For the fourth year in a row, U.S. oil reserves increased, according to a recently published Energy Information Agency report. Proven crude oil reserves jumped 15 percent in 2012 to 33 billion barrels, their highest level since 1976. The jump was the the largest by volume and percentage increase in oil reserves since 1970.

Natural gas reserves shrank, at least temporarily. Gains in Marcellus and Eagle Ford shale gas reserves were offset by declines in more mature fields, breaking a trend that had seen the natural gas figure increase for 14 years in a row. Natural gas reserves decreased 7.5 percent in 2012. But the decrease is viewed as only a temporary one.

“With natural gas prices higher in 2013 and technology continuing to advance, EIA expects U.S. natural gas proved reserves to increase in 2013,” EIA Administrator Adam Sieminski said.

Proved reserves are the volumes of oil and gas still in the ground that are considered possible to produce based on available technology and current energy prices. Improvements in technology and rising energy prices both lead to higher reserve estimates.