Canning basin: New life on Australia's last frontier?

Ross F. Reiser
Kimberley Oil NL
Alfred Cove, Western Australia

The Canning basin in the northwest part of Western Australia is one of the least explored, proven onshore petroleum provinces in the world, in a politically stable country with negligible sovereign risk, one in which the fiscal regime for petroleum operations is world-competitive, and in which the main language is English, albeit with a distinctive local flavor.

Canning is the same size as the Appalachian basin and about half the size of Alberta or Texas (Fig. 1 [55,728 bytes]). The Appalachian basin boasts over 500,000 wells and Alberta about 200,000. In stark contrast, the total well count in the Canning basin is less than 200, for both wildcat and development wells drilled since oil exploration began early this century.

To date, five small oil fields have been discovered in a basin in which all of the elements of a world class petroleum system can be shown to be present: abundant shows of oil and gas, excellent reservoirs, rich source rocks, and several regional seals.

Why the disappointing results? The answer probably lies in the very size of the basin and in the relative costs of drilling and seismic. Most wells can be drilled for less than A$1 million (U.S. $600,000 at recent exchange rates), and seismic costs A$2,000-6,000/km (U.S. $2,000-6,000/mile).

Many small companies that have explored the Canning were unable to afford adequate seismic grids prior to drilling. Consequently, even some recent wells lack adequate seismic control by modern standards, and certainly most early wells (drilled in the 1950s, 60s, and 70s) would be termed "stratigraphic tests" anywhere else in the world: They were drilled on long regional seismic lines, sometimes without even a cross line to confirm the existence of an anomaly.

It can be suggested that many of the dry holes failed to condemn the geological concept they were designed to test.

Regional setting

Over 15,000 m of prospective Paleozoic section is preserved in the Fitzroy trough, deepest of the basin's four major depocenters, while the Willara sub-basin 200 km to the south contains over 10,000 m of similar section.

The key shelf areas are the Lennard shelf to the north of the Fitzroy trough and the Broome platform, which separates the Fitzroy trough from the Willara sub-basin. The reader is referred to Purcell¹ for a detailed discussion of Canning basin geology.

Stratigraphy

Canning basin stratigraphy is presented in Fig. 2 [30,278 bytes]. Ordovician limestones and clastics, some with excellent reservoir characteristics, form the major objectives in the Broome platform area. A Silurian salt sequence that exceeds 1,000 m in places provides a regional seal in the Willara sub-basin, the Broome platform, and the Fitzroy trough and has formed salt domes in some areas.

The Devonian sequence is best known on the Lennard shelf, where it outcrops extensively, and in the northern Fitzroy trough. A generation of dedicated field geologists from the state and federal geological survey organizations has mapped the well preserved outcrop of an extensive Devonian reef complex and some of its basinal equivalents. The reef complex extends into the subsurface, as shown by well results and seismic data. The oil in the largest of the producing fields, Blina, is reservoired in grainstones and calcarenites associated with a Devonian reef.

The Permo-Carboniferous sequence is dominated by clastics, mostly normal shallow marine, but with two glacial sequences being preserved. The other four fields in the basin occur in this sequence.

Prospectivity

The Canning basin is not high on the list of fashionable oil exploration addresses for most oil companies, with the notable exception of Shell among the majors and several junior Australian companies, including Kimberley Oil (see Fig. 1 for Shell and Kimberley land holdings.) An advantage of this lowly rating is that the entry price for land in the Canning is commensurately low.

Shell, in a late 1980s survey of frontier basins, selected the southern Canning basin (Broome platform area, Fig. 1) as having significant potential.² Shell's Looma-1 well has underlined that potential with the discovery of several hundred meters of oil saturation in tight Ordovician carbonates and sandstones sealed by Silurian salt in a huge structure mapped on a 10 km seismic grid. The oil is thought by Shell to be sourced from the Willara sub-basin to the south.

Pessimists are depressed by the tightness of the reservoir section.

Optimists are encouraged by the generative potential of a source rock sequence that can generate the buoyancy pressures necessary to saturate several hundred meters of tight rock. Shell is obviously in the optimistic camp since it is currently acquiring A$5-10 million worth of seismic in the area.

Example prospect

The Selene Prospect in the far southeast of the Fitzroy trough (Fig. 1) is typical of the untested prospectivity common throughout the Canning. Selene combines rollover into the downthrown side of a listric fault, developed in Permo-Carboniferous clastics, with a possible reefal play on the upthrown side of the same fault (Fig. 3 [36,394 bytes]).

The limestone play is based squarely on the extensive field mapping referred to earlier. Playford & Cockbain³ recognized three basic facies associated with the Devonian reef complexes: platform, marginal-slope, and basin.

The platform facies was deposited as reef-fringed limestone platforms which stood a few meters to a few hundred meters above the surrounding inter-reef basins and covered areas of a few hectares to hundreds of square kilometers. The platforms were bounded by marginal-slope deposits of reef talus, the slopes of which approached vertical in some places. The slope deposits then passed into mainly horizontally-bedded, clastic basin deposits.

A structure map at the primary clastic objective level of Carboniferous age (red horizon) is shown in Fig. 4 [44,120 bytes]. Isopachs of the Upper Carboniferous (between red and green events) and Lower Permian (between green and yellow events) intervals suggest that the structure has had a history of growth from at least the Upper Carboniferous, in an area in which the major source sequences are thought to have reached maturity during the early Mesozoic.

Fig. 5 [104,776 bytes] shows the isopach of a possible Devonian limestone sequence and the seismic lines on which it is based. The seismic lines show the apparent abrupt pinchout of the (?) Upper Pillara limestone, a sequence in which major barrier and fringing reefs have been mapped in outcrop immediately to the north of the prospect. Based on the model of Playford & Cockbain,3 the steep dip of the pinchout (20°) is seen as indicating a marginal-slope deposit just seaward of the fringing reef of a limestone platform. This association is documented on the updip, basinward edges of rotated fault blocks on the hinge line between the Fitzroy trough and the Lennard shelf. The relationship is shown on the diagrammatic cross-section in Fig. 6 [21,490 bytes].

Volumetric calculations suggest that the structure could hold upside oil-in-place in excess of 400 million bbl, with possibly a quarter of that recoverable. The prospect is ideally placed to be charged from the major fault that bounds both the younger clastic and the older limestone plays and which should provide a conduit from the depths of the generative "kitchen" to the south.

Conclusions

1. The Canning basin has the elements of a prolific petroleum system that has yet to live up to its potential.
2. Political stability and fiscal regime make it an attractive place in which to look for, and to produce, oil.
3. Entry price is low.
4. Untested structures are common throughout the basin.

References

1. Purcell, P.G. (ed.), The Canning basin, W.A., Proceedings of the Geol. Soc. Aust./Pet. Expl. Soc. Aust. Symposium 1984.
2. King, M.R., Looma-1 reopens the Palaeozoic play in the South Canning basin, APPEA Journal, Vol. 38, No. 1, 1998, pp. 254-277.
3. Playford, P.E., and Cockbain, A.E., Revised stratigraphic and facies nomenclature in Devonian reef complexes of the Canning basin, Geological Survey of Western Australia, annual report for 1975, 1976.

The Author

Ross Reiser is operations director of Kimberley Oil NL, a small, public Australian oil explorer and producer. He was graduated as a geologist from the University of Queensland in 1966 and worked for Geological Survey of Queensland and Union Oil Co. of California before joining West Australian Petroleum Pty. Ltd. in 1970. He joined Kimberley Oil in 1977.

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