Study On The Relationship Between Graptolite Characteristics And Shale Reservoir Porosity And Permeability Of Wufeng-Longmaxi Formation In Wansheng Area

Graptolite shale facies is developed in Wufeng-Longmaxi Formation in southern Chongqing,and the formation has good shale gas accumulation conditions.In this paper,the black shale of Wufeng-Longmaxi Formation in Wansheng area is taken as the research object.Through literature review,field survey,organic geochemical experiments and porosity and permeability testing experiments,combined with the distribution characteristics,microstructure characteristics and evolution characteristics of graptolites in the target layer,the effect of graptolite on shale reservoir porosity and permeability and its contribution to shale gas accumulation have been comprehensively analyzed.Results are as follows:The bottom of Wufeng-Longmaxi Formation is deep shelf sediments,anoxic reduction environment and calm water,mainly black shale;The upper part is shallow shelf deposition with enhanced hydrodynamic force,mainly developing silty shale and argillaceous siltstone.There are 38 species belonging to 16 genera and 6 graptolite zones.Graptolite evolution trend showed simplified graptolite stipe,complicated graptolite theca and change of growth pattern.The shale samples are mainly clay,quartz,plagioclase,calcite and other minerals,and the percentage of brittle minerals is higher in Wufeng-Longmaxi Formation shale with high organic content.The organic matter of the shale is mainly typeⅠ,with a small amount of typeⅡ₁,and the organic matter abundance is between 0.57%and 5.9%.The organic matter maturity of the lower part of Longmaxi Formation is between 1.96%and 2.71%,which belongs to over-mature stage and is dominated by dry gas.Pores and fractures are well developed in the shale in the study area.High resolution scanning electron microscopy（SEM）was used to observe that organic pores,intragranular pores,intergranular pores and microfractures are mainly developed in the shale.Micropores and microfractures are mainly in micron and nanometer scale.Mercury injection experiment and gas adsorption experiment were used to analyze and test the contribution of macro-pores,mesoporous pores and micro-pores to the total pore volume,and the macro-pores are50nm～100nm.The porosity and permeability of shale samples range from 2.90%to 5.03%and from 0.023m D to 0.0049m D,which belong to ultra-low porosity and low permeability.Graptolites in shale are mainly preserved as carbonaceous thin films,a small part of graptolites are semi-stereoscopic pyritization and pyritization stereoscopic preservation.Organic pores,intergranular pores,quartz impression holes and graptolite edge cracks are developed in graptolite.Comprehensive analysis shows that there is a good correlation between graptolite abundance and TOC,shed light on that graptolites contribute to the enrichment of organic matter in shale.Graptolites mainly contribute macropores and mesopores to shale reservoir,and the pore connectivity in graptolite is good,providing storage space and seepage channel for free and adsorbed gas in shale reservoir.In the direction of vertical bedding,graptolite abundance is positively correlated with porosity and permeability.In the same bedding plane,graptolite abundance is positively correlated with porosity,but poorly correlated with permeability,which is speculated to be caused by sampling vertical to bedding plane.In conclusion,graptolite is an important factor for shale gas accumulation.