A massive cloud of explosive natural gas boiling out of the North Sea from a leak at Total's abandoned Elgin platform forced wider evacuations off the Scottish coast on Tuesday and an air and sea exclusion zone was declared.

Dubbed the well from hell by a Norwegian environmentalist who warned that the high pressure of the undersea gas exploited by the French firm made it especially hard to shut off, clouds of gas were seen by workers over the platform and a sheen of oil, also produced from the rig, was spreading over the water.

Total said it was unable to predict when or how the gas - and oil - leak would be contained. With memories still fresh of the environmental disaster in the Gulf of Mexico following the 2010 blowout at a BP oil well, managers and British officials assured the public that there was little immediate threat.

Total said shipping had been ordered to come no closer than two miles from the Elgin platform and aircraft no nearer than three miles if they flew lower than 4,000 feet - effectively shutting out helicopters but not affecting airline traffic.

Shell cut staffing at two of its nearby facilities, the Shearwater production platform, which continued pumping oil, and the Noble Hans Deul exploration rig, which suspended drilling.

Oil from Total's Elgin platform, which traders estimate at normally some 60,000 barrels per day (bpd) of light condensate, is exported via the BP-operated Forties pipeline system.

Its gas flows through the SEAL pipeline to the Bacton terminal. British gas prices for this week extended gains, rising close to four percent to 57.7 pence per therm.

COMPLEX PROBLEM

British officials said the gas, containing high levels of poisonous hydrogen sulphide - familiar from the smell of rotten eggs - should disperse in the atmosphere. But it poses a risk to anyone close to the source, making capping the well complex.

One option could be to ignite the gas - something the 238 crew on Elgin took extreme care not to do when heading for safety. But even that might go on for months, one environmental activist said, before a relief well could be sunk.

This is the well from hell, said the activist, Frederic Hauge, head of Bellona, a leading Norwegian group that closely monitors the oil industry. This problem is out of control.

Platform staff had struggled for 14 hours to contain the leak before having to evacuate early on Monday, said Hauge citing anonymous sources involved in the incident he said Bellona had spoken to. Staff were trying to close a well when problems arose, a view confirmed by trade union officials and engineers with knowledge of the matter.

They saw the sea bubbling with gas under the platform, said Hauge. This is quite shocking.

This situation is only going to get bigger and bigger.

A Total spokesman said technical teams were investigating the cause of the gas leak but declined to give further details beyond confirming that the gas leak was continuing.

Total removed all workers aboard the Elgin rig on Sunday, shutting down oil and gas production and reporting no injuries.

Shell described as a prudent precautionary measure the removal of some workers from two of its platforms nearby, in an area about 150 miles (250 km) off Aberdeen.

A partial downman of personnel is under way at the company's Shearwater platform and the nearby Noble Hans Deul drilling rig, a spokesman for Shell UK Limited said.

WELL FORM HELL

Environmental groups have voiced growing alarm over the impact of the leak.

I am extremely worried about the situation, Bellona's Hauge said.

At this stage it is impossible to get on the platform. The only solution, it seems, is to have a release well, but we don't know how deep the leak is. And how do we place a drilling rig close enough given the risk of explosion?, Hauge said.

An option would be to set the gas on fire to enable a rig to come close enough to drill the release well, he said: But this will still last for months before this situation can be solved.

The reservoir lies 6,000 metres, or nearly four miles, beneath the seabed and is a high-pressure, high-temperature reservoir, he said, making shutting off the flow of gas bubbling to the surface harder.

The pressure in the reservoir is 600-1,100 bar which is higher than at Deepwater Horizon's, which was a little over 800 bar, said Hauge, referring to the BP oil platform from which, two years ago, poured the worst spill in U.S. history.

Temperatures in the Elgin reservoir can reach 200 degrees Celsius (392 Fahrenheit), he said: We don't know how much gas there is again in the reservoir and there is a high levels of carbon dioxide and hydrogen sulphide, which makes it extra corrosive, he added.

However, the environmental impact of gas condensate leaks is substantially lower than from oil spills, Britain's energy ministry said.

An aerial surveillance flight confirmed there was a sheen on the water. The sheen, thought to be gas condensate, a petrol-like substance, is a direct consequence of the gas leak and usually evaporates naturally, a Total UK spokeswoman said.

The sheen is caused by an estimated hydrocarbon release of between 2 and 23.5 tonnes, according to the energy ministry.

(Additional reporting by Karolin Schaps; Editing by Jason Neely and Alastair Macdonald)