Chevron, the number two US oil company, secured Wednesday a five-year permit to explore for shale gas deposits in northeastern Bulgaria, the government said. Chevron last month won a tender to explore and eventually develop the huge 4,398-square-kilometre (1,698-square-mile) Novi pazar field in competition with Canadia\'s BNK. Bulgaria, which has been struggling to diversify its energy sources, is preparing two more tenders for shale gas exploration in the same region in northeastern Bulgaria with offers to be submitted by end-June. Initial company estimates, based on similar rock formations, showed potential reserves at the fields of between 300 billion and 1.0 trillion cubic metres of shale gas. Bulgaria is almost totally dependent on Russian gas supplies via Ukraine and sees shale gas as a possibility to wean itself off Moscow. The Balkan country also hopes to link its gas transportation network to neighbouring Greece in 2014 and receive gas from Azerbaijan via an interconnector system. Shale gas has become a major source of energy in the United States in recent years but its development is controversial as evironmentalists claim the methods used to extract the gas are very damaging, especially to water supplies.