U.S. oil companies are becoming less liquid — but not in a financial sense. With natural gas so plentiful, Big Oil is fast becoming Big Gas. Shareholders may be underestimating the impact of the shift on future returns while an unexpected advantage is tipping to the oiliest majors.

America’s energy titans are finding it ever harder to ramp up oil output. Exxon Mobil is on track to pump 5 percent less this year than in 2005, according to Barclays Capital forecasts. Instead, the company has turned to gas for growth. A big project in Papua New Guinea and the XTO acquisition are accelerating the shift. Gas, which accounted for 38 percent of Exxon’s output in 2005, could account for up to 48 percent next year.

Exxon isn’t alone. Rivals ConocoPhillips, and to a lesser extent Chevron, are also bloating with gas. The trend looks ominous. Oil offers much fatter margins and, in the United States, sells for almost three times more than gas for the same quantity of energy. Drillers are partly victims of their own success with new techniques boosting gas reserves by a third and driving down prices. Meanwhile, lawmakers have ditched proposals for a carbon tax — which promised to spur demand for cleaner-burning gas.

Of course, the market is not so grim everywhere. Multi-decade contracts for liquefied natural gas in Asia are typically hooked to oil and so can fetch double the U.S. price. This helps Chevron in particular, whose projects are more geared towards Asia.

But even these cushy overseas markets look less safe over the long run. The same drilling innovations that led to plunging prices in North America are being eagerly plied in Germany, Poland and China. Meanwhile other monster gas projects in Qatar and Australia are expected by the International Energy Agency to increase output by 50 percent by 2013.

With even oily Chevron drifting towards gas, Occidental Petroleum  –  which still gets 75 percent of its output from the black gold — may wind up the last of the majors to deserve the moniker Big Oil. It is pricier than its three biggest rivals — trading at 14 times expected 2010 earnings compared to an average of about 9.5. But its significantly lower exposure to gas justifies the premium, and then some.