Study On The Relationship Between Reservoir Performance And Mineral Composition Of Longmaxi Formation Shale In Southeastern Chongqing

This paper selects the shale of Lower Silurian Longmaxi Formation in the southeastern Chongqing as its object of the study.Based on the observation and analysis of the cores of four wells and five field profiles,according to the characteristics including lithology,sedimentary structure and paleontology,and divides the shale of Longmaxi Formation into upper and lower sections,including five layers:S1111,S1112,S1113,S1121 and S1122.On the basis of the work above,the author analyzes the composition of shale minerals in different sections.The findings are as follows:the average content of quartz is about 50%at the bottom of the Longmaoxi Formation(S1111);in contrast to quartz,clay minerals are less developed at the bottom of Longmaxi Formation,an average of about 25%,but gradually increases and reaches 48%at the top.In general,the content of quartz and clay minerals changes according to the change of the depth and it’s quite remarkable that the lower the depth is,the more quartz there will be while the lower the depth is,the less the clay minerals there will be.Through the microscopic observation of the cores and the field samples from shale of the Longmaxi Formation,the main pore types include grain pores,intergranular and inter-cyrstal pore,clay mineral interlayer pores,organic matter pores,dissolution holes,cracks,etc.According to the physical experiment,the shale of Longmaxi Formation shows the characteristics of ultra-low porosity and ultra-low permeability.The porosity is between 0.1%and 3.4%,with an average of 0.68%.The average permeability is just 0.0047 × 10-3μm2 and the permeability of 80%of the samples is lower than 0.005 × 10-3μm2.Vertically,the porosity of the black shale at the bottom of Longmaxi Formation(S1111)is the highest among all the sections of the well while the permeability is small and similar,so the change is not obvious in the vertical direction.On a flat surface,the average porosity of Qian shallow well No.1 is 1.65%,which is the highest in this area,and the bottom of the well in Longmaoxi Formation(S1111)has a porosity of 1.95%,which is the highest in the whole area.Based on the study of the minerals composition and reservoir properties of the shale,this paper studies the correlation between different mineral composition and reservoir properties.And found that quartz has an obvious positive correlation with porosity,while clay minerals is negatively correlated with porosity.On the one hand,as a kind of brittle mineral,quartz is prone to dislocation or dislocated between crystals under the action of temperature and pressure during the diagenesis and form a large number of interconnected intergranular pores;on the other hand,during the growth process due to the quartz crystal lattice defects,intergrannlar and inter-crystal pores of quartz are often formed,therefore,rich quartz play an important role in porosity of the shale.While the clay minerals are different,the clay minerals in this area are mainly constituted by illite-montmorillonite mixed layer of minerals and Illite-Smectite mixed layer of minerals are often the main minerals which plug the pores.The protective minerals-chlorite far less developed than the illite-montmorillonite mixed layer of minerals and that’s why it is difficult to effectively protect the already formed pore space,so the clay minerals are negatively correlated with the porosity.According to the mineral composition and reservoir property characteristics of the shale both horizontally and vertically,and combined with the structural characteristics of the area,the author holds that the Qianjiang area where Qian shallow well No.1 located in the southeastern Chongqing is the favorable area for the development of shale gas,and the black organic sandy shale at the bottom of Longmaxi Formation in Qianjiang area is the preferred area for shale gas extraction.