A longstanding assumption of American energy policy has been that natural gas would be plentiful abroad, and therefore readily available for importation, as production falls off in North America, where many fields are tapped out. But some experts are starting to question that idea, saying natural gas could be subject to the same explosion in overseas demand that has made oil so expensive. As it is, the supertankers that were supposed to deliver cargoes of gas from Africa and the Middle East to the United States are taking them to places like Spain and Japan instead, pushing up gas prices and depleting the nation's stockpiles as the hurricane season approaches.