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Площадь: 56043.4 км²
Qiongdongnan Basin
The Qiongdongnan Basin lies in the west continental margin of northern South China Sea where the basement is Yanshanian island arc belt intercalated with Proterozoic crystalline massifs. After Paleogene fault subsidence, Neogene depression and neotectonic movement, the basin exhibits a tectonic pattern of fault depression and depression and is separated into a north depression, a central uplift and a south depression (Fig. 1). During fault subsidence, the Eocene Series was deposited in a continental lake basin.
Fig. 1. Distribution of gas fields in the Qiongdongnan Basin
The Oligocene Yacheng Fm was deposited in semi-closed estuarine and shallow sea, and the Lingshui Fm in littoral and shallow sea. Afterward the Neogene and Quaternary Systems deposited are epicontinental, shelf and aktian deposits. Three sets of source rocks occur in the basin, i.e. Eocene lacustrine mudstone, Lower Oligocene Yacheng transitional coal measures and Upper Oligocene Lingshui shallow-seaebathypelagic mudstone (Fig. 2). The major source bed is the Yacheng Fm, which is divided into three members from the bottom up, i.e. the 3rd member composed of braided river deltaic deposits, the 2nd member of lagoon deposits and the 1st member of tide flat deposits. All the three members contain coal measures and marine mudstone.
Fig. 2. Gas reservoir section across the Yacheng13-1 gas field in the Qiongdongnan Basin
Coal measures mainly occur in braided rive deltas and tidal flats in those gentle half-graben slope zones, followed by fan deltas and tidal flats in steep slope zones, and dark mudstone mainly occurs in lagoons in deep trenches. Coal seams drilled in more than 10 wells mostly occur in deltaic plains and tidal flats and are characterized by multiple beds, small thickness and inconsistent lateral extension. At most 35 coal beds were interpreted in an individual well, 12 beds in the 3rd member, 7 in the 2nd member and 16 in the 1st member. On average 10 coal beds were interpreted in the 3rd, 4 in the 2nd and 9 in the 1st. The TOC of coal beds and dark mudstone are 32.37-81.30% and 0.24%-5.25% respectively and the kerogens are mainly of hybrid-humic types. For example, the Yacheng Fm in Well YC 13-1-2 has TOC of 1%e4% and some carbargilite and coals have TOC of 20%-40%. The TOC of Yacheng dark mudstone in Well YC 21-1-4 is about 1%. With a thermal gradient of 25.2%-60.8 0C/km, and 36.6-39.1 0C/km on average, the Qiongdongnan Basin is a hydrothermal basin. Source rocks in the basin have entered highly mature and post mature stages due to a deep buried depth and maintained a heat flow value of over 70 mW/m2 resulted from basin extension after Yacheng Fm deposition. For example, the main source rocks in the Ledong-Lingshui Sag have a Ro value of over 2.0%. Natural gas would dominate over hydrocarbon generation under these source and heat conditions. There are three reservoir-cap assemblages, i.e. basement assemblage, Paleogene assemblage and Neogene assemblage. So far oil and gas discoveries have been made in them except in the basement assemblage. The exploration of the basin began at the end of the 1970s, and some gas fields and gas-bearing structures (e.g. Yacheng13-1, Yacheng21-1 and Lingshui22-1) have been discovered. It is inferred that the Qiongdongnan Basin is a gastyped basin mainly producing natural 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: Armorican