CNOOC Ltd. has lifted its equity holding in China Oilfield Services Ltd. (COSL) to 50.91% under a continuing share purchase plan, according to a report carried by The Globe and Mail.

The increase moves the offshore operator's position past the 50% mark, consolidating its control of China's principal oilfield services contractor. COSL provides drilling, well services, marine support and survey work across CNOOC's offshore operations in Bohai Bay, the South China Sea and the East China Sea.

The purchases form part of an ongoing buy programme that CNOOC has executed in the open market. The company has not disclosed a final target stake or an end date for the programme, leaving open the possibility of further accumulation.

The added equity gives CNOOC tighter integration between its upstream development programme and its service arm as it advances work across its offshore Chinese portfolio. COSL's rig fleet and vessel fleet support CNOOC's production base in Bohai Bay, one of the company's core producing regions, and its deepwater developments in the South China Sea.

For COSL, the move places a majority of its shares with its parent-side shareholder and signals sustained state backing at a time when Chinese offshore drilling activity runs at elevated levels. CNOOC has kept its domestic exploration and development spending at high rates in recent years as it works to lift reserve replacement and hold production growth from Chinese waters.

The transaction also aligns with a broader pattern among Chinese state energy groups, which have used on-market purchases to increase cross-holdings in listed subsidiaries over the past year.

Watch item: further disclosures on whether CNOOC continues to buy COSL shares, and at what threshold the programme concludes. COSL's next fleet-utilisation and results filing will show how the tightened ownership structure tracks against the offshore workload.