Nearly half of the oil produced in the United States came as a result of hydraulic fracturing last year, the U.S. Department of Energy said.

The department's U.S. Energy Information Administration said hydraulic fracturing of about 23,000 wells in the country in 2000 yielded about 102,000 barrels of oil per day, which at the time was less than 2 percent of the total output.

"Even though hydraulic fracturing has been in use for more than six decades, it has only recently been used to produce a significant portion of crude oil in the United States," EIA said in a report.

According to data from the administration and its industry counterparts, the hydraulic fracturing, or fracking, of around 300,000 wells last year resulted in 4.3 million bpd, accounting for about 50 percent of the total domestic production.

EIA involves the injection of high-pressure water, a trace amount of chemicals and solid particles to coax oil and natural gas out of shale bed formations, giving drillers access to reserves previously out of reach.

Environmental groups have expressed concern about the potential environmental and health impacts of hydraulic fracturing fluids. More recently, the U.S. Geological Survey has said the disposal of wastewater associated with hydraulic fracturing may be triggering small-scale seismic events, particularly in Oklahoma.

EIA said most of the new oil from hydraulic fracturing is coming from shale basins in Texas and North Dakota.