UPDATE 2-Eni's Uganda ambitions hang on Tullow option

* Sale is dependent on Tullow waiving pre-emption rights

* Tullow has until Jan. 17 to make decision

* Eni targeting Africa to help boost oil output

* Heritage shares up 0.5 pct; Tullow up 1.4 pct (Adds detail)

LONDON, Dec 18 (Reuters) - Eni's (ENI.MI) ambitions to expand in Africa to help boost flagging output could be derailed if Tullow Oil uses a pre-emption right on Ugandan oil fields the Italian energy giant plans to buy for up to $1.5 billion.

Britain's Heritage Oil (HOIL.L) (HOC.TO) signed a deal on Friday to sell petroleum exploration fields in Uganda to Eni -- formalising a preliminary agreement made on Nov. 23.

But Eni acknowledged in a statement the deal is subject to Tullow Oil not taking up a right of pre-emption which gives it the option to acquire the assets on the same terms and conditions Heritage has agreed with Eni.

Tullow said earlier this week it may try to block the deal.

"If ... we can match the price (being offered for the Ugandan assets). We have a right of pre-emption. This means Tullow will acquire those assets ... at the right price," Tullow Vice President Tim O'Hanlon said in a newspaper interview. [ID:nLDE5BE26J]

Eni, which is already an operator in Angola, Ghana, Nigeria, Congo, Gabon and Mozambique, is targeting Africa to help it boost flagging output. In October, Eni cut its oil and gas output target for the full year to be in line with 2008's 1.797 million barrels of oil equivalent per day (boepd).

Heritage said last month it agreed to sell its interests in western Uganda's Blocks 1 and 3A to Eni. Tullow Oil holds the blocks with the British explorer as a non-operating partner.

Interest has grown in Uganda's hydrocarbon sector and Heritage and Tullow estimate the country's reserves at two billion barrels.

The entry of Eni, an integrated oil company with experience in building pipelines, terminals and refining capacity, heralds an escalation of development in Uganda.

Eni said development of the Ugandan fields will require synergies with Uganda's infrastructure programmes in which it intends to play a leading role alongside local authorities.

But the acquisition and capital expenditure will place an additional burden on Eni, one of the most leveraged of Europe's oil majors. [ID:nGEE5AP1P5]

Under the deal with Heritage, Eni has agreed to pay $1.35 billion in cash immediately, plus a further deferred payment of either $150 million or an interest in another oil-producing field independently valued at a similar amount.

Following completion of the deal, Heritage said it would consider returning some of the proceeds to shareholders through a special dividend of between 75 pence and 100 pence.

Shares in Heritage Oil closed down 0.1 percent to 419.5 pence, with Tullow Oil up 1.4 percent to 1,254 pence. Eni shares weredown 0.82 percent at 17 euros.

(Reporting by Matt Scuffham in London and Stephen Jewkes in Milan; editing by Gunna Dickson)