Influence of Early Silurian intra-shelf microgeomorphology on sedimentation and reservoir of deep shale gas in the Dazu Area,Western Chongqing
Based on petrography,3D seismic,elemental geochemistry,total organic carbon content and X-ray diffraction analysis,this study systematically discussed influence of different microgeomorphic u-nits on sedimentation and reservoir in the first sub-member of the first member of the Longmaxi Forma-tion,Dazu Area,West Chongging and analyzed the distribution of high-quality shale gas reservoirs.The result shows a 3-step microgeomorphic pattern in the early Longmaxi time in study area,which is divided into shelf plateau,shelf slope and shelf depression from north to south.Medium-to high-carbon laminated siliceous shale was mainly deposited at the shelf depression and transitioned into low-to ul-tralow-carbon unlaminated/weak-laminated clay shale or siliceous shale northward.Sublayer-1 and sub-layer-3 comprise mainly medium-to high-carbon laminated siliceous shale,whereas sublayer-2 and sub-layer-4 dominantly consist of low-to ultralow-carbon unlaminated/weak-laminated clay shale.The sedi-mentary microfacies types have carbonaceous siliceous,siliceous muddy and muddy siliceous deep-water shelf microfacies by shale lithofacies,which responded to transgression-regression sedimentary cycles with two times.Those sedimentary cycles are consistent with variations of redox and paleosalinity of sedimentary water column.From shelf plateau to shelf depression,the sedimentary microfacies transi-tioned from muddy siliceous to carbonaceous siliceous deep-water shelf,and the paleosalinity and reduc-ing of water column strengthen southward.At the transgressive phase,sublayer-1 and sublayer-3 were deposited and carbonaceous siliceous deep-water shelf microfacies expanded northward,which enhanced reducing condition and biological abundance of water column.High proportions of M8 and M11 lithofa-cies were developed in shelf depression and slope.High organic pores and brittleness indices improve the porosity.At the regressive phase,sublayer-2 and sublayer-4 were deposited and siliceous muddy deep-water shelf microfacies expanded southward.M5 and M6 lithofacies were mainly developed.Decrease of organic pores and increase of clay contents lead to total porosity falling.According to the measured data and quantitative prediction,the three reservoir evaluation parameters gradually increase towards the shelf depression,significantly controlled by the microgeomorphic pattern.These results provide guid-ance for the exploration and development of deep shale gas in Western Chongqing.
deep shale gasmicrogeomorphologysedimentary characteristicsreservoir characteristicsLongmaxi FormationEarly SilurianDazu AreaWestern Chongqing
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