Authigenic clay minerals are not only one of the reasons for the change of sandstone reservoir properties,but also one of the factors that cause reservoir sensitivity in the process of oilfield development.It is the key to evaluate reservoir correctly,formulate effective exploration and development measures,and improve the productivity of oil and gas well.This paper takes the Chang-10 oil formation of the Late Triassic Yanchang Formation in Ansai area of northern Shaanxi Province as the main research object.Based on previous studies,through field geological surveys,core observation and description,combined with the test data of thin section identification of cast,clay minerals X-ray diffraction analysis,physical property analysis,mercury injection analysis,etc.The characteristics of sandstone and the occurrence state,formation time and formation mechanism of authigenic clay minerals in Chang 10 oil formation were systematically studied,and the influence of authigenic clay minerals on deep sandstone reservoir quality was discussed.The following understandings were obtained:(1)The Chang 10 reservoir in the study area is mainly composed of fine-grained feldspar sandstone deposited by subfacies branch channels in the delta plain.The majority of sandstone debris is metamorphic rock debris,followed by volcanic rock debris.The interstitial materials are complex and diverse,mainly composed of turbidite,calcite and chlorite,and their physical properties are low porosity-extra low porosity,low permeability-extra low permeability.The pore types are mainly dissolution pores and intergranular pores,and the pore throat structure has the characteristics of micro-pore and micro-roar.(2)Chlorite,illite mixed layers and illite are the main authigenic clay minerals in the Chang 10 sandstone of the study area,and the authigenic chlorite is the main clay minerals,accounting for 88.3%.The occurrence state shows three types: film,ctenocybe(or bridge)and filling.The formation mechanism of authigenic illite mainly includes dissolution-recrystallization and feldspar alteration.(3)The formation of authigenic chlorite is related to the alteration of volcanic cuttings and the hydrolysis of biotite in the sandstone of Chang 10 formation in the study area.Authigenic chlorite is formed prior to intense compaction and dissolution and continues to develop with diagenesis until after the dissolution of feldspar or debris and the precipitation of authigenic quartz.Aemon/mixed layer is the substitution and substitution of different ions of soluble minerals(feldspar)in sandstone in an acidic environment,and eventually illite is produced in the periphery of montmorillonite,forming Aemon/mixed layer.The formation of Aemon/mixed layer is related to Clay mineral transformation and alteration of feldspar.(4)The relationship between authigenic clay minerals and physical properties in the Chang 10 sandstone in the study area shows that authigenic chlorite has a dual effect on the reservoir.The granular film and pore lining chlorite can inhibit compaction and compaction and cementation,protect the primary pores,facilitate the development of secondary pores,and play a constructive role in the reservoir.The content of chlorite in pore filling is less,which leads to the complexity of pore space,the reduction of physical property and the destruction of reservoir.Autogenous aemon/mixed layer and autogenous illite segment the primary intergranular pores in sandstone,reduce pore volume,and the throat becomes curved and roundabout,blocking the throat,resulting in the decrease of reservoir permeability,the deterioration of reservoir quality,and the impact of development effect. |