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CHINA: SEDIMENTARY BASINS

Globalshift.co.uk (source: Carnegie Endowment for International Peace)

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China has a number of productive sedimentary basins in the north, northwest  and northeast of the country and scattered through central and southern provinces in the south and offshore. Globalshift considers the Songliao Basin and and Bohai Basin (on and offshore) to be of most significance with the Tarim, Junggar, Ordos and Sichuan basins also important. Offshore the Pearl River Mouth Basin in the South China Sea has an extensive productive area.

The country lies between the Siberian Shield and the Pacific Oceanic Plate and can be broadly divided into two geological provinces. In the west lie compressional basins, including Junggar and Tarim, which have deep Palaeozoic sequences. In the east a series of Mesozoic and Cenozoic basins overlie the North and South China Cratons (Amur Plate and Yangtze Plate). They are bounded by major shear zones to the north and south which give way to the continental shelf that borders back-arc basins at the edge of the Philippines and Pacific Plates. Principal source rocks occur in Cretaceous and early Tertiary lake bed deposits with reservoirs throughout the overlying sequences.

The Songliao Basin

The Songliao Basin is located in northeast China in a landscape of forests, meadows and grassland. It is bordered by the Zhangguangcai Mountains to the east, the Greater Khingan Range to the west, the Kangping-Faku hill zone to the south and the Lesser Khingan Range to the north. These comprise Pre-Cambrian and Palaeozoic metamorphics and volcanics.

The mountains surround a large Mesozoic and Cenozoic depression containing multiple sedimentary systems dominated by a deltaic sandstone sequence deposited from a provenance in the north. Fields of the Daqing and Jilin Oil Provinces produce from these stacked sandstone reservoirs.

The Bohai Bay Basin

The Bohai Bay basin (also known as the North China basin), which extends offshore into Bohai Bay, is a rifted lacustrine basin formed on the basement of the North China Craton at the end of the Cretaceous. During the syn-rift stage a series of grabens and half grabens developed along northwest and northeast trending fault sets before becoming one large post-rift basin during the late Oligocene. Sediments were deposited in a lake setting within these grabens.

The post-rift sediments are dominated by fluvial deposits. The basin includes 6 sub-basins; Liaohe, Bozhong, Jiyang, Huanghua, Jizhong and Linqing. The Bozhong sub-basin covers most of the the offshore area and has been the depositional centre since the Oligocene with, unlike most of the other sub-basins, late-stage faults developed in the younger sediments. In the onshore regions the largest fields are in the Dagang, Liaohe, Huabei and Jidong Provinces.

The Tarim Basin

The Tarim Basin is in northwest China in the Xinjiang region. Its northern boundary is the Tien Shan mountain range and its southern boundary is the Kunlun Mountains on the edge of the Tibetan Plateau. The Taklamakan Desert covers much of the area. The basin was formed during the Carboniferous and Permian as the Tarim micro-continental fragment fused with other parts of the Eurasian continent. Compression around the margins is occurring as the micro-continent is forced under the northern and southern mountain ranges.

A thick succession of Carboniferous to Quaternary sediments occupy the centre of the basin, in parts exceeding a thickness of 15 kms. The source rocks are primarily Permian mudstones. Rivers originating in the north have flowed into the Tarim Basin accumulating potential reservoirs in salt lakes and marshes. Owing to the depth of burial Tarim is primarily prospective for gas with pipelines feeding the populated regions of Beijing, Shanghai and Guangdong.

The Junggar Basin

The Junggar Basin lies in the Uygur Autonomous Region of Xinjiang in the northwest of China between the Mongolian Altai Mountains in the north and the Borohoro and Eren Habirga mountains in the south. On its western edge the Dzungarian Alatau and Tarbagatay ranges separate it from the Lake Balqash depression in Kazakhstan.

