A government regulator said it has launched an investigation into the oil spill affecting the Fylde coast.
It comes as a clean up operation on coastline stretching from South Shore in Blackpool all the way up to Knott End is set to continue this morning.
The sea is still off limits, while the Knott End to Fleetwood ferry service remains suspended because the slipway is still too dangerous.
The shellfish beds at Knott Spit and Sea Centre South in Knott End have also been closed, while beach-goers were warned to keep away from the oil.
Around 11.5 tonnes of waste - made up of oil, sand, and stone - have been cleared away for council workers, who were joined for specialists, in recent days.
The Maritime and Coastguard Agency said samples of the oil showed the offshore storage facility, linked by pipework to the Douglas Complex in the Irish Sea, as the origin.
A spokesman this morning said between 'two and 20 tonnes' of oil leaked, and said authorities were informed 'immediately'.
And a spokesman for the Department for Business, Energy and Industrial Strategy (BEIS) said in a statement: "We are aware of an incident that resulted in the release of crude oil more than 30 kilometres from the coast of north west England on Monday, July 10.
"The Offshore Petroleum Regulator for Environment and Decommissioning is investigating the circumstances around the incident to determine whether a breach of environmental regulation occurred.”
The Health and Safety Executive (HSE) has also made enquiries.
Eni reported the spill to authorities immediately, and has been heavily involved in the clean up operation since the oil began washing up, it is understood.
The oil began washing up at Bispham on Monday, and was later reported as far south as the mirror ball in South Shore, and as far north as the River Wyre and Knott End.
In Blackpool, 23 25kg bags were filled with waste yesterday, compared to 72 the day before. In Wyre, around 200 were filled, up from 165 on Tuesday as more people were drafted in to help.