Accumulation Conditions And Favorable Exploration Zones Of Shale Gas In Wufeng-longmaxi Formation, Northeast Of Chongqing

Located on the northern margin of the Yangtze,the northeast of Chongqing widely developed Wufeng-Longmaxi marine organic-rich shale,which has been drawing increasing attention.On the premise of fully collecting predecessor's research results,combining field geological work,a series analyses were completed,including slice observation,mineral XRD analysis,organic geochemistry,scanning electron microscopy(SEM)analysis,and adsorption isothermal experiment,et al.Accumulation conditions of shale gas and adsorbed gas capacity have been studied systematically.In general,the conclusions mainly include the following aspects:(1)Wufeng-Longmaxi Formation was formed in shallow sea shelf sedimentary depositional environment,whose lithology is mainly made up of black carbonaceous shale,siliceous mudstone,silty carbonaceous shale,silty mudstone and with little sedimentary tuff as well as includes abundant grapholite fossils.Moreover,in the light of the color of rock,the distribution of grapholites etc,it is further divided into deep-water shelf,mirco-facies and shallow water shelf micro-facies,and shallow water shelf mirco-facies are merely occurred on the top of Wufeng formation and Longmaxi formation.(2)In the study area,the black shale of the Wufeng-Longmaxi Formation dominated by I-II1 types of kerogen,rich abundance of organic contents(TOC range from 1.33% to 8.46%).Vitrinite reflectance(Ro)was generally in 1.08 ~4.48%,has been in over mature evolutionary stages.(3)The distribution of black shale strata in northeast of Chongqing is relative stable.The middle of tian ba and jian shan townlet of Wuxi county is the largest in shale thickness,reaching above 80 m,being the deposition center of study area;the strata thickness away from the center position gradually reduces to about 40 m in shuang he country,Chengkou county.(4)The minerals in the black shale of the Wufeng-Longmaxi Formation are mainly composed of quartz,clay minerals,and small amount of feldspar,barite,pyrite,et al.The average amount of quartz was over 60%?Four pore types are recognized in the black shale of the Wufeng-Longmaxi Formation: organic pore,intergranular pore,intragranular pore and mirco-fracture?The shales are characterized by low porosity and low permeability,the porosity is mainly small than 1%,varying from0.06% to 14.42%,the permeability range from 0.0001×10-3?m2to 0.0540×10-3?m2.(5)In the study area,the Langmuir volume of the black shale was range from1.78 to 3.81 m3/t,with an average of 2.84cm3/t.Organic matter is the basic factor of influencing gas-bearing property of shale,there is a positive correlation between Langmuir volume and the TOC.However,to some extent the preserve conditions suppressed the gas content of the black shale?(6)Considering the thickness and deepness of organic-rich shale,TOC,Vitrinite reflectance,gas content and preservation conditions,the prediction of vantage area for shale gas exploration in Lower Silurian in the northeast of Chongqing was made,which located in the area of Tianba and Wuxi in Chongqing City.