This will take Exxon's working interest in the permit to 65%, while Drillsearch holds the remaining 35%.

In return for the additional stake, Exxon's subsidiary Esso Australia as operator will spend $US1.35 million on the 2008 work program, which involves reprocessing vintage 2D seismic data and shooting about 100km of new 2D data.

The seismic acquisition survey is expected to commence mid-year.

Drillsearch said it still considered WA-318-P to be prospective, even though it was farming-down its interest.

"Whilst Drillsearch currently holds 35 percent working interest in WA-318-P and has had to part with 10 percent of its equity in this project it is, nevertheless, pleased with the potential of this block and looks forward to an exciting and successful exploration program," the company said.

In October last year, the joint venture drilled the Marina-1 exploration well, which recorded hydrocarbons in at least five zones.

But analysis of wireline logs and pressure data indicated the reservoir was tight over those intervals.

At the time, Drillsearch said that even though the well failed to make a commercial discovery, hydrocarbon shows had encouraged the JV to further explore the permit.