The Microstructure Characters And Main Controlling Factors Of Continental Shale In Middle Western Sichuan Depression

The continental shale strata of the middle part in the Western Sichuan Depression are major targets to explore in China.However,because of its wide distribution,frequent facies changes,various rock types,great differences in evolution and extremely strong reservoir heterogeneity,it is hard to explore.Previous research usually lack analysis on the continental shale reservoirs,the microstructure of which is exactly the key to influence the reserving and gas-bearing properties.In this paper,the continental shale of T₃X⁵ in Middle WSD is selected as the object of study.Through the observation of outcrops and cores,together with the descriptions of normal and casting thin sections and logging data,this paper studies the rock types,mineral compositions,pore geometry and physical properties of reservoirs in detail.At the same time,scanning electron microscopy?SEM?,X-ray diffraction analysis,mercury injection capillary pressure?MICP?,and N₂ adsorption experiments were taken to get more useful data.Besides,the controlling factors of microstructures in different reservoirs were analyzed from four aspects,including sedimentary environment,diagenesis,tectonic process and abundance of organic matters.The results show that,the shale rich in organic matters in the study area is developed in the elevated semi-cycles of the middle sub-member of T₃X⁵.At that time,the tectonic background was relatively quiet and the climate was damp.Interdistributary estuary,shore or shallow lake muds are the main sedimentary micro-facies,which,commonly,are inside the mud-rich reservoirs characterized by sand wrapped by mud and large single layer thickness.The main pore types of reservoirs in our study area are organic pores and intergranular pores of clay minerals,among which miso-pores?2-50nm?were most developed.The average surface area is 2.1426 m²/g,with the average pore volume 0.0098 cm³/g,and average pore radius 20.99nm.The average porosity and permeability of the reservoir are 3.21%and 0.033mD,respectively,according to which the reservoirs are classified as ultra-low porosity and permeability reservoirs.Even though the pore throats are thin and the pore connectivity is relatively worse,the physical properties of shale reservoirs are still better than those of the sandstone reservoirs.Sedimentation,tectonic movements,diagenesis and abundance of organic matter all shows obvious controlling effects on the rock types,mineral composition,pore structures and physical properties of reservoirs in the study area.Sedimentation and sediment source determine the basic mineral compositions and control the reservoir rock types.During the diagenesis process,the mechanical compaction and the pressure solution altered the reservoir pore structures by reducing pore spaces.The dissolution and metasomatism improved physical properties of the reservoir by increasing micro cracks and intergranular pores of clay minerals.Tectonic process and organic matter abundance influenced the pore structures and physical properties of reservoirs by affecting the development of micro-fractures and organic pores,respectively.Among them,the fractures at the end of the fold and near faults are likely to develop,and the organic pores are relatively developed in the shallow muddy micro-facies of the middle sub-member.