of the People's Republic of China (PRC) has estimated that the Spratly area holds oil and natural gas reserves of 17.7 billion tons (1.60 × 1010 kg), compared to the 13 billion tons (1.17 × 1010 kg) held by Kuwait, placing it as, potentially, the fourth largest reserve bed in the world. The United States Energy Information Administration contests this, estimating almost no oil and less than 100 billion cubic feet of natural gas exists in fields near the Spratly Islands. Still, these large potential reserves have assisted in intensifying the territorial claims of the neighbouring countries. In 1968 and 1970, the Philippines started to take their territorial claims more seriously and stationed troops on three islands which had been claimed by the adventurer Tomas Cloma as part of Freedomland. In 1973, Vietnamese troops were stationed on five islands. On 11 March 1976, the first major Philippine oil discovery occurred off the coast of Palawan, near the Spratly Islands territory. In 2010, these oil fields supplied 15% of all petroleum consumed in the Philippines. In 1992, the PRC and Vietnam granted oil exploration contracts to US oil companies that covered overlapping areas in the Spratlys. In May 1992, the China National Offshore Oil Corporation (CNOOC) and Crestone Energy (a US company based in Denver, Colorado) signed a co-operation contract for the joint exploration of the Wan'an Bei-21 block, a section of the southwestern South China Sea that includes Spratly Island areas. Part of the Crestone's contract covered Vietnam's blocks 133 and 134, where PetroVietnam, PetroStar Energy (US) and ConocoPhillips Vietnam Exploration & Production, a unit of ConocoPhillips, agreed to evaluate prospects in April 1992. This led to a confrontation between China and Vietnam, with each demanding that the other cancel its contract. Commercial fishing The region is one of the world's most productive areas for commercial fishing. In 2010, the Western Central Pacific (excluding the northernmost reaches of the South China Sea closest to the PRC coast) accounted for 14% of the total world catch at 11.7 million tonnes. This was up from less than 4 million tonnes in 1970. The PRC has predicted that the South China Sea holds combined fishing and oil and gas resources worth one trillion dollars. There have already been numerous clashes between the PRC and the Philippines, PRC and Vietnam, and between other nations over "foreign" fishing vessels in exclusive economic zones (EEZs), and