HOUSTON, April 12 (Xinhua) -- The number of active drilling rigs in the United States decreased to 617 this week, down by three from the previous week and by 134 from this time last year, according to the weekly data released by Houston-based oilfield services company Baker Hughes on Friday.

       These active drilling rigs included 506 oil rigs operating in U.S. oil fields, down by two from the previous week; 109 gas rigs, down by one; and two miscellaneous rigs, unchanged from last week.

       The rigs included 598 land drilling rigs, zero inland water drilling rigs, and 19 offshore drilling rigs.

       Of them, 51 were directional drilling rigs, 554 were horizontal drilling rigs and 12 were 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.