LONG BEACH, Calif. -- An explosion of oil and gas development is coming in California, one that's likely to trigger a 21st-century gold rush, experts predicted at a conference here yesterday. Advisers to petroleum companies forecast that they are just a few years away from obtaining the key to extracting large volumes of resources from the Monterey Shale, a swath of land stretching from the middle of the state to Los Angeles County. Believed to hold as much as 15.5 billion barrels of recoverable oil, Monterey is the nation's largest shale site.