Fine Characterization Of Shale Reservoir Of Longtan Formation In Western Guizhou

The marine-continental transitional shale with large accumulated thickness,high organic mater content and thermal evolution degree,and certain exploration potential,is widely distributed in coal-bearing strata in western Guizhou Province of China.Guided by the theories of sedimentary petrology,geochemistry and petroleum geology,in this study,Longtan Shale in western Guizhou is taken as the main object,the spatial distribution,mineral composition and organic geochemistry were studied in detail.The development characteristics of pore structure and dynamic changes of shale reservoir are systematically studied and the shale reservoir is comprehensively evaluated.(1)Mesopores are the main contributors to the pore structure of the transitional facies shale in Longtan Formation.The carbon content of organic matter,maturity and mineral composition are the main factors affecting the development of pore structure.The reservoirs with low thermal evolution and high clay content mainly develop inorganic pores,while the macropores are relatively developed,and the micropores have the least contribution to the pore structure.In addition to inorganic pores,some organic pores are also developed in reservoirs with similar content of clay minerals and brittle minerals,and high abundance of organic matter and high degree of thermal evolution.The complex pore structure enhances the heterogeneity of reservoir.(2)With the increase of evolution degree,the pore structure of shale reservoir presents an intermittent change trend of first increasing,then decreasing and then increasing.The early diagenetic stage corresponds to the immature stage of organic matter evolution.The sediments are not yet consolidated,and only a small amount of biomethane is released through biological action.The reservoir pores are mainly mesopores and macropores,and compaction can inhibit the pore development,resulting in the decrease of pore volume.The middle diagenetic stage corresponds to the mature and high mature stage of organic matter evolution.Liquid hydrocarbon and gaseous hydrocarbon are continuously generated when the oil generation threshold is reached.The organic pores developed in kerogen and asphaltene lead to the significant increase of micropore volume.Kaolinite transforms into illite and illite-smectite mixed layer.Its rough surface and rich reservoir space make an important contribution to the development of mesopores.The late diagenetic stage corresponds to the over mature stage of organic matter evolution.The mineral composition remains stable,the gaseous hydrocarbon increases slowly,the collapse of organic matter due to the consumption of a large amount of organic carbon occurs in the interior of organic matter,the contraction joints of organic matter and mineral edge increase,and the macropores and micro fractures make an important contribution to the increase of pore volume.(3)The carbon content of organic matter is the material basis of hydrocarbon generation,which determines the hydrocarbon generation potential of source rocks.Through the rough surface and abundant pores of organic matter,it affects the adsorption and storage of shale gas,and is the most important index for reservoir evaluation.The shale of well XD1 in Xingren area is developed in lagoon facies,with high organic matter content and thermal evolution,high hydrocarbon generation potential and rich reservoir space.The balanced development of clay minerals and brittle minerals is conducive to gas adsorption and later fracturing development.The structure is relatively stable,which is conducive to the enrichment and preservation of shale gas.The reservoir performance is good and has certain exploration and development value.The shale of well YV1 in Panxian mining area is developed in delta facies,with high TOC content but low thermal evolution degree,undeveloped organic matter pores and relatively less reservoir space.High content of clay minerals,low porosity and permeability,tight rock structure are not conducive to late fracturing,and reservoir performance is general.