As energy companies began to sink horizontal wells into the Barnett Shale of north-central Texas a decade ago, a distinctive production curve appeared that has become a strong visual symbol in the debate about the future of the vast U.S. shale gas resources. Gas production shot up to 1,600 million cubic feet per day in the first three months of an average Barnett well's life -- four times the top output level for conventional, vertically drilled wells, according to IHS Inc., a leading energy analysis firm. And then just as quickly, production plunged.