The New York Times reports that "there is another reason for China’s interest in the South China Sea: the large quantities of oil and natural gas that might lie below these waters. In May, Beijing made its interest in those resources clear when it sent a drilling rig called Haiyang Shiyou 981 into waters claimed by Vietnam. The rig is owned by China National Offshore Oil Corporation, or Cnooc, the country’s biggest offshore energy producer."
"After the move provoked a standoff with Vietnam, the drilling rig was sent to undisputed waters near China’s island province of Hainan. In September, Cnooc said the rig had discovered a large gas field, which suggested that China had become more proficient at offshore drilling."
"The United States Energy Information Administration has estimated that 11 billion barrels of oil and 190 trillion cubic feet of natural gas lie below the seabed, including both proven and probable reserves. If those estimates prove correct, the South China Sea would be in the same league as Mexico, a midsize producer, and in the global top 10 in terms of gas."