A Study On The Microscopic Heterogeneity Of Mud Shale In The Longmaxi Formation In Southern Sichuan

Shale gas is an important unconventional oil and gas resource.With the acceleration of its exploration and development process,shale reservoirs show more obvious complexity and heterogeneity.In the current shale gas development,the problems such as low productivity of single well and great difference of production capacity are revealed.Because that in shale horizontal bedding are developed,clay is rich and clay particles are orientable,the lithofacies are single,organic and organic nano-pores are rich,drilling and development costs are high,the difference among wells are large,it is difficult to systematically characterize the heterogeneity of shale.Therefore,the study of micro-heterogeneity of shale reservoirs helps to clarify the distribution of high-quality reservoirs,explain the anisotropy of mechanical properties and the productivity difference of single well,and has a theoretical and practical value in the study of percolation mechanism.In this paper,the Longmaxi shale from wells such as Y101,Z101 and N10 in southern Sichuan basin is the main object of study,previous research results in the region are also taken into considerations.From the micro to the macro perspective,the heterogeneity and its influence factors of shale are discussed.The main conclusions and knowledge obtained are as follows:(1)The heterogeneity of shale macro scale was characterized by lithology,organic geochemistry and physical properties.The thickness and organic content in the plane are homogeneous of the Longmaxi shale in Southern Sichuan Basin.In the study area,the high quality shale in the 1st sub section of the first Member of Lower Silurian Longmaxi can be divided into four layers,from deep to shallow organic matter content gradually decreasing,in accordance with the classification of conventional reservoir heterogeneity,Jk,Tk,Kv show that the heterogeneity of layer and interlayer is moderate to weak.Based on mineral composition content,shale can be divided into four lithofacies:siliceous,calcareous,clayey and silty.TOC and sedimentary structure of different lithofacies are different.(2)By means of thin slice and X ray diffraction analysis,the microcosmic form of mineral and the microcosmic occurrence of organic matter in the shale of different lithofacies were compared.Mineral content and organic matter content are different in different layers.Dark layer is rich in organic matter and clay.Light colored layer is rich in brittle minerals such as quartz and calcite.Microcrystalline and amorphous structure of quartz are more common in siliceous shale;quartz in silty shale is small diameter,moderate sorting,sub circular to sub prism;there are less quartz particles in clay shale,mostly clay grade.The arrangement of clay minerals is mainly uniform distribution,without directivity,or continuous arrangement,strong orientated.The microscopic morphology of organic matter is dominated by honeycomb and floc amorphous kerogen,followed by banded organic and organic particles.(3)By means of scanning electron microscopy,nitrogen adsorption and high-pressure mercury injection,the heterogeneity of shale pore structure has been characterized by pore type,pore size distribution and fractal dimension.There are three kinds of pore types:the intragranular pore,the intergranular pore and the organic pore,and the main peak of the pore size distribution is 2?20nm and 100?300nm.Of the intragranular pore the morphology is complex,pore size distribution is large,the micro-heterogeneity is the strongest,followed by the intergranular pore and the organic pore.Due to its low organic matter content,aperture in the pores and pore,high content of clay minerals is easily compacted to reduce pores in the diagenetic process,so clayey shale has strong heterogeneity of pore structure,followed by calcareous shale,siliceous shale and silty shale.(4)The influence of shale micro-heterogeneity on rock physical parameters such as longitudinal and transverse wave anisotropy,compressive strength,Young's modulus,Poisson's ratio and brittleness index is systematically studied.With the increase of angle with the bedding,the longitudinal wave velocity and Young's modulus of shale are gradually reducing,and the compressive strength and brittleness index change as U type.In the case of different confining pressure and angle,the shale samples show 6 kinds of fracture modes,which have strong anisotropy.The anisotropy of shale is a comprehensive reflection of the micro-heterogeneity of mineral composition,organic matter and pore structure.