A single, vast shale deposit— the Marcellus formation, stretching from Tennessee to New York—might contain enough natural gas to supply the U.S. for more than 40 years at today’s consumption rates, according to recent estimates. Thousands of vertical wells have exploited the shale’s easy-to-reach deposits. But newer technology and improved procedures are making horizontal drilling cost-effective, greatly expanding the amount of gas that can be extracted economically.