Sedimentary environment identification and hydrocarbon source significance of black shale of the Ordovician Meitan Formation at Banqiao section in Zunyi,Guizhou
HUANG Tianhai1,2,3, XIAO Di1,2,3, TANG Hao1,2,3, LI Shuangjian4, ZHENG Jianfeng5, ZHOU Li1,2,3, TAN Xiucheng1,2,3
1 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Southwest Petroleum University,Chengdu 610500, China; 2 Research Branch of Southwest Petroleum University,Key Laboratory of Carbonate Reservoirs,CNPC,Chengdu 610500, China; 3 Sichuan Natural Gas Geology Key Laboratories,Southwest Petroleum University,Chengdu 610500, China; 4 Petroleum Exploration and Production Research Institute,SINOPEC,Beijing 100083, China; 5 PetroChina Hangzhou Research Institute of Geology,Hangzhou 310023, China
Abstract：The Lower Ordovician Meitan Formation in the Upper Yangtze area of South China is characterized as the thick mud shale deposits,in which the black shale is well developed. However,there is a great controversy on its sedimentary environment. Taking the Banqiao section of Zunyi as an example,this paper focuses on the comprehensive study on the sedimentary characteristics and geochemistry of the black shale and the associated rocks,especially the relative water depth,sedimentary hydrodynamics and physical and chemical properties of palaeo-seawater during the deposition of black shale of Meitan Formation. The results show that: (1)The black shale of Meitan Formation is characterized as the dark color and well-developed horizontal bedding,without macroscopically paleontological fossils. The interlayer of bioclastic limestone and calcareous siltstone with different thickness developed,indicating that the black shale of Meitan Formation was not the sedimentary product of deep-water environment,but deposited in the low-energy shallow muddy lagoon environment in the mixed platform. (2)The bioclastic limestone that is interbedded with the black shale section mostly develops the inverse grain structure with the fine grain at the bottom and the coarse grain at the top,and iron oxides can be seen at the top,while the calcareous siltstone,interbedded with black shale,mostly develops wave cross bedding and biological drilling,showing shallow water deposition characteristics. (3)The average values of V/(V+Ni),Th/U,V/Cr and Ni/Co of black shale are 0.71,5.52,1.07 and 2.04,respectively,indicating the hypooxidation-oxidation environment. The Sr/Ba ratio ranges from 0.11 to 2.20,with an average value of 0.55,indicating the fresh water-brackish water environment;(4)Macroscopically,obvious erosion surface can be seen in some sections of bioclastic limestone,and karst development can be seen through microscopic observation. At the same time,the REE distribution pattern tends to be horizontal,and the ∑ REE value is higher and Y/Ho is lower,showing the exposed karst environment. (5)TOC values of black shale ranged from 0.09% to 1.05%,with an average value of 0.39%. TOC values of original black shale ranged from 0.28% to 3.16%,with an average value of 1.25. The average value of RO is 2.095,and the organic matter is mainly sapropelic. Based on the above analysis,it shows that the black shale of Meitan Formation in Banqiao section of Zunyi is a set of effective hydrocarbon source rocks formed in a shallow water and suboxidation-oxidation environment and high paleoproductivity is the main reason for the organic matter enrichment. The research results provides strong evidence for identifying the sedimentary environment and hydrocarbon generation potential of black shale rocks series in Meitan formation in southern Sichuan,and also provides a new example for the identification of shallow-water sedimentary environment conditions of black shale.
HUANG Tianhai,XIAO Di,TANG Hao et al. Sedimentary environment identification and hydrocarbon source significance of black shale of the Ordovician Meitan Formation at Banqiao section in Zunyi,Guizhou[J]. JOPC, 2022, 24(6): 1193-1209.
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