The Sydney Basin is part of the Sydney-Gunnedah-Bowen Basin, a major foreland basin system which extends from southern coastal New South Wales to Central Queensland. The Sydney Basin is approximately 350 kilometres long and an average of 100 kilometres wide. The total onshore area of the basin is approximately 44 000 square kilometres with an offshore component of about 5 000 square kilometres which extends to the edge of the continental shelf. There are three major cities in the basin - Sydney (4 million people), Newcastle (600 000) and Wollongong (400 000). These cities are served by a reticulated gas distribution system, several oil refineries and a well developed transportation system.

The Sydney Basin has a long history of coal exploration and mining, with several thousand boreholes being drilled in the basin. Petroleum exploration has been modest with approximately 70 conventional petroleum wells, 35 coal seam methane wells and approximately 10 000 kilometres of seismic surveys. The basin is considered prospective for oil, gas, coal seam methane and oil shale. Although there have been no discoveries, there are numerous oil and gas shows from both petroleum and coal exploration drilling.

Geology

In the southwest of the Sydney Basin, basement consists of Lachlan Fold Belt strata. In the central and eastern portions of the basin basement composition is unknown, but believed to be Lachlan Fold Belt strata. Basement to the north has been thought to be an extension of Late Carboniferous New England Fold Belt strata that is located further north across the Hunter Thrust, however there is evidence that suggests that Lachlan Fold Belt strata may form basement to most of the northern Sydney Basin. The formation of the basin was previously thought to be the result of rifting, however recent interpretations of the structural history of the basin suggest that compression played a more dominant role, many major depositional cycles may have been initiated during foreland loading.

The earliest deposition in the basin consisted of volcanogenic sands and silts deposited in a marine shelfal environment with basaltic lavas erupted from island volcanoes in the lower Hunter Valley region. This marine deposition formed the Early Permian Dalwood Group/lower Shoalhaven Group. Subsequent compression led to folding of the New England Fold Belt to the north and the development of the northern Sydney Basin into a foreland basin setting. Sediment shed from the New England Fold Belt sourced alluvial fan deposits of the Greta Coal Measures in the north of the basin. Sedimentation at this time did not reach the southern parts of the basin.

Basement sag led to increasingly marine conditions toward the top of the Greta Coal Measures and the onset of a major transgression which in the west of the basin covered not only Permian sediments but also basement. This transgression, which included a minor regressive phase, formed the Maitland Group/upper Shoalhaven Group. The unit consists of mixed New England derived volcanogenic and Lachlan derived quartz rich, sands and silts. Regressive New England derived sands formed the Muree/Nowra Sandstone in the middle of the Group.

The Hunter - Bowen Orogeny followed, folding and faulting the New England Fold Belt and northern Sydney Basin. The volume of sediment shed from the uplifted New England Fold Belt lead to major long term regression. In the north, delta plain to fluvial conditions formed the Late Permian Tomago/Whittingham Coal Measures interrupted by the transgressive Kulnura Marine Tongue/Bulga Formation. In the south, the lower Illawarra Coal Measures were deposited in lower delta plain conditions interrupted by the Erins Vale Formation. Coal measures development was terminated by a marine incursion that formed the Dempsey Formation/Denman Formation/Bargo Claystone/Ball Bone Formation. A beach environment followed in the north, forming the Waratah/Watts Sandstone and prograded south forming the Darkes Forest/Angus Place Sandstone. Regression continued with the deposition of the Newcastle/Wollombi Coal Measures in a prograding meandering stream dominated alluvial plain environment while more distal environments in the south formed the upper Illawarra Coal Measures..

Coal measures deposition effectively filled the basin. This was followed by a lacuna and basin subsidence (more pronounced in the north) at the end of the Late Permian that lasted into the Early Triassic. Renewed uplift of the New England Fold Belt resulted in deposition of the Narrabeen Group onlapping the coal measures in an alluvial flood plain environment that graded into estuarine and fluvial flood plain environments. Sedimentation throughout this period varied from initially lithic New England derived to mixed load and finally quartz rich Lachlan derived sediments. In the south the redbeds of the Bald Hill Claystone were formed by lateritic weathering of the Gerringong Volcanics.

Compressive tectonism is thought to have continued into the Triassic with the northern Sydney Basin becoming the site of the frontal foreland fold and thrust belt. Uplift of the Lachlan Fold Belt, or subsidence of the New England Fold Belt, resulted in the tilting of the basin to the north and the deposition of the fluvial quartz rich Hawkesbury Sandstone. Based on regional comparisons and surface maturity data, at least 1-2 kilometres, and possibly up to 4 kilometres, of Jurassic to Cretaceous sedimentation is believed to have followed before being eroded off during tectonism associated with Tasman Sea rifting and/or underplating of the eastern continental margin from Aptian times.

