Marine Shale Gas Accumulation Characteristics Of Upper Paleozoic In Guizhong Depression And Its Periphery

Based on the analysis of organic carbon content,X-ray diffraction analysis of mineral composition,scanning electron microscope,nitrogen adsorption method,nuclear magnetic resonance technology,CT technology and isothermal adsorption exqperiment,this paper,taking key outcrop section,Huanye 1 well and Luye 1 well as examples,studied the accumulation characteristics of Upper Paleozoic shale in Guizhong depression and its periphery,discussing the characteristics of organic geochemistry,mineral compositions,reservoir physical property,pore structure and fractal characteristics,reservoir gas content and preservation conditions.Accordingly,the geological conditions,enrichment law and control factors of shale gas formation are revealed systematically,and finally the favorable areas for shale gas exploration and development are determined within various series of strata of Upper Paleozoic.The main achievements of this study are as follows:(1)There are three sets of Upper Paleozoic organic-rich marine shales in Guizhong depression and its periphery:Lower Devonian Tangding Formation,Middle Devonian Luofu Formation and Lower Carboniferous Yanguan Formation.The shale in research area has a great accumulated thickness,with high content of total organic carbon(TOC)and the organic matter type being type ? and type ?.The thermal evolution was in high mature-over mature stage,indicating the shale have a relatively good hydrocarbon-generating potential.(2)The Upper Paleozoic shale in the study area is mainly featured by high quartz content,low clay and carbonate minerals content,and they have low porosity and permeability,as well as a good breakthrough pressure,indicating that the shale has good reservoir capacity.The shale reservoir space can be divided into two major types,including pores(interlayer space of clay minerals,organic pores and et al.)and micro-fractures.The connectivity between nanoscale pores and micron-size pores is poor.However,the cylindrical and plate-like nanoscale pore system has good connectivity,in favor of the shale gas seepage and migration.The higher fractal dimension of the shale in Guizhong depression,the more complicated pore structure,which made a great contribute to the shale gas adsorption and accumulation capacity.As the content of micron-size pores in deep layer of Yanguan Formation shale is less and the connectivity of micron-size pores is poor,the gas seepage capacity is poor.Isothermal adsorption experiment results reveal that the maximum methane adsorption concentration of Lower Devonian shale,Middle Devonian shale and Lower Carboniferous shale was 3.70 m3/t,3.19 m3/t and 2.85 m3/t,respectively.The results indicate that Upper Paleozoic shale has high gas concentration under good reservoir and preservation condition.TOC of shale is positive related to its sorption capacity.Due to the low TOC,field analytical shale gas of Middle Devonian Dongganglin Formation from Luye 1 well was low.In addition,because of destructive preservation conditions in the region,field analytical shale gas of Lower Carboniferous Yanguan Formation from Huanye 1 well was low as well.(3)Middle-Lower Devonian shale in research area are ?-? type cap rock,which is the main regional cap rock,with medium-good sealing capability.They are well developed in northwest and eastern areas.Lower Carboniferous shale are important local cap rock,which is?-? type cap rock with good sealing capability.Faults are well developed in the Upper Paleozoic strata in the east and northeast of Guizhong depression,where meteoric water infiltrated intensely and medium-hot spring develop.These areas were strongly impacted by meteoric water infiltration and having poor hydrocarbon preservation conditions.The shale in the central and western regions of the depression,including central and northern of Mashan uplifted,as well as the central part of Hongdu shallow sag,have good hydrocarbon preservation conditions due to weak meteoric water infiltration,stable structure,weak denudation and stagnant zone in Middle Devonian strata.(4)In conclusion,this paper selects areas with favorable shale gas accumulation condition in various series of strata of Upper Paleozoic in Guizhong depression and its periphery.For the shale of Lower Devonian Tangding Formation,the favorable areas mainly distribute in Nandan-Hechi area of northwest Guizhong depression;the favorable areas of Middle Devonian Formation shale mainly distribute in Laibin-Liuzhou area of east Guizhong depression and Binyang area of south Guizhong depression;the potential shale gas exploration area of Lower Carboniferous Yanguan Formation,especially the deeper part,mainly distribute in Nandan-Hechi area of northwest Guizhong depression.