Chevron, the number two US oil company, has won a tender to explore shale gas deposits in northeastern Bulgaria, Economy and Energy Minister Traicho Traikov said.

"Chevron, which won the tender, offered the government a 30 million euro ($43 million) bonus for the licence to explore a shale gasfield near Novi Pazar. It won in competition with Canadian BNK," Traikov said.

Two more tenders for shale gas exploration in the same region in northeastern Bulgaria were still underway with offers to be submitted by end-June, Traikov added.

Initial estimates of the companies interested in shale gas exploration in Bulgaria put the country's deposits between 300 billion and one trillion cubic metres, he explained.

Bulgaria, which is almost totally dependent on Russian gas supplies via Ukraine, sees shale gas as a possibility of diversifying its energy sources.

Bulgaria also hopes to link its gas transportation network to neighbouring Greece in 2014 and receive gas from Azerbaijan via an interconnector system.