Sedimentary Environment Study And Shale Gas Geological Characteristics Of Carboniferous-Permian Dark Mudstone From Middle-Small Basins,Northern Xinjiang

Northern Xinjiang is an important shale gas exploration area in northwest China.It contains several dark mudstone and shales developed basins during Early Carboniferous,including Tacheng Basin,Fuyun Basin and Jimunai Basin.Dark mudstone is developed from Lower Carboniferous to Lower Permin.Littoral-neritic,delta,fluvial and lacustrine facies are the dominant facies.Many sections are measured and observed in the field.A systematic sampling and facies identification were done.A series analyses were completed,including elemental geochemistry,organic geochemistry,mineral XRD analysis,scanning electron microscopy(SEM)analysis,and kerogen related analyses,et al.Tectonic environment study shows that,the dark mudstone in Fuyun and Jimunai Basins deposits in an andesitic oceanic island arc setting during Early Carboniferous,and the Tacheng Basin is a continental island arc.It transforms from oceanic island arc,continental island arc to active continental margin during Late Carboniferous and Early Permin.The lithology also changes from mafic,andesitic to felsic source rock,with more older sedimentary constituent..During Carboniferous,Frequent volcanic and igneous activities in oceanic island arc or continental island arc bring many feldspar minerals.The feldspar content in the lower horizons is obviously higher than the upper.It bears a better reservoir because of common intercrystalline pore,solution pores,denudation fracture,et al within feldspar minerals.Shale gas geological characteristics study shows that,the dominant horizons and facies are Early Carboniferous Nanmingshui Formation(Cln)littoral-neritic facies in Fuyun Basin;Early Carboniferous Hebukehe Formation(Clhb)delta facies,Early Permin Molaoba Formation(Plm)lacustrine delta facies and Late Carboniferous Jimunai Formation(C2j)delta facies in Tacheng Basin;Early Permian Kalagang Formation(Plkl)lacustrine facies,Late Carboniferous Qiaqihai Formation(C2q)delta facies and Early Carboniferous Nalinkala Formation(Clnl)delta facies in Jimunai Basin.Elemental geochemistry study shows that,samples of delta facies bear a highest SiO2 average content and a lowest Al2O3 average content;on the contrary,samples of lacustrine facies bear a lowest SiO2 average content and a highest Al2O3 average content.The positive relationship is significant between total organic content(TOC)and Al2O3 contents in samples of lacustrine facies.The total organic content have a positive relationship with Boron content(salinity)and a negative relationship with Sr,CaO contents(carbonate)in samples from profile SEP and TSP.The total organic content of samples from littoral-neritic facies is mainly controlled by biological productivity,and redox condition secondly.On the contrary,the first factor is redox condition in the samples of lacustrine facies,and biological productivity secondly.Mineral composition of dark mudstone samples has a good balance among clay minerals content,quartz+feldspar content and carbonate content as related to shale gas adsorption and rock pressing crack.Rock specific surface increases with illite content,and decreases with kaolinite and chlorite contents.Samples from lacustrine or lacustrine delta facies bear a high clay mineral content,especially illite content.They also have a high rock specific surface,and are better for absorption of shale gas.