Advances in drilling technology are reviving the prospects of oil companies in shallow parts of the Gulf of Mexico, helping to squeeze more from older fields while the U.S. shale bonanza lures others onshore. Apache Corp and a handful of smaller independent companies are using seismic surveying and horizontal drilling - techniques perfected during the onshore fracking boom - to tap mature fields and find hidden reserves on the shelf. The methods appeal to investors hungry for the quick profits that cannot be delivered by deepwater drilling, where a dozen years of planning and billions of dollars in investment can be required to get oil pumping.