Study On The Densification Mechanism Of Low Porosity And Permeability Reservoirs In The Second Member Of Xujiahe Formation In Longgang-Yingshan Structure

In this paper,choosing the second member of the Xujiahe formation of Longgang-Yingshan structure for the object of study,by casting thin sections,scan electron microscope,cathodoluminescence,petrophysical testing,mercury injection method and so on,reservoir characteristics differentiation,the factors that influence reservoir characteristics differentia-tion and densification process differentiation were studyed.The reservoir physical property in Longgang area was worse than that in Yingshan area,the porosity in Longgang area ranges mainly from 1%to 6%,the porosity in Yingshan area ranks mainly from 4%to 9%.The sedimentation of the difference is the material basis of reservoir densification differentiation.The compositional maturity and texture maturity,sedimentary environment in Longgang were worse than those in Yingshan.Longgang area was mainly braided delta plain and front distributary channel,high content of soft rock debris,low content of feldspar.The sandstone of Longgang area was mainly medium-coarse detritus sandstone,the others were feldspathic litharenite and clastic arkose sandstone.Yingshan area was mainly beach bar and distributary channel sand body with superimposed sand body of beach dam,high water medium energy.Yingshan area were mainly medium detritus sandstone and feldspathic litharenite,the other was clastic arkose sandstone.Diagenesis intensity of the difference is the key to the difference of densification.The study section is B subage of mesogenetic stage.Diagenesis roughly experienced four processes,namely mechanical compaction,cementation,dissolution and recementation.The reservoir damage degree of early compaction and siliceous cementation in Longgang area is stronger than those in Yingshan,however,the reservoir improvement degree of dissolution is weaker,resulting ultimately that the reservoir physical property in Longgang area is worse.Through recovering burial history,thermal evolution history,porosity evolution history and diagenetic evolution,the initial porosity in Longgang area and Yingshan area were respectively 38.95%and 38.83%in early sedimentary stage.Due to mechanical compaction,the porosity decreased to 10.31%and 13.42%after experiencing a rapid subsidence stage of middle Jurassic,Longgang area had been largely into densification state at this time.After early cementation porosity reduced to 8.00%and 10.62%,Yingshan area had been also into densification state.Dissolution improved the reservoir quality,especially for the Yingshan area,porosity increased to 9.00%and 12.38%respectively.Recementation made further dense sanstone,porosity decreased to 3.90%and 6.98%respectively,Yingshan area had been into densification state again.Through the above study,reservoir densification reasons were summarized,strong mechanical compaction is the leading cause of reservoir densification,the siliceous cementation is a key factor of reservoir densification.