HOUSTON, June 3 (Xinhua) -- The number of active drilling rigs in the United States was 727 this week, unchanged from the previous week but up by 271 from last year, according to the weekly data released Friday by Houston-based oilfield services company Baker Hughes.

       These active drilling rigs included 574 oil rigs operating in U.S. oil fields, the same as the previous week; 151 gas drilling rigs, unchanged from last week; and two miscellaneous rigs, also the same as last week.

       The rigs included 710 land drilling rigs, one inland water drilling rig, and 16 offshore drilling rigs.

       Of them, 36 are directional drilling rigs, 666 are horizontal drilling rigs and 25 are vertical drilling rigs.

       So far, the Permian Basin in western Texas and eastern New Mexico has been the largest source of shale oil production growth in the United States, having become an engine of supply growth outside the Organization of the Petroleum Exporting Countries in the past years.