* Repsol says big oil fields possible offshore Cuba
* PDVSA also cites "great potential" in Cuba blocks
* Cuba says may have 20 billion barrels offshore
By Jeff Franks
HAVANA, April 5 (Reuters) - Repsol YPF (REP.MC) expects to have a Chinese-built drilling rig in Cuban waters by the end of the summer and start drilling immediately into a prospective undersea oil field that looks like it could be a big one, a geologist for the Spanish oil company said on Monday.
Repsol, in a consortium with Norway's Statoil (STL.OL) and a unit of India's Oil and Natural Gas Corp. (ONGC.BO), plans to drill one well with the high-tech Scarabeo 9 rig and another if the first is successful, said senior structural geologist Gonzalo Zamora at Cuba's Earth Sciences Summit in Havana.
"The perspectives are to find something, if not, we wouldn't be drilling. And the sizes are big," he told reporters after making a presentation at the conference that is heavily focused on Cuba's oil and gas development.
"To drill with so much risk like we have in Cuba, we have to look for a big prospect because the investment to develop it is very big," Zamora said.
He said seismic studies of the offshore blocks Repsol has leased have turned up "very interesting prospects" in several geological zones. The company drilled the only well in Cuba's offshore in 2004 and found oil in an "area of much potential," Zamora said.
It has not sunk a second well for various reasons, but contracted with Italian offshore oil services company Saipem for use of its Scarabeo 9 rig, which has been built in China and is now in Singapore undergoing preparations for the trip to Cuba.
Zamora was expected to arrive in Cuba by the end of the summer, but delays were always possible in the offshore oil business. Sources have told Reuters the rig should arrive at the communist-led island by mid to late August.
He said Cuba, which is anxious to tap into its unexplored sector of the Gulf of Mexico, wanted Repsol to commit to drilling two wells, but Repsol limited its risk by agreeing to do one well for sure.
Cuba has said it may have 20 billion barrels of oil offshore, although the U.S. Geological Survey has estimated a more modest 5 billion barrels.
After Repsol finishes with the Scarabeo 9, which is capable of drilling in 12,000 feet (3,657 meters) of water, the rig will be handed over to Malaysia's Petronas to drill in its Cuban offshore leases, then to ONGC Videsh, which is a unit of ONGC, for its Cuba exploration.
Venezuela's PDVSA may also be in line to get the rig for its Cuban blocks, where areas of "great potential" have been found, PDVSA senior basin analyst Jose Noya told reporters at the conference.
He said exploratory drilling could take place "maybe in a year."
"Once we have the equipment, it will be drilled. Probably we are in the line for drilling ... with the same rig, because it is a rig that can drill in 2,000 meters (of water)," he said of the Scarabeo 9.
Last summer, Cuban officials were saying they expected to drill seven wells, but Cuban senior oil adviser Manuel Marrero told the conference that the number now was five. He did not specify who would drill the wells. (Editing by David Gregorio)