Shale Reservoir Characterization And Oil-bearing Assessment For Chang 7 Member Of The Yanchang Formation In The Longdong Area,Ordos Basin

Previous studies on the Chang 7 lacustrine shale of the Yanchang Formation in the Longdong area paid more attention to the geological and geochemical characteristics of the source rock,hydrocarbon generation and expulsion simulation.In-depth quantitative characterization of the reservoir’s microscopic pore structure and oil-bearing status is still lacking,with relatively simple means.There is an urgent need to deepen the quantitative characterization of pore space,clarify its oil-bearing properties,and provide support for determining the occurrence mode of continental shale oil.This study takes the Chang 7shale reservoir in the Longdong area as the research object and comprehensively analyzes the petrology,geochemistry,and pore structure characteristics of the shale reservoir by integrating X-ray diffraction analysis,pyrolysis,Sohxlet extraction,microscopic observation under scanning electron microscope,low-temperature nitrogen adsorption experiment,carbon dioxide adsorption experiment,and water vapour adsorption experiment.By comparing the gas adsorption experiment results before and after extracting,based on microscopic observation under the scanning electron microscope,the oil content of shale was evaluated,and the occurrence characteristics of shale oil in pores of different scales were determined.The results demonstrate that:（1）The mineral components of the shale in the Chang7 member are predominantly terrestrial detritus,with less endogenous mineral content,the total organic carbon varies between 1.04%and 16.36%,and the main types of organic matter are type I and type II₁,and are in the mature stage;（2）The pore and fracture types of Chang 7 shale reservoir are diverse and their structures are complex.The reservoir space is mainly divided into three categories:mineral matrix pores,organic pores（fractures）and micro-fractures.It can be successively divided into 7 subcategories based on different genetic types:intergranular pores（clastic intergranular pores,clay mineral intergranular pores,and rigid particle edge pores）,intraparticle pores,organic matter pores,dissolution fractures,clay mineral interlayer fractures,marginal mineral particle fractures,and micro-fractures of organic matter due to hydrocarbon generation.（3）According to the characteristics of the nitrogen adsorption-desorption isothermal curve,the shale reservoir samples of Chang 7 member can be divided into three types,and based on six evaluation indexes,shale oil reservoirs are classified into Class I,Class II and Class III,corresponding good,general and poor reservoirs,respectively.（4）The oil content of Chang 7 shale is distributed in the low to the high oil-bearing interval.The shale oil mobility is closely related to the pore size.In micropores,shale oil mainly exists in an adsorbed state,which is easy to block the throat and difficult to form movable shale oil;In mesopores,shale oil is mainly in an adsorbed and free state,adsorbed oil is mostly distributed on the surface of organic pores,while free oil is mainly distributed in macropores with good connectivity;10 nm is the pore size threshold that affects the flow of oil in the Chang 7 shale.The research results provide certain support for further deepening the research on the microscopic pore structure characteristics,oil-bearing property,and shale oil occurrence status of the continental shale reservoir of the Chang 7 member of the Upper Triassic Yanchang Formation in the Longdong area,and have certain theoretical and practical reference significance.