The Balkan country, eager to reduce its near total dependence on Russian gas, had in April opened tenders for two offshore blocs, Silistar and Teres.

Shell, the only bidder for a five-year exploration permit at Silistar, which covers 7,000 square km, has pledged to invest 18.6 million euros ($21 million) in seismic surveys of the block.

Silistar is close to a block in Romanian waters, where Austria's OMV (>> OMV AG) has said it could produce up to 84 billion of cubic meters of gas, raising Bulgaria's hopes that it will be able to exploit reserves off its own coast to diversify its energy supplies.

"This is a very serious success for us and one of the most important steps towards diversifying our gas supplies," energy minister Temenuzhka Petkova told reporters on Tuesday.

However there had been no interest in exploring the smaller Teres block, she said.

French major Total (>> Total) along with OMV and Spain's Repsol (>> Repsol SA) are expected to start drilling for oil and gas in Bulgaria's biggest offshore block, Han Asparuh, between February and May.

($1 = 0.8901 euros)

(Reporting by Tsvetelia Tsolova; Editing by Adrian Croft and David Holmes)

Stocks treated in this article : Total, Repsol SA, OMV AG