China National Offshore Oil Corp (CNOOC), China's third largest oil producer, inked a deal to purchase a 50-percent stake in China United Coalbed Methane Co for 1.2 billion yuan($181 million) at a signing ceremony held on Wednesday.

The purchase brings CNOOC proven reserves of 50 billion metric meters. China United Coalbed Methane Co has 27 coal seam gas blocks in China with an annual output of no more than 200 million metric meters.

Earlier this month, CNOOC said it would spend $50.46 million to acquire 50 percent of the exploration rights in five Australian coal seam gas blocks owned by Exoma Energy Ltd.

According to International Energy Agency's latest prediction, China's coal seam gas reserves rank third in the world with about 36.81 trillion metric meters.