Source Rocks & Maturity

Reflected light organic petrology and limited geochemical analyses indicate that both the marine and terrestrial sediments of the Sydney Basin are high in dispersed organic matter (DOM) content. The Denman Formation ranges from 2 to 26 % DOM with an average of 8%, the Bulga Formation averages 11% and the Mulbring Siltstone 4%. Many samples in these units contain greater than 2% total organic carbon (TOC) and are classed as excellent source rocks. The Snapper Point, Rutherford and Pebbley Beach Formations all contain in excess of 1% TOC and could also be classed good source rocks.

Kerogen typing has shown that all of these source rocks are dominated by humic kerogens. DOM in these rocks is inertinite rich, with variable but lesser amounts of vitrinite and exinite. The exinites vary from 4 to 40% by volume and may form a significant proportion of the DOM in these rocks. The Cooper Basin, a basin with only terrestrial Permian sediments located in adjacent states, had initial recoverable reserves of oil, condensate and LPG of 210 million barrels. The Cooper Basin has inertinite rich DOM with less exinite content than the Sydney Basin and on the basis of quantity and kerogen type, the Mulbring Siltstone and Denman Formation have as good oil and gas potential as the sediments of the Patchawarra Trough in the Cooper Basin. Indeed the Sydney Basin humic kerogens are likely to be better oil source rocks than those of the Copper Basin (possibly by a factor of 2). The oil encountered in Picton 1 in the Sydney Basin has been interpreted as being sourced from terrestrial kerogens.

The sporadic occurrence of localised higher concentrations of resinite in the basin exinites, particularly in the Wollombi Coal Measures, Mulbring Siltstone and Greta Coal Measures is of importance. Limited pyrolysis data from the Bulga Formation show it to be a good mature, oil source rock. Gas chromatography of solvent extractable organic matter from the Mulbring Siltstone shows a minor proportion was derived from marine kerogens.

Vitrinite reflectance studies indicate that the Newcastle/Wollombi/upper Illawarra and Tomago/Whittingham/lower Illawarra Coal Measures are overmature in the deeper central portion of the Sydney Basin, making this area gas prone. The remainder of the basin is mature for oil except towards the northwest which is marginally mature, however the deeper marine units would be mature in this area.

Limited vitrinite reflection data from the Clyde and Yarrunga Coal Measures in the southern part of the basin indicate marginally mature to mature source rocks. These units would be mature further north, where they are more deeply buried.

Source rock geochemistry for the basin is limited, however it indicates that the coal measures are gas prone, and based on similar studies in the Gunnedah Basin, the marine units within and underlying the coal measures are likely to be oil prone. Timing of maturation is largely unknown as there are no published geohistory analyses to date. It has been assumed that maturation and migration began relatively early in the history of the basin and hence early structures are favoured as hydrocarbon traps.

Reservoirs & Seals

Potential reservoir sandstones occur at many levels in the Sydney Basin with porosities up to 20% and methane gas flows to in excess of 1 million cubic metres per day recorded, although large flow rates were generally not sustained. Gas flows have been recorded from the Narrabeen Group, Illawarra Coal Measures, Budgong Sandstone, Berry Siltstone, Nowra Sandstone, Conjola Subgroup and Clyde Coal measures.

Reservoir units include fluvial sandstones in the Narrabeen Group, fluvial and deltaic coal measure sandstones and marine shoreline and shelf sandstones. Terrestrial sediments in the basin commonly contain a high percentage of lithic volcanic detritus which has altered to clays during diagenesis. These clays generally cause low permeabilities in the lithic quartz and lithic sandstones even though the rocks have moderate to good porosities. Permeabilities commonly decrease during testing due to mobility of the clays within the pore spaces.

Low permeability reservoirs with high clay content present special well log interpretation problems necessitating 'shaly sand' interpretation techniques. In spite of inadequate well logs that have been run in exploration wells, a number of probable gas saturated intervals have been identified in various sands. These intervals show log responses which compare favourably with overseas examples of producing 'tight' gas sands but appear not to have been recognised when wells were drilled.

Oil has been reported in basal Narrabeen Group sandstones with porosities of 9% and permeabilities of 0.16-0.28 md. Similar quartz lithic sandstone occurs throughout the upper coal measures and in some cases these sandstones have been reworked by marine processes and cleaned up to varying extents (e.g. Waratah and Watts Sandstones). Notable quartzose fluvial sandstones with good reservoir potential occur in the lower Illawarra Coal Measures (Marrangaroo/Blackmans Flat Conglomerates, and basal Sydney Subgroup) and in the Wollombi Coal Measures (uppermost Watts Sandstone).

The upper part of the Erins Vale Formation contains lithic to quartz lithic tidal and low energy sandy beach deposits with secondary porosity observed in the beach facies associated with degraded algal matts and filaments.

The Maitland/Shoalhaven Group contains several reworked clean quartzose sandstones with excellent reservoir potential (Muree Nowra and Cessnock Sandstones and the Snapper Point Formation). The Farley Formation in the Dalwood Group is quartzose with a tendency to be silty, but it has reservoir potential for gas.

