HOUSTON, May 27 (Xinhua) -- The number of active drilling rigs in the United States decreased to 727 this week, down by one from the previous week but up by 270 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, down by two from the previous week; 151 gas drilling rigs, up by one from last week; and two miscellaneous rigs, the same as last week.

       The rigs included 710 land drilling rigs, one inland water rigs, 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.