HOUSTON, Jan. 12 (Xinhua) -- The number of active drilling rigs in the United States decreased to 619 this week, down by two from the previous week and by 156 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 499 oil rigs operating in U.S. oil fields, down by two from the previous week; 117 gas rigs, down by one; and three miscellaneous rigs, up by one.

       The rigs included 599 land drilling rigs and 20 offshore drilling rigs.

       Of them, 48 were directional drilling rigs, 561 were horizontal drilling rigs and 10 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.