Pore Structure Characteristics And Influencing Factors Of Lower Paleozoic Marine Shale In Northwest Jiangxi

Shale pore is the main place for the occurrence and accumulation of shale gas.The study of shale pore structure is of great significance to the evaluation of shale gas resources.In this paper,the shale samples from the lower Cambrian Wangyinpu Formation-Guanyintang Formation and the upper Ordovician Xinkailing Formation-the Lower Silurian No.1 section of the Lishuwo Formation in northwestern Jiangxi are selected as research objects.The results show that the Wangyinpu FormationGuanyintang shales have high organic matter abundance and thermal evolution degree with average TOC content above 6% and Ro between 3.2% and 4.15%.The shales mainly develop pyrobitumen pores,and its pore volume and specific surface area are mainly provided by micropores and mesopores.The main factors affecting the pore structure are the organic matter abundance and the thermal evolution.High TOC content not only provides a large number of pore volume for the target shale,but also provides a large number of specific surface area.Due to the extremely high degree of thermal evolution,organic matter graphitization occurs in the Wangyinpu—Guanyintang shales,and its pore development and connectivity are affected.The Xinkailing Formation-No.1 section of the Lishuwo Formation shales have low organic matter abundance and high thermal evolution degree with average TOC content less than 2% and Ro between 2.5% and 3.2%.Organic matter type is mainly unstructural kerogen,in which the development of organic matter pores is poor.And a large part of the target shale pores,such as intergranular pores and intragranular pores,originate from brittle and clay minerals.The main factor affecting the pore structure of the target Formation is mainly the organic matter abundance.The low abundance of organic matter leads to the lower pore volume and specific surface area of the target formation than that of the Wangyinpu—Guanyintang Formation.Inorganic minerals have certain influence on the pore of two sets of target shale.Among clay minerals,the content of illite has the best correlation with specific surface area and pore volume of Wangyinpu Formation-Guanyintang shale,and the content of illite mixed layer and chlorite have the best correlation with specific surface area and pore volume of shale of upper Ordovician Xinkailing Formation-lower Silurian Lishuwo shale.Brittle minerals play a positive role in the development of two sets of shale pores.Especially the upper Ordovician Xinkailing Formation-Lower Silurian Lishuwo Formation shale,because of its low organic content and poor organic pore development,a large part of the shale pores originates from intergranular pore and intragranular pore formed by the dissolution of brittle minerals.