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Площадь: 353739.5 км²
East China Sea Basin
The East China Sea Basin, located in the continental shelf of the East China Sea, with the Yanshanian continental margin as granite, experienced fault subsidence in the Paleocene Eocene, depression in the Oligocene-Miocene and neotectonic movement in the Pliocene-Quaternary in Cenozoic Era. There mainly developed Cenozoic sedimentary formations from the Paleocene Series to the Quaternary System in the basin. The basin experienced five episodes of tectonic reworking and some tectonic inversions in particular (Figs. 1 and 2). There are two depressions and two uplifts in the basin, i.e. the west depression belt (the Changjiang Depression and the Taibei Depression), the central uplifted belt (the Hupijiao Uplift, the Haijiao Uplift and the Yushandong Uplift), the east depression belt (the Fujiang Sag, the Xihu Sag and the Diaobei Sag), and the east uplifted belt (the Diaoyu Island uplifted fold belt, i.e. the shelf break uplift). The basin is the product of back-arc spreading from the subduction of the Pacific Plate. With the underthrust zone retreating to the east, the basin experienced tectonic migration from west to east and shifting of depocenters eastwards, resulting in the differences in structure and depositional evolution from sag to sag. Paleocene fault subsidence mainly occurred in the west depression belt and Eocene-Oligocene fault subsidence in the east depression.
Fig. 1. Distribution of gas fields in the East China Sea Basin.
In this basin, the Paleocene Series is composed of lacustrine mudstone, transitional coal measures and marine mudstone; the Eocene Series consists of transitional coal measures and estuarine mudstone. Coal measures composed of coal seams, carbargilite and dark mudstone are the major source rocks in the basin. Coal seams of the Eocene Pinghu Fm were encountered in 16 exploratory wells, with an average thickness of coal seams in each individual well of 16.9 m, single-layer thickness of 0.3-1.2 m in general and the maximum cumulative thickness of 50.6 m. The maximum cumulative thickness of coal seams and carbargilite is 75.8 m and the cumulative thickness of dark mudstone is 200-1800 m. Pinghu dark mudstone is medium to high in organic matter abundance, with a maximum TOC of 1.97%. Coal seams have high abundance of humic hybrid-humictyped organic matter, TOC of up to 57.07%; coal-measure source rocks contain some hydrogen-rich maceral. Coal-measure source rocks mainly occur in deltaic plains and coastal plains and tectonically in the west and north of the Xihu Sag.
Fig. 2. Geologic section showing hydrocarbon accumulations in the Xihu Sag, East China Sea Basin
The thermal gradient in the basin and its surrounding areas, low in the north and high in the south, is 25e43.5 0C/km, and 32.7 0C/km on average. The heat flow value is high in the east and low in the west. As a result, most source rocks at the bottom of the Yueguifeng Fm in the Jiaojiang Sag in the west part of the basin have matured with Ro > 0.7% and entered the oil-generating window. Most source rocks in sub-sags have entered the wet gas window with Ro > 1.3%, and some deep source rocks have entered the dry gas window with Ro > 2.0%. In comparison, the east depression belt in the basin contains source rocks with relatively high maturity. For example, most source rocks at the bottom of the Pinghu Fm in the Xihu Sag have entered the dry gas window with Ro > 2.0%. The basin mainly generates natural gases under the co-control of source and heat. There are several reservoir-seal assemblages in the basin, namely, the basement assemblage, Mesozoic assemblage, and Paleocene, Eocene, Oligocene and Miocene assemblages, in which Eocene and Oligocene assemblages are main reservoir-forming ones. The basin experienced several episodes of tectonic inversions, so reverse anticlines are the major trap type.
The petroleum geologic survey in the East China Sea Basin began in the 1970s and Pinghu oil and gas field was discovered in the 1980s. To date, a number of gas fields have been discovered in the Xihu and Lishui sags, demonstrating that the basin is rich in gas.
Data source: Analysis of the orderly distribution of oil and gas fields in China based on the theory of co-control of source and heat. Zhang G.C., Jin Li, Lan Lei, Zhao Zhao. 2015
Следующий Бассейн: Qiongdongnan