(Repeats story published Thursday; no change to text)
* Estimate of potentially large shale oil, gas potential in South Australia
* Analysts say Linc shale potential highly speculative
* Linc CEO expects early well production by end of 2014
* Linc seeks partner to develop shale asset
* Shares hit 18-month high
PERTH/MELBOURNE, Jan 24 (Reuters) - Shares in Linc Energy soared on Thursday, a day after the firm announced it was sitting on potentially large resources of shale oil in South Australia, although some experts urged caution since the estimate is at very early stage.
Two respected independent consultants estimated Linc's 100-percent owned Arckaringa Basin could hold between 103 billion and 233 billion barrels of oil equivalent (boe) in formations comparable to liquids-rich shale plays in the United States.
But as the share price shot up by more than 30 percent at one stage on Thursday, and Australian media trumpeted the "$20 trillion shale oil find", some urged caution on the shale play in the little explored Arckaringa Basin.
"It's at a very early stage... it's a theoretical play," said Peter Strachan, an analyst with Stock Analysis in Perth.
The huge bump in Linc's share price even took the company's chief executive, Peter Bond, by surprise.
"We didn't expect it to go this nuts," Bond told Reuters, while adding that he chalked up the gains to the company being "massively underpriced."
"A lot of what you are seeing is a re-rating of the company based on people starting to understand that the assets we have are far better than what they have been giving us value for," he said.
Bond, Linc's largest shareholder, owns 39 percent of the company's shares.
HOW MUCH IS RECOVERABLE?
At the heart of excitement around Linc's new find were figures displayed prominently on the first page of the company's announcement: 103 to 233 billion barrels of oil equivalent.
But the estimates are what are called "unrisked prospective resources", which give no indication as to how easily the reservoirs can be tapped or whether oil and gas can be profitably produced.
Some analysts zeroed in on another estimate -- the 3.5 billion barrels that are likely to be recoverable -- that was on a chart on the third page of its release and which they said gave a more accurate picture.
"I think the industry is becoming ill-disciplined in talking about volumes of oil that are in place (which) is so different to what is actually recoverable," Johan Hedstrom, an energy analyst with Bell Potter Securities in Sydney, said.
Linc's Bond said media reports perhaps should have focused on the likely recoverable figure but defended the announcement.
"Most of them don't know the work we've done," Bond said, adding that Linc is confident there is a large amount that is recoverable.
"How much is recoverable is always the question. Is it 3 billion barrels or is it 203 billion barrels? Even if it's 3 or 4 billion barrels... that's a massive find in this part of the world. No matter how you cut it, it's still a massive outcome," Bond said.
Linc has appointed Barclays Capital to help it find a partner with experience in shale drilling and the company expects to announce a partner in the next six months, Bond said.
Linc's shares soared to a high of A$2.83 on Thursday and last traded up 23 percent at A$2.67.
Its market value has more than quadrupled over two months as Linc, mainly known for its efforts to develop underground coal gas-to-liquids projects, recently started producing oil in the United States.
IT'S NOT JUST THE ROCKS
Even if Linc ends up finding huge amounts of oil and gas in the Arckaringa Basin, it will still face huge challenges, shared by other developers in Australia's fledgling shale industry.
Bond said he expects production could begin by the end of 2014, but some analysts said commercialisation, even in ideal circumstances, would take about five years.
Geoff Barker, partner at Resource Investment Strategy Consultants (RISC) in Perth, said even if the geology was right, the cost issue was formidable in Australia.
"Even if you've got rocks which have the necessary conditions for success, the big challenge we have is one of cost. Our cost structures are very high."
Costs can be several times what U.S. shale developers pay -- drilling a shale well in the U.S. costs about $10 million, while in Australia prices are 50 percent higher at about $15 million.
And unlike the United States, where a "shale gas revolution" turned it from an importer to a prospective exporter in a matter of a few years, Australia has little infrastructure in the far-flung locations where most shale exploration is occurring.
But high risks can mean high rewards.
"There's really two times when global oil and gas companies...come to large resources like this. Firstly it's at the early stage, where they pay a smaller amount to take on a higher amount of risk, but the reward is potentially there.... Or they pay a fortune when it's completely de-risked," said Ian Davies, managing director of Senex Energy, an Australian company which also has shale acreage. (Editing by Ed Davies)