U.S. crude oil and natural gas proved reserves rose to record highs in 2017, driven by stronger energy prices and the continuing development of shale formations, the U.S. Energy Information Administration (EIA) said Nov. 29.

Crude reserves increased 6.4 billion barrels (Bbbl), or 19.5%, to 39.2 Bbbl at year-end 2017, marginally higher than the previous record of 39 Bbbl set in 1970.

“Total U.S. oil reserves in 2017 exceeded a brief, one-year, 47-year-old record, highlighting the importance of crude oil development in shales and low permeability plays, mainly in the Southwest,” the statistical arm of the U.S. Department of Energy said.

Natural gas reserves jumped 123.2 trillion cubic feet (Tcf), or 36.1%, to 464.3 Tcf last year. The previous record was 388.8 Tcf, marked in 2014.

The new record for natural gas extends a longer-term trend of development, mainly in shale plays in the northeast, the EIA said.

Reserves of both oil and gas were about double their levels from a decade ago.

The agency said these new proved reserves records were established in 2017 despite crude production at levels not seen since 1972, and record natural gas output.

U.S. crude futures averaged $50.85 per barrel in calendar 2017, compared with $43.47 in 2016.