz sand facies of the Digby Formation. Sandstones within the delta front facies of the upper Watermark Formation and the shallow marine facies of the lower Black Jack Group also have good reservoir characteristics. Regionally extensive shale units occur throughout the Gunnedah Basin sequence and all the potential reservoirs in the sequence are considered to be adequately sealed.

In the north of the basin compression, associated with the Hunter-Mooki Thrust, appears to have developed a number of north-south trending anticlines. These anticlines generally contain faulted cores, however the faults commonly do not penetrate higher than the top of the Triassic sequence. In the south of the basin a change in the direction of the Hunter-Mooki Thrust appears to have resulted in compressive left lateral wrenching and the development of a number of en-echelon anticlines.

There have been two significant sub-economic gas discoveries, numerous gas shows and a few oil shows recorded in the Gunnedah Basin. Wilga Park 1 flowed gas at a rate of 1 million cubic feet per day at 28 pounds per square inch through a 3/8 inch choke in the Porcupine Formation. Coonarah 1 and 1A were drilled 11 kilometres west of Wilga Park. Coonarah 1 flowed dry natural gas at a rate of 340 000 cubic feet per day from the Digby Formation, although the target reservoir was intruded and the gas was reservoired in the underlying unit. Coonarah 1A was drilled beside Coonarah 1 after the drill string became irretrievably stuck. In Coonarah 1A the Digby Formation reservoir was again intruded and gas bearing, and the western sands of the Black Jack Group were water wet, however a tight sand in the Black Jack Group recovered 140 metres of brackish water. Bellata 1, a well drilled by the Department as a stratigraphic test, encountered numerous small gas shows mainly from porous and permeable sands interbedded with coal seams of the Black Jack Group. Oil staining and a petroliferous odour but no fluorescence were recorded from the basal Pilliga Sandstone in Bellata 1. There was considerable argument as to the possibility of contamination being the source of these shows. However, a recent examination of core from Moema 1 drilled in 1975 also showed hydrocarbon staining and fluorescence in the basal Pilliga Sandstone. An oil show was also reported from the Early Permian Boggabri Volcanics in Bohena 1. Gas shows are commonly reported from coal drilling throughout the basin.

Coal Seam Methane

Both the Maules Creek Formation and the Blackk Jack Formation contain coal seams that are prospective for coal seam methane. These units contain numerous coal seams with exceptionally thick seams and net coal thickness reaching 50-60 metres in the depocentre located adjacent to the Hunter-Mooki Thrust. The Hoskisson seam averages greater than 16 metres in the southeast of the basin. Although inernitite is the dominant coal maceral in the basin, vitrinite content in the Hoskisson seam increases toward the southeastern depocentre to over 40% suggesting that maceral content improves toward the depocentres of coal formation. Thermal maturity, as indicated by vitrinite reflectance, is generally marginal for coal seam methane, however maturity is often increased in the vicinity of intrusions that are common throughout the basin.

Forcenergy Inc has drilled a number of wells in the Gunnedah Basin in the hopes of discovering gas resources in a combined coal seam methane/ conventional gas play. This exploration along with Sydney Gas Ltd's exploration in the southern Sydney Basin, are the most comprehensive exploration efforts in the state for many years and are the best chance for petroleum production in New South Wales in the short term.

Please refer to the following publications for further details.

Structural Diagram

Structural elements of the Gunnedah Basin

20 Kb

Stratigraphic Table

Gunnedah Basin Stratigraphic Table To Download (Image)

46 Kb

Exploration Data (Gunnedah & Surat Basin)

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

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