Study On The Densification Mechanism Of The Second Member Of The Xujia Formation In Hechuan Area

The secend member of Xujiahe Formation in Hechuan Area is representative tight sandstone reservoir.Based on the theories of Sedimentology,Petroleum Geology,Oil geology,and Oil&Gas Geochemical,through the analysis of experimental technology such as core observation,microscopic rock section observation,conventional mercury injection,scanning electron microscopy,electron probe,X-ray diffraction,fluid inclusions and so on,to in-depth studied the petrology feature and diagenesis and diagenetic evolution of reservoir,preliminary studied the densification mechanism of reservoir of The secend member of Xujiahe Formation in Hechuan Area.Research suggests that the reservoir of the secend member of Xujiahe Formation in Hechuan Area developed coastal and shallow lake subfacies,sand were washed clean so the matrix contents low(<5%normally),has better reservoir particle separation,high psephicity,as well as high textural and composition maturity.The type of debris is in many styles,including metamorphic rock debris,deposit rock debris and magmatic rock debris,the metamorphic rock debris are primary,because research area is controlied by the various source system.Fine to medium feldspar lithic sandstone and fine to medium lithic feldspar sandstone are the main stone type.Reservoir porosity is concentrated in 0%?14%,average 5.98%,mainly concentrated in 4%-6%.permeability is mainly concentrated in 0.01mD?0.1mD.It is typical low porosity,low permeability tight reservoir.The pore type is mainly composed of crude residual intergranular pore,then the feldspar and debris dissolved pore in local.The reservoir sandstone has experienced complex diagenesis.It is in the middle diagenetic B sub period now.According to the thin section observation and determination of trace besides porosity evolution elements that the porosity is reduced to about 10%at the end of late Jurassic,and the reservoir has been basically densification.To a certain extent,the latter dissolution improved the porosity of reservoir,but it worse again because of the second silica and calcite cementation.The clay mineral such as autogenous illite more seriously blocked the pore roar,and the reservoir permeability was greatly reduced.In the end,it becomes today's tight reservoir.The composition,particle size,content of the clastic particles and space distribution in the study area are controlled by sedimentation,so it is the basic factor affecting the physical properties of the reservoir.On the other hand,diagenesis is the final cause of reservoir density.Compaction,silica cementation and a small amount of calcium cementation cut the porosity a lot,while illite and other clay minerals seriously blocked the pore roar,and ultimately lead to the tight reservoir.