The basin was formed in a similar fashion to Tarim. Source rocks are Permian lacustrine sediments which provide oil to basin margin alluvial fan reservoirs of Permian, Triassic and Jurassic age. The Karamay Thrust Belt, which forms the northwestern margin, is the site of main oil fields. Smaller fields lie in Himalayan structures in the southwest. Carboniferous basement rocks on the southward extension of the Karamay Thrust Belt have also proved productive.

The Ordos Basin

The Ordos basin in central north China s surrounded by mountains including Yin and Daqing in the north, Qinling in the south, Helan and Liupan in the west, and Luliang and Taihang in the east.The Great Wall divides the basin into two parts, a northern arid desert and a southern semiarid plateau. The Yellow River lies on its western border from which tributaries flow into desert lakes or salt marshes.

The basin is a depression of the North China Craton with sediments present from Pre-Cambrian age up to the present reaching up to 10kms in thickness. Oil, mainly in the south, and gas, mainly in the north, are produced from Triassic, Jurassic, and Ordovician sediments. Fields include Yanchang, Sulige, Ansai, Jing'an, and Jingbian.

The Sichuan Basin

The Sichuan Basin is a lowland region in southwestern China bordered by the Qionglai Range in the west, the Longmen Mountains in the northwest and the Daba Mountains in the northeast. It underlies the central and eastern portions of Sichuan province and neighbouring Chongqing Municipality. Flat, fertile and heavily populated, it is a significant gas producing region.

The majority of the gas in the basin is non-associated and produced from carbonate reservoirs of Sinian, Paleozoic, and early Mesozoic age and also from low-permeability Upper Triassic fluvial sandstone reservoirs and Permian and Triassic coalbeds. Modest quantities of oil and associated gas are also produced from lacustrine rocks of Jurassic age. Key gas and oil provinces are Chongqing, Chuanzhong, Chuanxibei and Chuandongbei. Large sour gas discoveries (called Yuanba and Puguang) are very significant.

Offshore

China's offshore shelf is wide, extending at least 320 kms for much of its 3,800 km length, although a substantial part is disputed with neighbouring countries. The area covers, from north to south, the Bohai Gulf, the Yellow Sea, the East China Sea, the South China Sea and the Beibu Gulf (Gulf of Tonkin in Vietnam). All the basins are Tertiary rifts overlain by deltaic sediments emanating from rivers draining the Chinese continent.

The shallow waters of the Bohai Gulf east of Tianjin overlie the offshore extension of the Bohai Bay Basin (North China Basin), 200 kms east of Beijing. This produces over half of China’s offshore oil. South from here beyond the Yellow Sea, the East China Sea overlies a large basin west of Shanghai. The Xihu trough is the most prospective area, holding many discoveries mainly of gas and condensate.

Further south the shallow waters of the Pearl River Mouth Basin in the South China Sea is an important producing area despite problems with typhoons, which sweep through the area in late summer. The adjacent Yinggehai basin to the southwest, south of Hainan Island, is a gas-prone area with large non-associated gas fields. These two areas deliver the bulk of China’s offshore gas with the Yacheng 13-1 field supplying gas to fuel Hong Kong’s electricity generators. In 2014 gas production began from the Liwan gas project, the first deep water development in China.

Finally the Beibu Gulf lies in the South China Sea between Hainan Island and the mainland. It is underlain by the Beibuwan basin. Many oil accumulations have been developed here but all are of a small size.

Other areas

Several smaller basins produce some oil and gas from similar stratigraphies in the centre and south of the country including the Subei Basin in Jiangsu and Anhui Provinces and the Nanyang Basin in Henan Province in the east, and the Qaidam Basin in Qinghai Province and Jiuquan Basin in Gansu Province in the west.

There are also many potential tight oil and gas reservoirs within the conventional oil and gas basins. The most promising shale gas resources are in the Sichuan and Tarim basins. In the northeast tight gas has been developed in the Ordos (Changqing) Basin’s, Sulige and Changebei fields. There is also potential for tight oil primarily in the Songliao, Bohai Bay and Ordos basins.