A major recent gas discovery deep under the South China Sea could reopen a frontier for oil and gas exploration that some multinational companies abandoned decades ago after shallower wells turned up dry, said the chairman of China National Offshore Oil Corp (CNOOC), the Wall Street Journal reported.   The discovery, announced in June by CNOOC's Canadian partner, Husky Energy Inc, is a 'tremendous breakthrough for us,' the newspaper quoted CNOOC chairman Fu Chengyu as saying. Husky, controlled by Hong Kong billionaire Li Ka-shing, said the field, China's first deep-water discovery, is located about 240 kilometers south of Hong Kong beneath 1,500 meters of water. Husky estimates it contains four trillion to six trillion cubic feet of recoverable natural-gas reserves. CNOOC has 5.8 trillion cubic feet of booked natural-gas reserves, wrote the Wall Street Journal.