Both crude oil and natural gas reserves in the United States increased in 2013, according to a new report from the U.S. Energy Information Administration (EIA). This report also included the first update of the EIA’s “Top 100 U.S. Oil and Gas Fields” list since 2009.

All told, the top 100 oil fields in the United States contained an estimated 20.6 billion barrels (56%) of crude oil and lease condensate in proved reserves in 2013. For natural gas, the top 100 fields represented 239.7 trillion cubic feet (68%) of natural gas proved reserves. According to the EIA, there were several particularly important changes in the updated rankings. Specifically:

“…oil fields in the Eagle Ford and Bakken shale plays and natural gas fields in the Marcellus Shale have become significant contributors to total U.S. reserves. Fields in the Marcellus and Eagle Ford plays appear at the top of the list in their respective categories, whereas in 2009 the Marcellus fields were ranked in the bottom half of the list, and Eagle Ford fields (discovered in 2008) did not appear in the top 100. Alaska’s Prudhoe Bay, previously the oil field with the largest amount of reserves, fell to third, behind fields in the Eagle Ford and Permian Basin.”

Furthermore, the definition of “proved reserves” brings in a dependence on economic conditions (i.e. market prices and operating costs) that can result in significant variations in reserve values over time. The EIA defines proved reserves as “estimated quantities of oil and natural gas that analysis of geologic and engineering data demonstrates with reasonable certainty are recoverable under existing economic and operating conditions.” Therefore, while the estimated total amount of crude oil or natural gas in the ground might remain constant, the proved reserves could vary year to year.

To give an example from the EIA’s analysis:

“The Securities and Exchange Commission (SEC) requires companies to assess proved reserves based on the average of the prices of a crude benchmark on the first day of each month for the year. Using, for example, front-month West Texas Intermediate (WTI) futures closing prices for the first trading day in each month of 2013, this value was $97.28 per barrel (bbl) for 2013. The comparable value for 2014, for which EIA is now collecting proved reserves data from operators, was slightly lower at $94.42/bbl.”

Photo Credit: U.S. Energy Information Administration (2015)