A BP scientist identified a previously unreported deposit of flammable gas that could have played a role in the Gulf of Mexico oil spill, but the oil giant failed to divulge the finding to government investigators for at least a year, according to interviews and documents obtained by The Associated Press.

While engineering experts differ on the extent to which the two-foot (60-centimetre)-wide swath of gas-bearing sands helped cause the disaster, the finding raises the spectre of further legal and financial troubles for BP. It also could raise the stakes in the multibillion-dollar court battle between the companies involved.

A key federal report into what caused the worst offshore oil spill in US history is set to be released as early as today.

“This is a critical factor, where the hydrocarbons are found,” said Rice University engineering professor Satish Nagarajaiah. “I think further studies are needed to determine where this exactly was and what response was initiated by BP if they knew this fact.”

At issue: BP petrophysicist Galina Skripnikova in a closed-door deposition two months ago told attorneys involved in the oil spill litigation that there appeared to be a zone of gas more than 300 feet (91 metres) above where BP told its contractors and regulators with the then-Minerals Management Service the shallowest zone was located.

The depth of the oil and gas is a critical parameter in drilling because it determines how much cement a company needs to pump to adequately seal a well. Federal regulations require the top of the cement to be 500 feet (152 metres) above the shallowest zone holding hydrocarbons, meaning BP’s cement job was potentially well below where it should have been.

Cement contractor Halliburton recently filed a lawsuit against BP asserting that Skripnikova’s statements prove the oil giant knew about the shallower gas before the explosion and should have sought a new cement and well design. BP has denied the allegations.

Skripnikova’s job involved analysing data from BP’s Macondo well to determine the depth and characteristics of oil and gas deposits, which in turn is used in a process called temporary abandonment, when wells are sealed so they can be used for production later.

Based on the initial information, regulators approved BP’s well sealing plan, which called for placing the top of the cement at roughly 17,300 feet (5,273 metres) below the surface of the water. The cement was pumped April 19, the day before the explosion. But Skripnikova said that after she flew back from the rig she and others re-examined the analysis, and on the day of the explosion she identified the shallower gas zone. That would have meant the cement should have been placed at just under 17,000 feet (5,181 metres) below the surface of the water.

She said she did not relay that information to drilling engineers on the Deepwater Horizon and did not warn them to hold off proceeding with the abandonment. She suggested in her deposition that she thought the information would be passed up the chain. BP was already US$60 million over budget and stopping operations at that point and coming up with a new cement design would have meant millions of extra dollars in costs.

Later in the deposition, Skripnikova backtracked and said the new analysis was not discussed among her team until the day after the explosion.

“Do you believe that BP complied with MMS regulations with its selection of where the top of cement should go in the cement job that was done on April 19?” an attorney asked Skripnikova.

“I don’t know,” she responded.

Before her deposition, none of Skripnikova’s findings appear to have been passed on to federal regulators or the numerous government investigations since the disaster. Skripnikova was never questioned at public hearings before the presidentially appointed oil spill commission. Nor was she questioned before the joint investigative panel of the US Coast Guard and the agency that regulates offshore drilling, which is readying its final report. Her name and the information she has is not in BP’s internal investigation report released last September.

BP spokesman Scott Dean insisted in a statement Tuesday to The Associated Press that when assessing top-of-cement requirements before the accident, BP did not identify the zone in question as bearing oil or gas. Dean said “BP has provided material concerning this zone to the parties in the multidistrict litigation and to government investigators”.

However, Dean declined to say when that notification was done. Dean added that a subsequent third-party analysis by oilfield services firm Schlumberger supports BP’s pre-accident assessment.

An investigator with the presidential oil spill commission, which released a report on the disaster months ago and has since disbanded, told AP that BP did not specifically reveal the higher probable gas zone during the course of the panel’s investigation. The investigator, who spoke on condition of anonymity because he was not authorised to speak publicly, said an independent petrophysicist reviewed the data available to the panel and did not express concern about gas being at a shallower depth.

A spokeswoman for the Bureau of Ocean Energy Management Regulation and Enforcement, formerly known as MMS, declined to say to what extent the revelations would play in the investigation team’s final report. The Justice Department, which has been conducting its own investigation, declined to comment.

Some experts believe the error played a role in the disaster, while others say the blast still could have occurred even if the cement was placed higher in the well. Investigators have previously faulted misreadings of other key data, the failure of the blowout preventer from stopping the flow of oil to the sea, and other shortcomings by executives, engineers and rig crew members. One thing the experts agree on is BP should have spoken up sooner.

“I don’t think anybody had any idea what they were doing, because had they known, they would have stopped it,” said Rice’s Nagarajaiah.

University of California at Berkeley engineering professor Bob Bea, who spent decades studying and working on offshore oil rigs, said that the previously undisclosed gas zone was yet another “critical flaw” — one of several made by BP and its contractors.

Bea said the shallower gas could have travelled through channels in the cement and helped to further weaken it before the blowout.

Such a situation would have been detected if BP had conducted what’s called a cement bond log to test the strength of the cement, a test the company chose not to do. Bea also said the company did not wait long enough for the cement to set.

“It would have been remarkable … for that cement to have been able to perform its required function,” Bea said.