* Repsol rejected CNOOC's JV proposal last month - source
* Repsol says not eyeing tie-up
(Adds detail, analyst, background, Repsol shares)
By Joseph Chaney and Jonathan Gleave
HONG KONG/MADRID, July 21 (Reuters) - China's CNOOC has approached Repsol YPF about forming an international oil and gas production venture, sources familiar with the matter said on Tuesday, but the Spanish oil major said it was not interested in such a tie-up.
State-owned CNOOC wants to form a joint venture which would take a stake in Repsol's often troubled Argentine unit, YPF, and interests in some of Repsol's other assets, a source familiar with the matter said.
"(CNOOC) is interested in something more than YPF," the source told Reuters, adding that a deal could be worth roughly $15 billion.
Repsol rejected an initial joint venture proposal from China's top offshore oil group, which has been charged by Beijing with securing overseas energy assets to fuel China's economy, made a month ago, a second source said.
Repsol denied it was in talks with on a joint venture or that it was considering such a move.
The company has been mulling a part sale of its interest in YPF, which has failed to meet Repsol's hopes at the time of its acquisition of the South American oil and gas producer, for some time.
David Thomas, oil analyst at Citigroup, said investors would be nervous if Repsol agreed to offer stakes in choice assets such as its Santos basin interests in Brazil, in order to offload some of its YPF stake.
Repsol shares closed down 0.03 percent against a 0.88 percent rise in the DJ Stoxx European oil and gas sector index.
CNOOC is eyeing a "partnership with Repsol involving upstream E and P (exploration and production) assets in its global portfolio," said the first source, who has direct knowledge of the deal, but who was not authorised to speak publicly about the process and so wished to remain anonymous.
CNOOC is one of a number of companies, including fellow state-owned Chinese energy giant CNPC, parent company of PetroChina, which have approached Repsol about taking a stake in YPF, sources have previously told Reuters.
Sources have said CNOOC is eyeing a 25 percent in YPF, while CNPC could be eyeing as much as a 75 percent stake.
Goldman Sachs, the bank advising Repsol on the sale of YPF, and CNOOC declined to comment. (Additional reporting by Tom Bergin in London)
(Editing by Will Waterman and David Cowell)