According to a new report by audit firm Ernst & Young, U.S. oil and gas reserves increased by 9 percent in 2013, with the bulk of added reserves due to independent producers that specialize in exploration and production, rather than major integrated companies that also run refining operations.

Oil reserves in the United States went up to almost 25.4 billion barrels, marking a rise of 2.1 billion barrels or 9 percent on the previous year and an increase of 52 percent compared to 2009, the Houston Chronicle reported. The estimated reserves featured in the report were based on 2013 end-of-year reserves data submitted by the 50 largest publicly traded companies. These are proved reserves that companies are ready to drill over the next five years, as defined by federal standards.

The majority of gains in terms of reserves come from independent companies like Anadarko Petroleum Corp. and Devon Energy. By contrast, integrated companies with refining businesses appear to be facing difficulties in finding and extracting oil and gas in shale regions, commented Herb Listen, U.S. oil and gas assurance leader at E&Y.

Natural gas reserves went up last year as well, and by the same rate of increase as for oil — 9 percent. This means that total U.S. gas reserves exceeded 178 trillion cubic feet in 2013. These estimates are significant because they come on the heels of a slight dip in reserves in 2012, the report noted.