HOUSTON, April 5 (Xinhua) -- The number of active drilling rigs in the United States decreased to 620 this week, down by one from the previous week and by 131 from last year, according to the weekly data released by Houston-based oilfield services company Baker Hughes on Friday.

       These active drilling rigs included 508 oil rigs operating in U.S. oil fields, up by two from the previous week; 110 gas rigs, down by two; and two miscellaneous rigs, down by one.

       The rigs included 600 land drilling rigs, zero inland water drilling rigs, and 20 offshore drilling rigs.

       Of them, 51 were directional drilling rigs, 557 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.