Offshore Sydney Basin - Geological Overview
In 1997-1998 Department undertook a review of the offshore Sydney Basin with a view to assessing the petroleum exploration potential in this area. The offshore extension of the Sydney Basin stretches from the coastline eastwards some 70 km to the shelf-break water depth rarely exceeds 150 m. The total area covers approximately 9500 sq km.
Within this area some 5 km of Permo-Triassic sediments are overlain by a thin-veneer of post-break-up Cainozoic sediments with the possibility of some Late Mesozoic sediments preserved across down-faulted basement blocks. As a result of previous exploration campaigns a comprehensive regional seismic grid of mainly 1981 to 1991 vintage seismic data covers much of the region. These data identified a broad structural framework in which the onshore Sydney Basin trends are truncated by major thrust zones running parallel to the margin. These thrust zones represent extensions of the Hunter-Mooki Thrust which forms the boundary between the onshore northern Sydney and Gunnedah Basins and Palaeozoic hard-rock New England terrain to the north. Compressional movements including overthrusting along this boundary resulted in several episodes of foreland loading, the associated sedimentation being instrumental in the formation of the Sydney Basin.
Major offshore thrusts and thrust related ramps are intersected by north-west structural trends associated with transform and extensional faults formed during the Latest Cretaceous - Early Tertiary sea floor spreading episode of the adjacent Tasman Sea Basin. These provide both for strong fault partitioning of the Permo-Triassic sediments and a variety of trapping mechanisms.
Although no wells have been drilled in the offshore area, the onshore stratigraphy is well known, although poorly evaluated from a petroleum perspective. A number of thick shallow to deep marine shale and siltstone sequences occur in the Lower to Middle Permian. These units are separated by a number of near shore and shoreline marine sandstone sequences and also a brief terrestrial depositional event represented by the Greta/Clyde Coal Measures. These sequences are believed to be strongly represented offshore and offer potential reservoir objectives particularly where deposited in shoreline facies with source and seal rocks provided by the surrounding marine shales and siltstones.
The Upper Permian sequence is dominated by the well known Sydney Basin coal measures. The Sydney Basin contains approximately 50% of Australia's black coal resources. These also contain sediments deposited during marine incursions. The north eastern portion of the coal measures are dominated by large conglomeratic tongues.
The Permian is overlain by the Triassic Narrabeen Formation and Hawkesbury Sandstone. Both are massive fluvial sandstones sequences above which in the central onshore basin lies the Wianamatta Shale, providing regional seal.
The major exploration risk perceived in the offshore Sydney Basin is the presence of suitable reservoirs, the sequences being considered dominated by volcanoclastics with labile constituents. More recent petrophysical studies identify three distinct provenances for Permo-Triassic sediment supply, with intervals of cleaner sands identified within the transgressive marine sandstones. These were deposited in a number of discrete environments; within the Upper Permian either as marine reworked sands and delta front sands and in the Lower Permian as beach sands. Additionally, the Narrabeen Group, sealed by the Bald Hill Claystone/Gosford Sub-Group, may provide reservoir in the near offshore Sydney region over structures such as the Biggus Prospect. Across the eastern flank of the Offshore High more open marine conditions are likely and cleaner sands are likely even in Lower Permian sequences.
As onshore Sydney Basin reservoir studies have shown even where primary poro-perm characteristics are low with the appropriate mud program and with reservoir stimulation the potential of low permeability sands cannot be dismissed, analogies being drawn with the successful production in North American of tight gas sands and conglomerates.
Oil has been recorded from some 55 locations within the onshore basin. These occurrences include oil shows, oil bleeding from core, oil staining, and fluorescence in core. Oil has also been reported from surface oil seeps in areas such as Terrigal, Richmond and Shellharbour; and seeping into coal mines in the southern coalfield. Gas occurrences have been encountered during coal mining operations as well as from 34 onshore petroleum and coal exploration wells with flows ranging up to 2.5 MMCFPD with an average of about 100 MCFPD. The two principal coal measure sequences - the Newcastle and Tomago (which correlate with the southern Illawarra) - are known to underlie most of the onshore portion of the basin and interpreted from seismic to extend across much of the offshore areas. Geochemical analysis shows that these coal measures constitute regional source rock sequences principally for gas but also with minor liquids, levels of maturity being predicted across the proposed gazettal areas within the gas and condensate windows.
In all some 10 prospects and leads have been identified by current operators, including the Biggus Prospect where estimates of gas reserves exceed 1.0 TCF. Traps are identified associated with thrust plays, overthrust duplex structures as well as in extensional features such as faulted anticlines. A major structural high block, the Conjola High, provides regional targets for Middle to Lower Permian sequences. Although the age of cover across this regional high remains contentious, where covered by a more complete sequence, reservoir targets within the Triassic Narrabeen Group and Permian coal measures provide obvious objectives. Alternatively, the presence of the Lower Permian sandstone sequences deposited in the east in open marine conditions raise the issue of the less conventional reservoir objectives.
Located as it is adjacent to Australia's most populous metropolis, with over 5 million inhabitants and major industry located within the Newcastle-Sydney-Wollongong area, the Sydney Basin offers an unique marketing opportunity. With the main gas supply contract to Sydney terminating in 2006 and a shortfall in supply anticipated from 2000 additional sources of gas are actively being sort. Major utilities such as AGL and Pacific Power are keen to negotiate gas supply contracts with active explorers, prior to discovery and production.
Please refer to the following publications for further details.
Structural Diagram
| Offshore Sydney Basin Structure Diagram |  |
60 Kb |
Stratigraphic Table
| Offshore Sydney Basin Stratigraphic Table To Download (Image) |  |
74 Kb |
Exploration Data - Sydney Basin (Onshore & Offshore)
| Seismic Shot Points (October 2001) - Data |  |
238 Kb |
| NSW well locations |  |
44 Kb |