The offshore Sydney Basin permit PEP 11 (Figure 1) is over 200 km in length (125 miles) covers over 8,200 km2 (2 million acres) and lies immediately offshore Sydney in the state territorial waters of New South Wales. Water depths in the permit range from 40 m to a maximum of 200 m.
Exploration over the region has been intermittent over the last 30 years. A number of seismic surveys have been carried out over the area including two regional surveys of 1,934 km and 1,742 km completed in 1981 and a third semi-detailed survey of 603 km in 1991. These surveys enabled the delineation of six major structural leads (Figure 1). No wells have been drilled offshore Sydney Basin to date. Although no wells have been drilled offshore, over 70 wells have been drilled in the onshore sector. Whilst most of these wells were sited without the benefit of seismic data, almost all had gas shows and over 40% were reported to have flowed gas, mostly from the Early Triassic Narrabeen Group. Numerous oil seeps have been reported from underground coal workings and offshore. The presence of sea surface oil slicks has long been reported off the coast from Terrigal. Recent interpretation of Landsat satellite imagery has indicated some sea slicks, including one that directly overlies the South Baleen prospect. A combination of depth to basement calculations from aeromagnetic data and seismic interpretations have defined the major tectonic elements of the Sydney basin and concluded that much of the multiple coal measure sequences present in the onshore black coal mining districts of Sydney would be present and mature for hydrocarbon generation offshore.
Prospects & Leads
South Baleen
Located in the northern sector of the permit, the South Baleen prospect is characterised by a strong structural configuration. Recent reprocessing of key lines by Bounty Oil has improved the data over the prospect (Figure 2), confirming the presence of a substantial feature. Bounty is presently evaluating additional reprocessing to mature the feature. The configuration of South Baleen is a fault-controlled anticline located on a crestal position along the Offshore Uplift. The feature has approximately 300 mSec of closure and covers approximately 74 km2. The validity of the South Baleen prospect is further confirmed by the presence of an oil slick detected on satellite imagery directly above the feature.
Additional Prospects
A review of the existing data set and previous work (ESP (1982), Santos (1992) and Ampolex (1992)) has confirmed both the presence of several large leads and some smaller prospects. These are shown in Figure 1. Evaluation of the older data sets in conjunction with the reprocessed data is ongoing.
Proprietary Seismic Processing
Seismic quality ranges from poor to fair in both surveys. Generally the data has a low signal to noise ratio at depth; the signal is contaminated by strong multiples from hard bottom reflections and reverberating refractive energy as well as apparent random noise trains from shallow layers. These multiples and coherent noise trains in turn appear to mask any primary reflectors.
Recent work by Bounty has applied a proprietary processing technique to both the 1981 and 1991 data in an effort to reduce the effect of the coherent noise.
The software is based on the application of a truly hyperbolic integral transform process, along with a number of other advanced seismic processing routines based on proprietary mathematical formulations. These include 900 depth migration, zero offset forward modeling and a collection of random noise filtering programs based on sophisticated implementations of predictive filtering.
Figure 2 shows a portion of one of the 1991 dip lines in the northern part of PEP 11, before and after the reprocessing. The line crosses the South Baleen prospect. The attenuation of multiple energy and the associated improvements in data continuity and frequency are clear, in particular in the shallower part of the section. The consequent improvement in the structural imaging is also clear.
Geologic Setting
For details of the technical aspects of the geology of the offshore Sydney Basin the reader is referred to the paper by Alder et al, 'Prospectivity of the Offshore Sydney Basin: A New Perspective', published in the 1998 APPEA Journal.
Reservoirs The main productive fairway in the offshore Sydney Basin is considered to lie within Permian and Triassic sediments of the Newcastle Syncline, and in structures present towards the crest of the 'Offshore Uplift'. The primary target reservoirs of the Sydney Basin are sandstones of Late and Early Permian age and Triassic sandstones of the Narrabeen and Wianamatta Groups. The Triassic sandstones generally have high porosity, given their more quartz-prone composition, while a lesser degree of burial is assumed for Jurassic and Early Cretaceous times.
Later Permian sandstones that form part of the coal measures sequence within the Late Permian Illawarra Group offer short-range hydrocarbon accumulation sites for any liquid or gas generation from the co-deposited coal measures. Early Permian sandstones of the Muree, Snapper Point and Wasp Head Formations provide similar reservoir potential adjacent to the Greta Coal Measures and the deeper Clyde Coal Measures.
Reservoir potential may also occur in smaller sand units interbedded with individual coal horizons; potential could also reside in the coals themselves if cleat geometry enables sufficient production rates. Finally, potential exists for commercial flows from fractured basement volcanics or within the Gerringong Volcanics of Later Permian age.
Source Rocks
The most likely source rocks in the basin are the Permian coal measures - a thick sedimentary section represented by locally stacked coal seam sequences of both Early and Late Permian age. Total Organic Contents (TOC) of the coals in the Late Permian often exceed 10-20%. In the onshore wells, high inertinite and vitrinite contents of the coals indicate that they are primarily a gas source, but their chemistry also indicates an age affinity with the Toolachee Formation of the Cooper Basin of SW Queensland and South Australia (the Toolachee Formation is generally regarded as the source horizon for most of the Cooper Basin oilfields).
Seals
Shales and siltstone within the Permian coal measure sequence will provide intraformational seals. Regional seals are provided by Early Triassic-Narrabeen Group shales and by the Middle Triassic shale sequence of the Wianamatta Group.
Conclusion
The Offshore Sydney Basin permit PEP 11 is over two million un-drilled acres with multiple prospects immediately adjacent Australia's largest gas market. The South Baleen prospect is a large structure that still looks very good after state of the art seismic reprocessing.
MMR Price at posting:
60.0¢ Sentiment: Buy Disclosure: Held
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