In the high mountains of Xining, Northwest China's Qinghai Province, where secrets of deep-sea oil and gas exploitation and development are hidden, China National Offshore Oil Corporation (CNOOC) has been seeking energy signals from trillions of years ago to reveal those unknowns.
Continent-ocean transition is caused by crustal movement and sea level eustacy. By observing features of the outcrop and the rocks, sedimentation models can be built to effectively allow exploitation and development of offshore oil and natural gas.
Home to rich deep-sea oil and natural gas reserves, the South China Sea is a main energy replacement area.
In 2021, a geological survey in Xining, Qinghai, indirectly pushed forward discovery of deep-sea gas fields like Liuhua 29-1 and Liuhua 34-2.
With the aim of building a massive output gas field in the South China Sea, a field expedition including experts in exploitation, development, production and well-drilling entered the mountains, seeking new methods of oil and gas development from the rocks themselves.
Going out and finding the answer
For workers in the petroleum industry, field geological surveying is like a bridge connecting geological phenomena with theories and practices of oil and gas exploitation and development, and supporting the development of oil and gas resources.
Field geological surveying at oil fields in eastern areas of the South China Sea is not rare. In the early years after its establishment, CNOOC sent out survey teams to Yingde and Huizhou in South China's Guangdong Province. The company also launched surveys in South China's Hainan Province, North China's Jilin Province and Eastern China to discover distribution features of coastal sand stones, landforms of volcanic rocks with high permeability and porosity as well as traces of tidal sand bodies.
The field investigation at the eastern part of the South China Sea in those early days mainly focused on marine deltaic sedimentation.
Several marine deltaic sedimentation oil fields had been discovered following opening of the Huizhou 21-1 oilfield, the first of its kind in the eastern South China Sea. The oilfields have been a main source of oil and gas in the region, and have led to oil reserve replacement.
Zhang Zhenbo, a technical expert at CNOOC, said that facing new issues like the continental deltaic and deep-sea gravity flow sediment, CNOOC staff members had to add to their understanding gained from previous surveying and post-development by carrying out field investigations.
To acquire knowledge of the new areas, a five-year survey route plan was made in oilfields in the eastern part of South China Sea, and in 2022 new investigation routes in Xining of Northwest China's Qinghai Province, Southwest China's Yunnan Province and North China have been determined.
Carrying out field investigation and figuring out puzzles
CNOOC staff members recently carried out a survey in Huangnan Tibet Autonomous Prefecture in Xining, Qinghai. With years of continent-ocean transition, the region has typical and various kinds of deep-sea sedimentation outcrop sections, making it a perfect place to study that kind of sedimentation.
Moving forward along the ridge, the survey team stopped in front of a huge rock where trenches could be clearly seen on the massif section.
After analyzing the features of the sand body sedimentation and the rules of particle size changing, and measuring the thickness and status of the sand bodies, Zhang said that there is a group of similar targets in the deep-sea area in the eastern South China Sea, so that the continuous study of the geological condition in Xining might help develop a new deep-sea exploitation pattern.
Breakthroughs were made in the Liwan 3-1 gas field in 2006, leading to CNOOC's obtaining several commercial discoveries in surrounding areas. However, the Liuhua oilfields have complicated underground sand body structures, making them quite difficult to develop.
Engineers gained new knowledge and findings of the complicated inner structure and the formation mechanism through their survey in Xining, which is expected to help them improve their work in deep-sea oil and gas development.
Recording findings of field investigations
Returning from the field investigations, Zhang reminded the survey team members to write down their findings in the mountains and adopt them in basic study of the Liwan, Liuhua and Panyu deep-sea gas fields.
Bai Haijun, a geophysical engineer at CNOOC, said that he learned that changes in the thickness of the deep-sea sedimentation reservoir stratum can be quite fast, which is similar to the deep-water fan with strong amplitude in the Liwan 3-1 gas field. Meanwhile, the survey also deepened Bai's knowledge of the sedimentation process of the gravity flow. He knew that a more accurate assessment of the gas field could be obtained by combining geological sedimentation evolution models with the seismic amplitude attributes.
Shu Liangfeng, the reservoir stratum prediction engineer, agreed with Bai's views. He said that multiphase density current can be generated in the narrow continental shelf areas, and that an in-depth deep-sea sedimentation mode study can guide prediction of Liuhua 29-1 gas field's deep-sea reservoir and make assessment more objective.
Exploitation and development of deep-sea oil and gas has been a hot and difficult issue in both China and abroad. It is also a key to improving oil and gas output in the South China Sea. In recent years, CNOOC has stepped up efforts in the field and discovered several deep-sea gas fields such as Liuhua 29-1, Liuhua 29-2 and Liuhua 28-2.
CNOOC will continue to seek methods of deep-sea oil and gas development through non-stop exploration.
(Executive editor: Ma Zheming)
