Underscoring Cnooc Ltd.'s CEO -4.34%massive $15.1 billion deal for Canada's Nexen Inc. NXY +51.82%is an uncomfortable fact for Beijing's policy makers: China's economy remains intensely energy hungry.

The world's No. 2 economy imported 5.6 million barrels of oil a day in the first half of this year, according to customs data, up 11% from a year earlier despite its modestly slowing economy. It likely won't stop there: The International Energy Agency has forecast China's net imports of oil could surpass 12 million barrels a day by 2035.

By contrast, the U.S. is likely to buy less foreign oil, with imports eventually falling below levels last seen in the 1990s, thanks to rising domestic ethanol and shale oil-and-gas production, according to an analysis by BP PLC. BP.LN -2.11%.

China faces similar trends with other energy sources. June natural-gas imports, in the form of pipeline gas from Central Asia or liquefied natural gas imported by sea, increased by 58% from a year earlier in the first half of 2012 to 7.27 million metric tons.

In coal, China—long a net exporter—has become a net importer in recent years. Imports in the first half of the year rose 61% to 112 million tons compared with a year earlier.

Investment bankers say China in recent years has pushed to acquire stakes in resource companies, particularly energy, to secure supplies for its voracious economy. Such investments also help diversify its vast foreign-exchange reserves.

Shale gas—which is trapped in rock formations and is plentiful but difficult to extract—provides a potential solution in the eyes of Chinese officials. China recently set a target of producing 6.5 billion cubic meters a year of shale gas by 2015 from virtually zero this year and hopes to produce between 60 billion and 100 billion cubic meters a year by 2020. The U.S. Energy Information Administration said last year that China has an estimated 1,275 trillion cubic feet, or 36 trillion cubic meters, of technically recoverable shale-gas reserves, making it the largest repository of shale gas in the world.

China uses price controls on gasoline and electricity to insulate consumers and business from price shocks, measures that contribute to waste and excessive consumption. But officials are openly considering making pricing more market-oriented to encourage efficiency. They are experimenting with market prices for electricity in some places and they are considering ways to more quickly pass on the rises and falls of oil prices to drivers.

China isn't using all the oil to power its economy, though: It is pouring large amounts into in strategic oil reserves, as a hedge against future shortages, and into commercial stockpiles. China doesn't publish data on its oil-reserve volumes.