With its ongoing push for modernization, China seems to have an unquenchable thirst for energy. But with a shortage of domestic sources, it's been aggressively looking overseas: Seven of the 10 biggest foreign mergers or acquisitions by Chinese companies this year have been in the energy sector as state-owned Chinese energy companies rapidly expand their portfolios.

China National Petrochemical Corporation invested $10 billion in oil and gas fields in Mozambique and Kazakhstan in 2013. Sinopec (HKG:0386) spent $3.1 billion on a 33 percent stake in Apache Corp.'s (NYSE:APA) Egypt fields, CNN reported Wednesday.

More than 20 percent of oil and gas deals worldwide in 2013 have involved a Chinese firm, according to Brian Lidsky, a managing director at Houston-based data provider PLS Inc. That’s a new record for the Chinese, building on the trend seen in 2012. China National Offshore Oil Corporation purchased Canada’s Nexen Inc. (NYSE:NXY) for $15 billion last year, for the latter’s oil sand and shale gas resources.

The deals are partly fueled by China’s shift from its past reliance on coal as a primary energy source -- which has caused dangerous levels of pollution.

Chinese firms in their aggressive overseas expansion have been willing to sign energy contracts with terms most other multinational oil firms like Exxon Mobil Corporation (NYSE:XOM), BP PLC (NYSE:BP) and Royal Dutch Shell PLC (NYSE:RDS.A) would not even consider. For example, some deals involve more than 90 percent of a firm’s profit being taken away in royalties, taxes and other fees.

The trend is unlikely to end any time soon, since China's growing population needs more energy to keep up its modernization and urbanization efforts. It’s currently the second-largest oil consumer after the United States, but it will soon surpass the U.S.

Chinese firms have shied away from wholesale purchasing of U.S. companies since a 2005 bid from CNOOC, another state-owned enterprise, to buy California-based Unocal for $18 billion had to be abandoned due to political pressure in the U.S., CNN reported.