HOUSTON, Dec. 29 (Xinhua) -- The number of active drilling rigs in the United States rose to 622 this week, up by two from the previous week but down by 157 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 500 oil rigs operating in U.S. oil fields, up by two from the previous week; 120 gas rigs, unchanged from last week; and two miscellaneous rigs, the same as last week.

       The rigs included 602 land drilling rigs and 20 offshore drilling rigs.

       Of them, 46 were directional drilling rigs, 565 were horizontal drilling rigs and 11 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.