Exxon Corporation and Pertamina, Indonesia's national oil company, plan to begin building offshore plat forms that will enable them to drill into a natural gas field beneath South China Sea, near the Indonesian island of Natuna. The field, one of the world's largest, was dis covered in 1973, but has remained untapped as engineers could not do away with the unwanted carbon dioxide in the valuable methane. The gas field is estimated to contain 210 trillion cubic feet of gases, 'including about 150 trillion cubic feet of carbon dioxide, a potent greenhouse gas. Exxon's solution is to separate the carbon dioxide from the methane, compress it, and inject it under high pressure into porous rock formations about 914.4 m below the ocean floor.