Origin's advisor, Macquarie Bank, will seek competing bids, but is unlikely to find anything to match BG's offer, which is at a 40% premium to Origin's previous closing price.
The Australian Financial Review's Street Talk column speculated that Santos might be able to join forces with a technology partner, such as ConocoPhillips, and a energy retailer seeking Origin's 3 million retail customers (the AFR cited TRUenergy and Babcock & Brown Power as possible candidates), to match the bid.
But this seems unlikely. Last time Santos tried to acquire a major CSM producer - Queensland Gas Company - the Australian Competition and Consumer Commission ruled the bid out of bounds.
Indeed, the ACCC might also find that BG is asking for too much in seeking to buy Origin while also holding equity in QGC.
At the very least, BG might have to stop building its stake in QGC and renegotiate its deal with that company.
BG can probably find a way of clearing the ACCC's hurdles, but it must also get its bid past the Foreign Investment Review Board.
On the face of it, this shouldn't be a problem, at least with Origin's upstream assets.
These assets mostly fall into three groups - Queensland CSM, Cooper Basin oil and gas, and Bass Strait gas. There are few foreign-owned companies active in either Queensland CSM or in the Cooper.
And while Bass Strait has long been dominated by the ExxonMobil-operated joint venture with BHP Billiton, many new projects have been initiated by Australian companies.
Indeed, Exxon operates only in Gippsland, while the parts of Bass Strait where Origin is most active - the Otway and Bass basins - are dominated by Australian companies operating several similarly sized projects. Hard to make a national interest case on these grounds.
In energy retail, Origin has 23% of the eastern Australian market, second only to AGL. This shouldn't pose a problem, but even if it did BG could get around this by agreeing to sell the network to Babcock & Brown Power, or some other local player.
No doubt BG wants the retail and electricity generation assets, as it is familar with this integrated energy model in its own UK operations, but CSM is the big prize, and BG will do what it must to secure its LNG ambitions.
However, the biggest cloud over the deal may be increasing nervousness in government, sections of business, and the community at large over rapidly growing Chinese investment in Australia.
Many Chinese companies will be keeping a close eye on BG's bid, looking for precedents. But is hard to see how the Foreign Investment Review Board could justify rejecting BG's bid.
It's also unlikely that Origin shareholders will reject BG's handsome offer.
If somehow the bid were knocked back what would happen?
Several analysts and journalists have suggested that this could lead BG to move on Santos, either in its own right or in partnership with energy retailer and gas producer AGL Energy.
But Santos's 15% shareholding cap is not due to be removed until the end of November, and BG's actions in recent months suggest it is in a great hurry - the company reportedly has more forward contracts for LNG than it has gas to supply them.
PNN'sbest guess is that BG would look at a deal with Origin, and perhaps with Santos, that involved long-term gas sales and a major shareholding with both partners having upstream and downstream equity.
BG is probably more interested in securing the gas than in acquiring Australian companies, and might be prepared to settle for firm gas supplies from companies it has safeguarded against hostile takeovers.
Not that PNN believes BG's bid for Origin is likely to fail - just that the British major will get its gas come what may.
