Characteristics Of Shale Diagenesis And Pore Evolution Of Chang 7₃ Sub-Member Of Yanchang Formation In The West Center Of Yishan Slope In Ordos Basin

Driven by the shale oil and gas revolution,the research of shale rocks diagenesis has achieved important progress,and has become the cutting-edge field of sedimentation and petroleum geology.The role of shale rocks diagenesis not only controls oil and gas to hide and move,but also has an important impact on shale mineral composition,storage micro-characteristics,and mechanical characteristics.This paper is based on the research results of domestic and foreign scholars.For the shale of the Chang 7₃ sub-member of the Yanchang Formation in the middle of the Yishan slope in the Ordos Basin,various modern analyzes and testing technology were performed using X-ray diffraction（XRD）,,scanning electron microscope（SEM）and TOC content measurement The mineral assemblage types and organic geochemical characteristics of the Chang 7₃ sub-member shale are systematically discussed.According to the lamina thickness,mineral composition,sedimentary sub-environment and organic matter content of the Chang 7₃ sub-member shale of the Yanchang Formation.Dividing the shale lithofacies combination of the Chang 7₃ sub-member.Discuss the type and evolution process of shale diagenesis.Identify shale pore types and characteristics.Analysis of factors affecting shale pore development and evolution law.The results show that:The shale mineral composition of the Chang 7₃ sub-member is mainly clay,and the content of carbonatite is low.The organic matter content ranges from0.46 to 27.3%,with an average content of 7.5%.The types of organic matter are mainly type I and type IIA.The average reflectance of vitrinite is 0.84%,and it is in the low-mature to mature stage.It can be divided into five types of lithofacies combination:high TOC laminar shale（HLS）,medium and low TOC laminar shale（MLS）,medium and low TOC felsic shale（SLS）,medium and low TOC tuffaceous shale（LV）,massive mudstone（LM）.The shale diagenesis of the Chang 7₃ sub-member has undergone various diagenetic processes such as compaction,cementation,dissolution,mineral transformation,and organic matter pyrolysis,and is in the middle A stage of diagenesis.Three types of pyrite are formed during diagenesis:（1）pyrite framboid and euhedral pyrite formed before compaction;（2）A small amount of euhedral pyrite filled inside the mineral dissolution pores during the thermal evolution of organic matter;（3）Few ring-shaped pyrites developed on the edge of algae bio-fossils under the action of bacteria and organic matter decomposition.The sources of authigenic quartz siliceous mainly include:（1）Transformation and precipitation silicon of clay minerals;（2）Felsic dissolution to produce silicon.Different types of pores developed in different lithofacies of Chang 7₃ submember shale.High TOC laminated shale（HLS）and medium-low TOC laminated shale（MLS）mainly have inter-crystalline pores and organic pores;Medium-low TOC felsic shale（SLS）mainly develops intergranular pores and dissolution pores;Medium-low TOC tuffaceous shale（LV）mainly develops tuffaceous devitrification pores;Massive mudstone（LM）mainly develops micro-fractures.The pore diameter of Chang 73 sub-member shale mainly developed in the range of 19.41 to 24.64nm.The specific surface area is mainly developed in the range of0.71 to 1.88m²/g.The pore volume is mainly developed in the range of 0.003 to 0.0108cm³/g.The main factors affecting the development and evolution of Chang 7₃ sub-member shale pores are lithofacies combination type and diagenesis.Select TOC content,hydrocarbon generation potential and brittle minerals（quartz,feldspar,etc.）to evaluate the sweet spot section of the shale in Well G135 of the Chang 7₃sub-member of the Yanchang Formation.The Chang 7₃ sub-member shale can be divided into four sweet spots:（1）Well section from 1805 to 1811m,the lithofacies combination in this well section is dominated by massive mudstone（LM）.The average content of S₁ is6.7mg/g,the average content of S₁+S₂ is 34.6mg/g,the average content of TOC is 6.1%,and the average content of brittle minerals is 19.8%;（2）Well section from 1814 to 1821m,the lithofacies combination in this well section is dominated by medium-low TOC laminar shale（SLS）.The average content of S₁ is 3.8mg/g,the average content of S₁+S₂ is 30.6mg/g,the average content of TOC is 8.5%,and the average content of brittle minerals is 21.7%;（3）Well section from 1828 to 1832m,the lithofacies combination of this well section is dominated by high TOC laminar shale（HLS）.The average content of S₁ is 4.3mg/g,the average content of S₁+S₂ is 34.1mg/g,the average content of TOC is 9.4%,and the average content of brittle minerals is 20.8%;（4）Well section from 1837 to 1843m,the lithofacies combination in this well section is dominated by high TOC laminar shale（HLS）.The average content of S₁ is 6.7mg/g,the average content of S₁+S₂ is 54.7mg/g,the average content of TOC is 11.2%,and the average content of brittle minerals is 20.3%.