Subdivision Of Organic Matter Types And Pore Development Characteristics Of Longmaxi Formation Shale In Chongqing Area

Gas in Lower Silurian Longmaxi shales of the Sichuan Basin or around it has been commercially exploited.The gas contents of the Longmaxi shales vary across different blocks or stratus in a specific area.Shale gas is mainly stored in organic matters in shales.Therefore,organic matter type and organic matter pores may be the dominate controlling factors of gas enrichment.However,the organic matters in Longmaxi shales were recognized as type ? or type ?,which is too coarse to describe finite differences among shales in various blocks.In addition,the reflectance of solid bitumen was extensively used to evaluate maturity of Longmaxi shales.It has been found that the roughness of solid bitumen surface has an influence on its reflectance value.Therefore,the pores in solid bitumen may also have an effect on reflectance.In order to define organic matter composition and pore characteristics of Longmaxi shales,furthermore,to understand surface imperfections have effects on solid bitumen reflectance measurements.Various techniques including organic matter maceral,chemical structure are used in this study to subdivide type of organic matter,study pore characteristics of different organic matters by using optical microscope and scanning electron microscope.Influence of surface imperfections on bitumen reflectance is also discussed.The following are the main outcomes of this thesis:The types of organic matter in Longmaxi shales are subdivided by three methods:maceral percentage determination,infrared spectroscopy determination and solid nuclear magnetic resonance.It is found that method of maceral percentage determination is better than other methods.The types of organic matter are subdivided into graptolite-amorphous type,bitumen-amorphous type and amorphous type.A division scheme is made based on incident light and reflected light,which guides for organic matter type subdivision of Lower Silurian Longmaxi shales in the Sichuan Basin or around it.Pore characterstics of graptolite,bitumen and amorphous are observed by SEM,JmicroVision software is used to calculate the surface porosities and pore size distribution of different organic matter.Organic matter of graptolite rarely develops organic matter pores and isolated pore occasionally.There are a lot of nano-scale pores in bitumen organic matter,amorphous rarely develops pores.The surface porosities of graptolite ranges from 0.102%to 5.671%,with an average of 2.45%.The surface porosities of bitumen ranges from 35%to 79.1%,with an average of 68.2%.The surface porosities of amorphous ranges from 2.12%to 9.84%,with an average of 5.61%.The pore size distribution range of bitumen is the widest,ranging from 4.5nm to 210nm,and mainly concentrated between 20nm and 76nm.Graptolite is next,distributed between 10nm and 56nm,and concentrated between 10nm and 24nm.The pore size distribution range of amorphous is the smallest,distributed from 10nm to 37nm,and concentrated between 20nm and 25nm.The degree of pore development on the surface of bitumen has a certain influence on bitumen reflectance measures.Therefore,the reflectance of different bituminous organic matter in the same sample of selected shale samples is determined.The results show that the measured value is low in rough bitumen while it is high In the bitumen organic matter with pyrite.When measuring bitumen reflectance,the maceral of bitumen with smooth surface and without pyrite interference should be selected to ensure more accurate results.