* Eni says cooperating with prosecutors, denies anywrongdoing
* Former CEO Paolo Scaroni also being probed - source
* Shares down 2 percent (Rewrites first paragraph, adds analyst comment, shares,background)
By Stephen Jewkes, Oleg Vukmanovic and Emilio Parodi
MILAN, Sept 11 (Reuters) - Eni SpA chief ClaudioDescalzi has been placed under investigation over allegedcorruption relating to a big Nigerian oil deal, the company saidon Thursday, just four months after he took the helm at the oilmajor, Italy's biggest listed company.
Milan prosecutors opened a probe earlier this year and havenow widened the net to include Descalzi, in a case relating to a$1.09 billion acquisition of Nigeria's OPL-245 offshore oilblock in 2011.
Eni confirmed Descalzi was being investigated after a reportin Italian daily Corriere della Sera said he was being probedover the Nigeria deal.
"Eni is cooperating with the Milan prosecutor's office andis confident that the correctness of its actions will emergeduring the course of the investigation," it said in a statement.
Descalzi could not immediately be reached for comment.
A long-standing executive at Eni and former head of its coreexploration and production division, Descalzi took over in Mayfrom Paolo Scaroni, himself under investigation for allegedcorruption in Algeria, although the company said in January ithad found no evidence of illegal conduct by the group inrelation to the north African country.
Eni said its Operations and Technology Officer RobertoCasula was also being probed. Court sources told Reuters onThursday Scaroni was also under investigation in the Nigeriancase. Scaroni and Casula could not immediately be reached forcomment.
Milan brokerage Akros noted Eni had been involved in asimilar case in Nigeria in 2009, when it paid around $400million to settle the dispute.
"We believe that the potential negative impact on Eni may beworth 500 million euros or around 1 percent of the currentmarket capitalisation," it said in a note.
At 1436 GMT shares in Eni, Italy's biggest company by stockmarket value, were down about 2 percent, underperforming theEuropean oil and gas sector stocks index, with tradersciting concerns about the investigation.
IN DISPUTE
Ownership of the OPL 245 field has been in dispute for morethan a decade.
Former Nigerian oil minister Dan Etete awarded the block in1998 for $20 million to Malabu Oil and Gas, a company in whichhe was a leading shareholder. Malabu however only ever paid $2million for the stake, in 1999.
The field was eventually sold on to Eni and Shell in 2011 for a total of $1.3 billion, including a signature bonusof around $207 million.
Malabu received around $1.09 billion from the sale, whilethe Nigerian government kept the rest, a British court documenthas shown.
Campaigners for greater transparency in political andbusiness dealings, who asked Britain to look into the matter,allege Shell and Eni used the Nigerian government as ago-between to obscure the fact they were dealing with Etete.
Eni, the biggest foreign oil and gas producer in Africa, hasalways said it dealt exclusively with the government of Nigeriaand Shell over the acquisition.
Elsewhere Ebeka Obi, a Nigerian intermediary for Etete, hasbrought a court case in Britain against Malabu for unpaid feesrelating to what he says was his help in brokering the Shell-Enideal.
A judicial source on Thursday said the London court hadseized Malabu funds worth $83 million. This follows the seizureof $110 million from a Malabu account in Switzerland a fewmonths ago, the source said.
Descalzi, who was head of the group's exploration andproduction (E&P) unit at the time of the Nigerian deal, wasappointed chief executive of Eni in May.
Prime Minister Matteo Renzi came to office in Februarypledging to clean up Italian business and introduce ethics rulesat publicly-controlled companies, aiming to eject directorsfound guilty of financial crimes.
But shareholders at many of Italy's big state-controlledcompanies, including Eni, threw out the proposals by votingagainst their inclusion in company bylaws.
OPL 245 could contain up to 9.23 billion barrels of crudeoil, more than enough to keep China running for two and a halfyears.
Eni has operated in Nigeria since the early 1960s and thecountry accounted for around 8 percent of its total output lastyear. Chronic oil pipe sabotage in the country has recentlyaffected the group's hydrocarbon production in the country. (Additional reporting by Giancarlo Navach; Editing by DavidClarke and David Holmes)
