Although no major new refineries have been built in the U.S. since the 1970s, U.S. refining capacity has risen by 14 percent since 1997 due to refiners modifying equipment to increase product flow and adding new distillation units, according to a report from the Energy Information Administration.

The average annual refining capacity has increased by 185,000 bpd over the same time period, which is close to the impact of adding one and a half refineries per year, the report states.

The number of refineries and refining companies has declined in recent years, due to mergers and an increase in individual refinery capacity.

The second and fourth largest refiners in 1997 merged to become ExxonMobil Corp., and BP, the ninth largest in 1997, acquired Amoco, which was number three that year. Valero Energy Corp. was ranked 51st based on refining capacity in 1997, but by 2009, Valero is the largest U.S. refiner with 14 refineries and two million bpd of capacity. The share of U.S. refining capacity at the top four companies increased from 26 percent in 2007 to 40 percent in 2009, while the top 10 share went from 51 percent to 69 percent.

The capacity of the average refinery increasing from 94 million bpd capacity in 1997 to 118 million bpd capacity in 2009. The average capacity per refining company has risen from 196 million bpd to 310 million bpd over the same time frame.

Net refining margins and return on investment reached historic high levels from 2004 to 2007, which prompted many refiners to expand, the report states. However, refining margins have declined sharply over the past several quarters, and petroleum product demand has declined as renewable fuels make inroads into the market, according to the report.