(Reuters) -- Chevron's offenses in the drilling of a well that caused an offshore oil spill in November did not amount to negligence, Brazil's oil regulator said Thursday.

The finding may help the company fight criminal charges and an $11 billion lawsuit.

Chevron and its drilling contractor Transocean committed operational and safety violations and improperly designed the well that led to the leak, according to a four-month investigation by the regulator known as the ANP.

But the accident does not appear to have irreparably damaged the deep-sea Frade field and the mistakes did not qualify as negligence, Silvio Jablonski, a senior ANP official, told a Brazilian Senate hearing.

Negligence is a sensitive legal term, Jablonski said. It needs to be reserved for things that are beyond correction and improvement - and that's not the case here.

The hearing came a day after a federal prosecutor filed criminal charges against the companies and 17 of their employees. Both companies have denied the charges, which carry prison terms of up to 31 years.

The November accident caused no discernible damage to the environment, Jablonski said, spilling 3,000 barrels at most - less than 0.1 percent of BP's 4.9-million-barrel Gulf of Mexico disaster.

Still, the stakes are huge for the companies and Brazil itself, which risks scaring away international investors and choking off an oil boom.

Chevron shares fell 2.56 percent on Thursday to $105.35. Transocean shares lost 2.87 percent.

FAULTY DESIGN

The faulty design and reinforcement of the well shaft are one of the main causes for the November leak, according to the ANP report cited by Jablonski.

The ANP has completed its report and delivered it to Chevron for rebuttal, said Rafael Williamson, director of corporate affairs at Chevron, who spoke at Thursday's hearing.

Chevron says it acted according to accepted industry practice, stopped the leak within four days and continues to work at controlling any residual flow.

Chevron and the ANP agree that higher-than-expected pressure in an oil reservoir caused oil to surge into the well. Before it could rise to the drill rig on the surface, causing a dangerous and possibly deadly gusher, a valve in the sea floor known as a blow-out protector sealed the well.

The pressure trapped in the well ruptured unreinforced rock in the bore hole far below the sea floor. The oil then flowed through porous rock and crevices to cracks in the seabed, where it bubbled to the surface.

Chevron could have prevented the accident by reinforcing more of the bore hole with cement, Jablonski said, but the well design did not properly anticipate actual oil pressure and rock strength.

Williamson said Chevron had sufficient information to explore and drill. The company is studying the ANP report.

EXCESSIVE CHARGES

Eduardo Santos de Oliveira, the prosecutor who filed the criminal and civil charges, said Jablonski and other speakers at the hearing were trying to cloud the issue and diminish the significance of the November spill.

An influential senator from President Dilma Rousseff's party criticized the charges as excessive and irresponsible in an interview prior to the hearing.

Senator Jorge Viana told Reuters the charges create a climate of insecurity that could damage badly needed investment in the oil sector.

If the charges, plus an $11 billion civil suit filed against Chevron and Transocean in November, were fairly applied to other polluters, the industry would shut down, said Viana.

Brazil is entering a phase in which we need investment from all over, said Viana.

Rousseff, a former energy minister who sat on the board of state-controlled oil company Petrobras, has remained mostly silent about the Chevron case.

She has warned that foreign companies in the oil sector need to respect Brazilian regulations, but refrained from demonizing the Chevron executives or the company in public.

Rousseff sees the case is a legal issue, not a government one, a senior official told Reuters on Thursday. He spoke on condition of anonymity to be able to frankly describe her views on a sensitive matter.

(Additional reporting by Brian Winter; Writing by Reese Ewing; Editing by Alden Bentley, Jim Marshall and Bob Burgdorfer)

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