It is the biggest update to the potential for Leschenault in years.

The target sat within Empire Oil & Gas (40%), mapped but largely left in the bottom drawer until a change of management at companies saw Pilot take a 60% interest.

RISC recently completed an audit of Leschenault, a giant structure that straddles EP 416 and EP 480 between the cities of Mandurah and Bunbury.

It concluded that the Lesueur Sandstone could host prospective resources of 150-970 billion cubic feet while the Sue Sandstone could host 120-625Bcf.

In terms of best unrisked prospective resources that would be 1.6Tcf.

RISC estimated a geological chance of success of 10% for the Sue Sandstone and just 5% for the Lesueur Sandstone given the early stage nature of the southern extent of the Perth Basin, which hosts the large but undeveloped Whicher Range tight gas discovery.

Because both levels can be drilled with a single well, operator Pilot estimates that the combined chance of success is 14.5%.

While high risk exploration, Pilot says demand in the southern Perth Basin market for gas is sufficient that development is almost certain in the event of success.

The Dampier to Bunbury pipeline runs across EP 416 and EP 480, and the Western Australian government has recently started re-investigating the potential of extending the pipeline some 300km to the east to extend gas reticulation to regional centres such as Katanning and Albany.

Pilot said that while the southern portion of the basin has only been lightly explored, the Waitsia gas-condensate discovery made by AWE and Origin Energy in the northern party of the basin shows that large new discoveries are possible, even close to the most drilled out area.

There are less than two dozen wells drilled onshore and offshore in the southern Perth Basin, and five of those are at Whicher Range.

The Leschenault structure has been confirmed by seismic data over an area of up to 240sq.km, which RISC has defined as a robust three-way dip feature that relies on closure to the west by a major bounding fault and exhibits two culminations, both of which offer potential drilling locations for a single vertical well to test the two conventional reservoir targets.

The new interpretation has increased the potential of the structure thanks to a review of the seismic data and airborne data, and a basin-wide assessment of existing discoveries and associated reservoir parameters.

The Permian Willespie Formation at Whicher Range is a direct analogue for the Sue Sandstone, and has similarities with the Dongara Sandstone to the north. There are also oil shows in the regional Sue Coal Measures, including on one of the offshore wells, and it is expected to be the source for any trapped hydrocarbons.

The Sue Coal Measures are equivalent to the Irwin River Coal Measures that source much of the gas in the north of the Perth Basin.

The Permian Sue Sandstone and the Triassic Lesueur Sandstone are thick and laterally extensive, and the uplift in the area means Leschenault should have superior reservoir properties given the estimated target depths between 500m and 2500m.

By comparison, Whicher Range sits at some 4000m.

Lake Preston-1 encountered gas in the Sue Formation from 4000m, but the formation is likely to be uplifted at Leschenault.

Of the four wells within the permits, Preston-1 and Lake Preston-1 were drilled off-structure and did not test valid prospects, while Pinjarra-1 well did not reach its planned depth due to an interpretation error.

The black soils in the area and the presence of the Bunbury Basalt have posed serious issues for past explorers in the 1960s.

Pilot's most relevant well, 2012's GSWA Harvey-1, did not reach the Sue Sandstone primary reservoir objective, but did intersect the Lesueur Sandstone, with excellent porosity of up to 20% on the flank of the structure.

To help derisk any drilling Pilot is planning a geochemical survey next year.

If it can find micro-seepage of hydrocarbons broadly conformable to Leschenault it will suggest that the major risk, that the seal has been breached by the fault, is no longer a concern.

"RISC's report confirms that Leschenault offers the potential for a major new gas discovery, just a few kilometres from regional pipeline infrastructure and within a proven petroleum system," Pilot managing director Iain Smith said.

"To the best of our knowledge Leschenault is the largest undrilled prospect within the Perth Basin, for which independently verified prospective resources are available."