CNOOC Bohai gas discovery reaches 5,770 m at ultra-deep well

By Elena Vasquez

CNOOC drilled a well to 5,770 metres below the seabed in the central Bohai Sea, offshore China, and tested it at about 800,000 cubic metres of natural gas per day, the company said.

The oil and gas well lies more than 5,000 metres below the seabed in ultra-deep volcanic formations, next to the Bozhong Sag, which CNOOC calls the largest hydrocarbon-rich sag in the Bohai region.

CNOOC reported a final depth of 5,770 metres and more than 120 metres of gas-bearing pay zone within volcanic rocks. Although it described the well as an oil and gas discovery, the published test rate applies to natural gas.

During testing, the well achieved daily output rates of around 800,000 cubic metres, equivalent to 28 million cubic feet (mcf). The company did not present that rate as sustained commercial output and provided no reserves estimate, final investment decision or startup schedule.

What did the Bohai well establish?

CNOOC said the result indicates new exploration potential in offshore ultra-deep volcanic rocks and contributes to its continuing natural gas exploration in the Bozhong Sag. It also defines a specific geological play around volcanic gas, rather than a general increase in offshore exploration potential.

CNOOC chief scientist and chief geologist Xu Changgui said: “The success of this well has opened up a new play for offshore ultra-deep natural gas exploration and expanded the scope of deep oil and gas exploration in Bohai.”

The takeaway is straightforward: CNOOC has a high-rate gas test in volcanic rock, but the announcement does not define a producing asset.

How did CNOOC locate gas in ultra-deep volcanic rock?

The geology has made reservoir prediction difficult in the Bohai region. CNOOC said it has improved its understanding of offshore ultra-deep resources and developed its own offshore deep, multi-source, broadband 3D seismic exploration technologies.

The company also established new approaches for identifying quality reservoirs in ultra-deep buried hills. According to CNOOC, those methods helped it locate areas in the Bozhong Sag with concentrations of volcanic natural gas and drill the high-yield oil and gas well.

Xu said CNOOC has strengthened theoretical work on deep and ultra-deep oil and gas enrichment in basins with high heat flow and strong activity, while expanding research into key technologies. He also said China’s offshore exploration has moved from shallow formations into deep and ultra-deep formations.

Xu also said the result could advance the wider Bohai effort: “It is of positive significance for accelerating the ultra-deep exploration process in Bohai and for building a major gas province of one trillion cubic metres in Bohai.”

The trillion-cubic-metre province remains a prospect tied to further exploration in Xu’s statement, not a production forecast from the single well.

The takeaway is that CNOOC credits improved imaging and reservoir selection with finding the accumulation; the province-scale ambition still rests on more exploration and delineation.

Is the discovery a production project yet?

The disclosed work remains a discovery and well-testing success. The announcement did not identify a development sanction, final investment decision, production startup date or output forecast for the new Bohai well; its 800,000 m³/day result is a test rate.

In a separate operational note, CNOOC said it started production at the Xijiang Oilfields 24 Block development project in December last year. The company did not connect that startup to the ultra-deep volcanic well.

The takeaway is that Xijiang 24 is a separate production milestone and does not turn the new Bohai discovery into a producing asset.

What should readers watch next?

The watch item is CNOOC’s next delineation and development decision: whether further work establishes the size and quality of the volcanic reservoir, and whether the company approves a plan with a startup date and production basis. Until then, the 800,000 m³/day flow marks a successful test, not a forecast for the Bozhong Sag.