The agreement was signed during a three-day state visit to China by Abu Dhabi Crown Prince Sheikh Mohammed bin Zayed.

Under the agreement, the companies will share the latest know-how in developing ultra-acidic natural gas fields, according to ADNOC's statement.

They will consider using CNOOC's engineering arms, Offshore Oil Engineering Co Ltd and China Oilfield Services Ltd, as contractors for design, purchase and construction as well as oilfield service providers for ADNOC.

The two firms will also look at potential opportunities to work with both these companies on offshore oil and gas field assets in Abu Dhabi, the statement said.

They will look into working together on LNG marketing and purchasing.

In oil refining and petrochemicals, ADNOC will consider opportunities for investing in CNOOC's existing refineries, and potential collaborations in new integrated refining and petrochemical assets in China, the statement said.