(Repeats story from Thursday)
* A fifth of projects at risk with oil at $80/barrel- Rystad
* Oil prices drop puts economics of some projects in doubt
* Canadian oil sands, LNG projects at high risk of delays
* Some North Sea, Arctic and Gulf of Mexico fields on hold
By Ron Bousso
LONDON, Dec 4 (Reuters) - Global oil and gas exploration projects worth more than $150 billion are likely to be put on hold next year as plunging oil prices render them uneconomic, data shows, potentially curbing supplies by the end of the decade.
As big oil fields that were discovered decades ago begin to deplete, oil companies are trying to access more complex and hard to reach fields located in some cases deep under sea level. But at the same time, the cost of production has risen sharply given the rising cost of raw materials and the need for expensive new technology to reach the oil.
Now the outlook for onshore and offshore developments - from the Barents Sea to the Gulf or Mexico - looks as uncertain as the price of oil, which has plunged by 40 percent in the last five months to around $70 a barrel.
Next year companies will make final investment decisions (FIDs) on a total of 800 oil and gas projects worth $500 billion and totalling nearly 60 billion barrels of oil equivalent, according to data from Norwegian consultancy Rystad Energy.
But with analysts forecasting oil to average $82.50 a barrel next year, around one third of the spending, or a fifth of the volume, is unlikely to be approved, head of analysis at Rystad Energy Per Magnus Nysveen said.
"At $70 a barrel, half of the overall volumes are at risk," he said.
Around one third of the projects scheduled for FID in 2015 are so-called unconventional, where oil and gas are extracted using horizontal drilling, in what is known as fracking, or mining.
Of those 20 billion barrels, around half are located in Canada's oil sands and Venezuela's tar sands, according to Nysveen.
For a graphic on the outlook for global oil and gas projects click here: link.reuters.com/nab63w
Geographically, the projects on the balance are widespread.
Chevron's North Sea Rosebank project is among those with a shaky future and a decision on whether to go ahead with it will likely be pushed late into 2015 as the company assesses its economics, analysts said.
"This project was not deemed economic at $100 a barrel so at current levels it is clearly a no-go," said Bertrand Hodée, research analyst at Paris-Based Raymond James. He estimates a development cost of $10 billion for Rosebank, with potential reserves of 300 million barrels - meaning the Chevron would only recoup $33 a barrel.
Even with oil at $120 a barrel, the economics of some projects around the world were in doubt as development costs soared in recent years. Chevron's Rosebank project has already been delayed for several years.
In response to a question from Reuters, the company said "the Rosebank project is in the Front End Engineering and Design phase. The review of the economics and the additional engineering work is progressing... It is premature to make any statements on an FID date."
Hodée said any offshore project with a development cost above $30 a barrel would most likely be put on hold in current oil prices.
Norway's Statoil this week said it had postponed until next October -- a six-month delay -- a decision to invest $5.74 billion in the Snorre field in the Norwegian Sea as its profitability was under threat.
New oil fields typically require four to five years to be developed and billions before the first drop of oil is produced.
Any cutbacks in oil production bodes ill for international oil companies that are already struggling to replace depleting reserves as exploration becomes harder and discoveries smaller. It also points to tighter supplies by the end of the decade.
Projects in Canada's oil sands, which require expensive and complex extraction techniques, are the most unlikely to go ahead given their high investment requirements and relatively slow returns. Total recently decided to postpone the FID on the Joslyn project in Alberta, the cost of which Hodée estimated at $11 billion.
Shell's liquefied natural gas (LNG) project in Canada's British Columbia, already under pressure from a looming supply surge, faces further strain in the current price environment, analysts said. According to research by Citi, the project requires oil at $80 a barrel to break even.
Royal Dutch Shell's chief financial officer Henry Simon indicated in October that it was "less likely" to go ahead with unconventional projects in West Canada if oil falls below $80 a barrel.
Asked by Reuters what the company's current thinking was, a Shell spokesman would not comment on "internal decision-making."
Even in the Gulf of Mexico, one of the most attractive oil production areas in the world, projects are facing challenges.
BP last year put on hold a decision on its Mad Dog Phase 2 deep water project in the Gulf of Mexico after its development costs ballooned to $20 billion and the oil major is now expected to further delay an investment on the field's development.
"BP were talking positively about bringing it back, but now it may be put on hold," BMO Capital Markets analyst Iain Reid said.
BP's chief financial officer Brian Gilvary however said in an analysts briefing in October that he expected Mad Dog Phase 2 to be sanctioned in the first quarter of 2015.
Statoil's Johan Castberg field in the Barents Sea, which was expected to get its FID in 2015, seems unlikely to get the go-ahead at the moment given it has an estimated project cost of $16-$19 billion, Hodée said.
Statoil said that the final project design is due in the summer of 2015. Its giant Johan Sverdrup field in the North Sea is still on track for development with a price tag of $32.5 billion. (Additional reporting by Oleg Vukmanovic in Milan and Balazs Koranyi in Oslo; Editing by Sophie Walker)