The Shale Gas Accumulation Conditions Of Taiyuan Formation In The Southern North China Basin

The Taiyuan Formation in the Southern North China Basin is a potential shale gas layer.The shale of this formation has the characteristics of large cumulative thickness and high organic matter abundance.According to organic geochemical test,geochemical element test,mineral composition analysis,porosity,and permeability test,argon ion polishing and scanning electron microscope,high-pressure mercury injection,low-pressure gas adsorption(N2,CO2),nuclear magnetic resonance,methane isotherm adsorption,field desorption,well-logging interpretation and drilling core data,the sedimentary environment,spatial distribution,organic geochemical characteristics,mineral composition,and lithofacies classification,reservoir characteristics and gas bearing of the Taiyuan Formation shale in the study area were systematically analyzed,and the shale gas accumulation conditions of the Taiyuan Formation in the Southern China North Basin was discussed.Studies have shown that the Taiyuan Formation the Southern North China Basin was in the epicontinental sea environment with multiple transgressions.During this period,the climate was humid and dry,and the overall temperature was warm and humid.Sequence stratigraphy plays a guiding role in the prediction of high-quality shale reservoir section in the environment of land sea interaction.The lagoon facies mudstone was thick with local coal seams or coal lines,and the organic matter content was high.It was the main facies belt for the development of thick shale.It was mainly developed in S1 and S2 highstand system tracts in the East and middle of the basin.The organic matter type of shale in Taiyuan Formation is mainly?.The main distribution range of TOC content and R_o was 1.0%to 3.0%(average value was2.21%)and 1.20%~4.13%.The organic matter content was moderate to high.The mineral composition of shale was mainly clay and quartz,and the brittleness index is44%on average.Clay minerals were primarily composed of illite-montmorillonite mixed-layer and illite.Shale facies were classified mainly as siliceous clayey shale facies(CM-1),clayey(siliceous)mixed shale facies(M-2),and clayey siliceous shale facies(S-3).The pore types of shale in Taiyuan Formation were mainly organic matter pore,intergranular pore,clay ore intergranular pore,and pyrite nodule intergranular pore.The development degree of organic pores was generally low.A large number of organic matter hydrocarbon generating contraction fractures were developed on the edge of shale with high thermal evolution.The pore size of the shale in Taiyuan formation was generally multi-peak distribution,and the pores of various scales were developed.Mesopores and macropores were dominant in the total pore volume,which provided 89.98%of the total pore volume.The specific surface area was mainly provided by micropores and mesopores,which provided 99.41%of the total specific surface area.Micropores and mesopores had poor connectivity,high overall bound water saturation,and low movable water saturation.The main accumulation conditions of shale gas were analyzed from five aspects:favorable facies belt,shale distribution characteristics,thermal evolution degree,diagenesis on gas bearing property and later preservation conditions.The shale gas of Taiyuan Formation in Southern North China Basin should be accumulated in lagoon facies with a burial depth and thickness when Ro was at the range of 2.5%and 3.5%.