Proved U.S. crude oil and lease condensate reserves continue recent ticks upward, reversing a years-long decline.

The nation’s proved reserves increased year-over-year from 2011 to 2012 by 4.5 billion bbl—a 15.4% increase, according to a recently published U.S. Energy Information Administration (EIA) report. It was the largest single-year increase in proved reserves since 1970 when Alaska’s North Slope reserves—10.3 billion bbl—went in the books. That was the largest one-year increase ever, the agency noted.

The makeup of the nation’s reserves is changing dramatically, thanks to the continuing success of unconventional shale
plays. Overall extensions to existing fields, some 5.2 billion bbl—particularly in Texas and North Dakota—altered the map of where U.S. reserves are located. The rise in shale play reserves more than offset meaningful declines in conventional reserves in such large, conventional oil-producing states as Alaska and California.

Other states with shale plays, such as Oklahoma, Colorado, New Mexico and Utah, also registered big proved reserves increases.

The South Texas Eagle Ford surpassed the Bakken Formation of North Dakota to become the largest tight oil play in 2012, the EIA report said.

U.S. wet natural gas proved reserves decreased, however, by 26 trillion cubic feet (Tcf), or 7.5% in 2012, due to lower gas prices. Gas prices had a 34% decline in the 12-month, first-of-the-month, average spot price for gas delivered at Louisiana’s Henry Hub between 2011 and 2012. EIA found that lead to large negative net revisions—45.6 Tcf—to reserves of existing fields, offsetting almost gains from extensions of existing fields.

But, “EIA anticipates that natural gas proved reserves for 2013 will be affected positively by the recovery in natural gas prices from 2012 to 2013,” the report added.

The sprawling Marcellus Shale play beneath Pennsylvania and West Virginia surpassed the Barnett Shale play in North Texas as the largest shale gas play in the nation, EIA said.

Overall oil and gas production increased in 2012, the agency found. “Crude oil and lease condensate production rose about 16%, and, despite the drop in proved natural gas reserves, wet natural gas production rose about 6%,” the report added.

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