HOUSTON, Dec. 20 (Xinhua) -- The number of active drilling rigs in the United States stands at 589 this week, unchanged from last week but down 31 from a year ago, according to the weekly data released Friday by the Houston-based oilfield services company Baker Hughes.

These active drilling rigs include 483 oil rigs operating in U.S. oil fields, up by one from last week; 102 gas drilling rigs, down by one from last week; and four miscellaneous rigs, the same as last week.

The rigs include 573 land drilling rigs, two inland water rigs, and 14 offshore drilling rigs.

Of them, 49 are directional drilling rigs, 527 are horizontal drilling rigs and 13 are vertical drilling rigs.

The Permian Basin in western Texas and eastern New Mexico has been the largest source of shale oil production growth in the United States, and an engine of supply growth outside the Organization of the Petroleum Exporting Countries in past years. ■