OIL firms have to go where the oil (and gas) is. Increasingly, that place is the US.

Over half of global upstream oil and gas mergers and acquisitions since the start of 2011 targeted US assets, up from an average of 37 per cent in the prior four years, according to consultancy IHS Herold. Besides a relatively open market for energy assets, the shale boom has drawn in companies seeking reserves and know-how.

But this shift in the center of gravity for oil M&A poses a quandary for one group of important buyers: China's national oil companies, or NOCs.

Their traditional strength - the backing of Beijing - isn't work in the US. When Washington derailed CNOOC's $18.5 billion bid for Unocal in 2005, much was made of the potential for vital US resources and technology being transferred to the Chinese government at a time when peak oil fears were taking hold.

In theory, the shale boom should have eased concerns about scarce resources. But this wasn't the main issue with Unocal: Most of its reserves were in Asia. US objections had more to do with CNOOC's hostile approach cutting across Chevron's own designs on the company, says Robin West, founder of consultancy PFC Energy. Unocal eventually accepted Chevron's embrace.

Yet China's NOCs can't ignore the US Indeed, half their overseas acquisitions of upstream assets this year have been stateside, up from 7.6 per cent last year, according to IHS. Their interests lie primarily in seeking expertise in developing shale resources that they can apply back home.

Luckily for the NOCs, this can mostly be achieved with buying non-operating stakes in shale prospects or forming joint ventures with US exploration and production companies; CNOOC's deals with Chesapeake Energy in recent years are an example.

"You acquire control when you think you can run the assets better," says Trevor Houser, a visiting fellow at the Peterson Institute for International Economics. Besides politics, the risk with buying a company is that employees leave, taking their expertise elsewhere. Even Exxon Mobil worried about this when it acquired XTO Energy in 2010, one reason it gave the business its own division. That risk would likely be even higher for a Chinese acquirer.

For US E&P companies and their investors, suffering from the collapse in gas prices, this situation is bittersweet. The buying power of Chinese NOCs and their desire to learn about shale resource development makes them important potential buyers for assets, such as those Chesapeake has for sale.

Full takeovers remain unlikely, though. The US might not worry quite so much about energy security these days -but rivalry with China spans much more than that.