HOUSTON - Exxon Mobil Corp. on Thursday posted its biggest profit increase since 2003, as rising production helped the largest U.S. oil company take advantage of gains in energy prices. Second-quarter net income jumped 91 percent to $7.56 billion, or $1.60 a share, from $3.95 billion, or 81 cents, a year earlier, Irving, Texas-based Exxon said Thursday in a statement. Per-share profit was 15 cents higher than the average of 17 analyst estimates compiled by Bloomberg. Exxon and Royal Dutch Shell, Europe's biggest energy company, capitalized on a 31 percent gain in crude prices, as measured by U.S. oil futures, with increased production. Exxon's output climbed 8.4 percent to the equivalent of 4 million barrels of oil a day, and Shell had a 5 percent gain. Exxon's profit increase was its largest since the first quarter of 2003. 'This will probably be the last quarter that we'll see large increases in earnings per share unless oil prices increase significantly from where we are now,' said Brian Youngberg, an analyst at Edward Jones in St. Louis. Shell said Thursday that its profit rose 15 percent to $4.39 billion. In April, Exxon reported a 38 percent jump in first- quarter net income, marking the company's first profit increase since the third quarter of 2008. Worldwide demand for crude rose an estimated 3.2 percent from a year earlier in the second quarter, according to the International Energy Agency in Paris. Exxon said July 8 that it plans to buy back $3 billion of its shares this quarter following its acquisition last month of natural-gas producer XTO Energy Inc. The transaction increased outstanding shares at Exxon by about 9 percent. Exxon also said it will look to pay off or refinance $11 billion of assumed debt. Exxon has said the XTO deal is a long-term bet that gas from shale formations and other so-called unconventional resources will meet rising U.S. demand through at least 2030 as output from conventional wells falls. 'They're going to take their XTO expertise and redeploy them in their own properties and try to get more bang for their bucks, and that's what they are very well known for,' said Fadel Gheit, an analyst at Oppenheimer Co. in New York who has a 'market perform' rating on Exxon shares. He said low gas prices have prevented the deal from helping the company's stock. Chevron Corp., the second-largest U.S. energy company, is scheduled to report earnings Friday. 