EL&C Baillieu research director Ivor Ries told the Sydney Morning Herald that even if half of Origin's contingent resources were proved, BG would bag an amazingly good deal if it could acquire its target at the current offer price of $15.50 per share.

"If you apply a 50 percent rule, Origin would end up having about 20 trillion cubic feet of gas, which would be an extremely profitable LNG producer," he said.

Ries added that it would be surprising if Origin's board accepted the BG bid and that they would be canvassing for alternative proposals

"I think they would struggle to get someone to put cash on the table but you wouldn't rule out them selling part of their reserves to another party, let's say one of the big North Americans or a Chinese or Japanese party for a large cash sum."

Fat Prophets resources analyst Gavin Wendt added that a rival bid was a high possibility.

"BG is giving itself room to move. This may not be the last offer on the table."

Meanwhile, the Australian reported analysts as saying the chances of BG winning control of Origin Energy at a hefty discount to yesterday's closing price of $16.42 a share were slim, given more than one-third of Origin's shares were held by retail investors, who generally acted in concert with the board.

The remaining two-thirds of the Origin shareholder base is held by institutional shareholders who are likely to bet on BG putting up a better offer.

BG chairman Frank Chairman told a media conference in Sydney yesterday that BG saw no need to increase the offer though it would reserve the right to do so.

The UK gas major had, in a statement, slammed Origin's CSG reserves and resources estimate as not presenting a "complete and balanced assessment of the scale and risk of the company's reserves".

BG added that Origin's well density was below that of its peers and that not enough wells had been drilled to give confidence in the reserves changes.

BG said it expected to lodge its bidder's statement within two weeks, and shareholders would receive the statement and a copy of the offer within three weeks.

The company said the offer was likely to be open for two months and might be extended, if required.

Origin is yet to respond to BG's hostile takeover offer but has advised shareholders not to take any action.