Interbedded red and green claystones within the Narrabeen Group are effective seals to individual fluvial sandstone bodies. The Bald Hill Claystone at the top of the Narrabeen Group acts as an effective regional seal. Siltstones, claystones and mudstones of the coal measures sequences act as local seals to interbedded fluviodeltaic sandstone units such as the Waratah/Watts Sandstone and Marrangaroo Conglomerate.

Thick sequences of marine siltstones in the Maitland/upper Shoalhaven Group and Dalwood Group act as regional seals to their contained sandstone units, such as the Nowra/Muree Sandstone.

Oil Shale

Oil shale occurs as high grade torbanite yielding approximately 300 litres shale oil per tonne and as lower grade cannel coal. Torbanite deposits in the upper part of the Late Permian coal measures have been exploited along the western margin of the Sydney and Gunnedah Basins. The largest deposits have been recorded at Joadja in the south, Newnes and Glen Davis in the central west and Baerami in the southern Gunnedah Basin. Deposits have also been recorded in the Greta Coal Measures in the Hunter Valley.

Oil Shale was mined in New South Wales between 1860 and 1952, generally to produce a kerosene that was used for lighting purposes, but during World War 2 petrol was refined from shales mined at Glen Davis. One tonne of Glen Davis oil shale had an average yield of 203-550 litres of crude oil which gave approximately half this amount of refined petrol.

Coal Seam Methane

The Sydney Basin has favourable geological attributes for the development of a coal seam methane industry:

• There are enormous reservoirs of coal seam gas at economically acceptable depths throughout the basin.
• The majority of the coal seams are sufficiently thermally mature to place them well within the gas generation window.
• The cumulative reservoir thickness is similar to, and in many cases exceeds, the minimum requirements established by successful overseas projects. There are many areas where cumulative coal thickness is well in excess of 20 metres, and numerous individual seams with reservoir thickness greater than 2 metres.
• The interburden distances between reservoirs are small, with the majority of production horizons in the coal measures sequence occurring within a 100 to 200 metre interval.
• Many seams have competent lithologies bounding the production interval. This will assist in ensuring that stimulation treating pressures are contained and the risk of breakout minimised.
• Gas contents are, in many areas, in excess of the minimum requirements for commercial production. Some seams have contents of 18 cubic metres per tonne.
• Gas compositions are attractive. The average methane content ranges between 90 and 95 percent and in some areas, the gas contains up to 5 percent ethane.
• The majority of isotherm determinations conducted to date, in combination with the desorption data, indicate that the seams are gas saturated. This, together with the fact that many seams do not contain large quantities of water, indicates that gas production would initiate shortly after water production commenced.
• These relatively low volumes of groundwater would assist in maintaining low production costs and the relatively good quality of the water would minimise the cost of its disposal.
• Cleat systems are well developed in most seams and many of these systems have not been affected by infilling of mineral matter.
• Permeabilities are similar to those in other parts of the world where commercial production is undertaken. A number of seams have permeabilities which range from 5 to 10 millidarcies and some have permeabilities up to 30 millidarcies.

The deep underground collieries in the Sydney Basin are currently experiencing problems controlling the inflow of gas into mine workings. In the Hunter Coalfield the control of gas is likely to be a significant factor in the profitability of future underground operations. The current underground methods of reducing gas inflow are reaching technological limits. The recovery of gas using surface based stimulation technology may well be a viable alternative.

Sydney Gas Ltd has recently been granted a petroleum assessment lease over its Johndillo coal seam methane pilot project near Camden in the south of the basin. The granting of this type of lease will allow gas produced from the project to be sold rather than flared.

Coal Liquefaction

Coals in the Denman-Scone area of the upper Hunter Valley have been investigated for conversion to liquid fuels. All coals tested proved suitable as feedstock for liquifaction regardless of the process used. The conversion yields were satisfactory, the H/C ratio was satisfactory and the nitrogen, sulphur and oxygen levels compared more than favourably with northern hemisphere coals which have been tested for liquifaction uses.

Please refer to the following publications for further details.

• New South Wales Coal Seam Methane Potential
• New South Wales Coal Industry Profile 2005
• New South Wales Petroleum Potential

Structural Diagram

Sydney Basin Structure Diagram

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Stratigraphic Table

Sydney Basin Stratigraphic Table To Download (Image)

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Exploration Data - Sydney Basin (Onshore & Offshore)

• Sydney-Gunnedah Basin Coal Seam Methane Exploration Fairways & Sweetspots 2006
• Available Core/Cuttings
• Wireline Logs
• Seismic Shot Points (October 2001) - Image

Seismic Shot Points (October 2001) - Data		238 Kb
NSW well locations		44 Kb

View Offshore Sydney Basin Geological Overview

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