Research On The Geological Characteristics Of The Marine Shale Of The Lower Silurian Longmaxi Formation In The Northeastern Part Of Chongqing

The northeastern Chongqing province,performancing strong tectonic activities,and the geological feature of the shale gas lack of relatively systematic research.The research object of this paper is marine shale in Lower Silurian Longmaxi Formation,northeastern Chongqing province,China,and Combined with regional geological data and the results of previous studies,through field reconnaissance,sample collection and laboratory analysis et al.methods,systematicly researching the shale formation,sedimentary fecies,organic geochemistry and the characteristics of reservoir of the shale in Longmaxi Formation.According to the outcrops,cores,well logging data,sea level fluctuation and characteristics of sedimentary cycle,identified the top and bottom boundaries of Longmaxi Formation,Longmaxi Formation devided into Lower sub-member and upper sub-member in northeastern Chongqing province,and the rich organic shale mainly concentrated in Lower sub-member.Longmaxi Formation influenced by later tectonic movement obviously in the study area,in the north of Chengkou-Wuxi eroded away,to the south,the residual thickness of the formation gradually thickening,and the residual thickness in Wanxian-Yunyang-Wuxi is max.Through the analysis of rock color,mineral composition,rock structure characteristics and biological content of the single well coring in northeastern Chongqing province,2 subfacies(Inner shelf and Outer Shelf)with 4 microfacies(Muddy Silty Shelf,Muddy Shelf,Silty Muddy Shelf and Contour Current)are clarified in the study area,and the Muddy Shelf is the most advantageous sedimentary face for the enrichment of shale gas.The analysis of geochemical characteristics showes that the organic matter types of black shale are mainly ?-? 1 in Longmaxi Formation,northeastern Chongqing province,TOC values are more than 2%,Wuxi-Shashi-Mingtong has the highest organic carbon,the values are more than 3.5%;Ro values are more than 2%,and the maturity of organic matter is good for the production of shale gas in Tianba-Xinhua,Longxi-Futian,Ro values are more in 2%?3%.The black mudstone thickness distribute stablely(40m?90m),good continuity in horizontal,zonal distribution in NE,especially the black shale are the thickest in Wuxi-Yunyang(60m?80m),with good shale gas reservoir hydrocarbon generating conditions.Reservoir characteristics of the study results show that quartz content is more than 60%in Longmaxi Formation,brittleness,for gas well fracturing.Reservoir space types are dominated by micro pores and cracks,poor physical property,effective porosity generally is less than 2%,permeability is less than 0.1×10-3 ?m2;The variation range of gas content is large,the average value of the total gas content in Wuxi 2 is 2.1m3/t,while the average value of the total gas content in Wuqian 2 is only 0.05m3/t.Combined with the effective thickness of shale,abundance of organic matter,organic matter maturity,mineral composition and shale depth,classify and evaluate the shale in Longmaxi formation in the study area,Tianba-Shashi-Xin hua-Wenfengzhen block(Class I)and Longxi-Futianzheng block(Class II)are divided into favorable blocks.