(Adds details on Marathon Petroleum results, Valero CEO quote, shares, industry background))
By Yashaswini Swamynathan and Anirban Paul
Feb 1 (Reuters) - U.S. refiners Valero Energy Corp and Marathon Petroleum Corp posted quarterly profits on Thursday that beat estimates and said they expected strong global economic growth to spur demand for oil in 2018.
Shares of Marathon and Valero, which have gained more than 45 percent in the past 12 months, were marginally up in early trading on Thursday.
Analysts say refiners are likely to outshine other energy segments in 2018 as they benefit from a surge in U.S. oil production and rack up among the biggest gains from the recent corporate tax cuts.
U.S. oil output is expected to surpass 10 million barrels per day this month, supported by strong demand and improving global oil prices.
"Looking ahead, we continue to see a favorable fundamental environment, with abundant crude oil supply and strong products demand being supported by global economic growth," Valero's Chief Executive Joe Gorder said in a statement.
Shareholders and analysts, however, are keeping a close watch on investment budgets at oil companies, arguing that prices are still 40 percent lower than their 2014 highs and do not justify big new outlays.
Companies should instead make the most of current capacity, they say.
On Thursday, Valero said it expects to invest $2.7 billion in 2018, about $300 million more than it spent in 2017.
Marathon said it expected to spend $1.6 billion in 2018, compared with the $1.7 billion it had earmarked for 2017.
The companies have increased their quarterly dividends by about 15 percent.
In the fourth quarter, operating income at Marathon's refining business surged more than four-fold, while Valero's increased 52.2 percent, supported by higher gasoline and distillate margins.
Valero reported a $1.9 billion gain to net income and Marathon a $1.5 billion increase as the companies benefited from President Donald Trump's recent tax reforms.
Excluding items, Valero earnings beat analysts' average estimate by 8 cents per share and Marathon by 4 cents, according to Thomson Reuters I/B/E/S. (Reporting by Yashaswini Swamynathan in Bengaluru; Editing by Savio D'Souza and Sriraj Kalluvila)