Bulgarian aims to auction off the rights to six shale gas exploration blocks this year, though opposition groups said they would try to block the move.Most of the blocks for shale gas development are in the north of the country. Bulgarian officials estimate there are at least 1 trillion cubic feet of natural gas locked in the Novi Pazar deposit in the country\'s northeast, where Chevron aims to start drilling soon.France in June became the first country in the European Union to ban hydraulic fracture, or fracking, in which energy companies use a mixture of water, chemicals and abrasives to gain access to natural gas deposits locked in shale formations.Environmental groups and residents near Bulgarian shale deposits have expressed concern about the potential damage from shale gas exploration. The opposition Bulgarian Socialist Party has said the issue should go before a public referendum before licenses are awarded, the Sofia News Agency reports.Bulgarian Energy Minister Traicho Traikov, however, said there is enough natural gas locked in shale deposits to feed domestic demand for the next millennium.Poland and Ukraine are viewed as having some of the largest shale gas deposits in Europe.