Total's Elgin field was discovered by Elf in 1991 but it was ten years before production began in March 2001.
It shares a processing and export platform in the North Sea's Central Graben Area with the adjacent Franklin field, located 6km south-east of the Elgin rig.
Before the leak, the Elgin-Franklin hub was delivering around 10% of the UK's gas and a significant amount of ultra-light oil (condensate) production.
It is the world's largest "high-pressure/high-temperature" development with temperatures reaching as much as 190C and pressure hitting highs of 1100 bar.
It is also considered to be the deepest producing field in the North Sea at a water depth of 93m/305ft.
The well, around 5km below the sea bed, is thought to be bringing up 'sour gas' which is very flammable and poisonous.
This combination of pressure, heat and depth - coupled with the volatility of the gas - makes the North Sea operation one of the most complex of its kind in the world.
In 2008, the well was producing 20,000 barrels of oil equivalent per day and gas at a rate of 2.6 million standard cubic meters per day.
At peak production the fields were turning out approximately 220,000 barrels of oil equivalent per day, or around 7% of total UK production.
There is thought to be approximately 60 million cubic metres of condensate and 50 billion cubic metres of gas left in the Elgin-Franklin field.
First production from these two adjacent gas fields was brought on-stream in March 2000 and is expected to continue until at least 2022. To date, Total has invested over £1.65bn in the two fields including £28m for R&D
Oil produced at the facility is exported through the BP-operated Forties pipeline system to Grangemouth where it is processed.
Its gas flows through the Seal pipeline to Bacton in Norfolk. According to data from the National Grid, the gas flow through the Bacton Seal terminal fell from around ten to seven cubic metres.
The leak saw a sharp rise in gas prices for day-ahead delivery with figures going up by 8.5%.
Industry leader Jake Molloy said the incident was the worst scenario that could happen on a North Sea rig and due to the unique circumstances of the platform made it almost impossible to find a quick fix solution.
The gas is leaking from the pipelines coming up from the well, known as the annulus. Gas started leaking from the G4 Well Head on the platform after an incident on Sunday afternoon.
However, the leak was discovered to be coming from a "dead well" which had been out of use for nearly a year.
Staff monitoring the well noticed that pressure was building up between the annulus.
It is thought to be the first incident of its kind in the North Sea with no clear indication of how quickly it can be resolved.
Mr Molloy said: "The likelihood of an ignition is remote, or at least has been minimised as far as it possibly can be.
"Beyond that no one knows. We're dealing with the unexplained, the unencountered.
"So I can sympathise with Total to some extent here, because it's unprecedented in the North Sea.
"How do they tackle it? You've got the two extremes. Do we drill a relief well, or just hope the gas dries up?
"I think hoping the gas dries up is probably overly optimistic. So we're looking to do some kind of intervention, some tertiary well to take the pressure away.
"I see Shell have taken the precautionary measure of downmanning the Shearwater. That's probably to do with the good weather we're having. In very still conditions the gas appears to be building into a cloud.
"We're looking at potentially a major event if the gas ignites, but everybody should be safe. That's the bottom line.
"It's still there. You're looking something on the scale of Piper Alpha here. Freeflowing gas into a platform. On the positive side, nobody's there. So the human side has been dealt with.
"But the potential remains for an ignition source and for the complete destruction of that installation.
"God willing, they get in there and kill this well sooner rather than later."
David Hainsworth, Total's safety manager, admitted that the situation would be difficult to get under control and could last for weeks.