Study On Sedimentary Facies And Reservoir Characteristics Of Baikouquan Formation,Northwestern Margin Of Mahu Sag,Junggar Basin

The Baikouquan Formation(T1b)of the Mahu Sag in the Junggar Basin holds a scale of hundred million tons of reserviors,and its accumulation conditions are superior.The shale conglomerate reservoir under the control of fan delta sediments is developed in the main body of the Baikou Formation,while the pore structure has a strong heterogeneity,which is poorly studied.In this paper,core observation,thin-section identification,scanning electron microscopy(SEM),combined with energy spectrum analysis and X-ray diffraction,cathodoluminescence,high pressure mercury injection and other experimental analysis methods are used to identify the characteristics of the reservoir.The main knowledges based these works I had done are following:(1)The glutenite reservoir in Baikouquan Formation can be divided into five main rock types: fine gravel-small conglomerate,sandy fine conglomerate,coarse sandstone containing fine gravel,medium-fine lithic sandstone,and silty mudstone.The gravel composition is complex and its particle size distribute widely,reflecting the characteristics of near-source rapid accumulation.The gravel composition is mainly tuff(average 32.7%),granite(average 8.7%),andesite(average 8.1%),and a small amount of rhyolite,felsite,quartzite and mudstone.The fillings are mainly clay minerals,and the types of cement include carbonates,zeolites and a small amount of silicon.(2)Combining the core observation and sand-ground ratio statistics,it is believed that the sedimentary period of Baikouquan is the fan-delta sedimentation under the water-inflow system;the glutenite deposited in this period is divided into three sedimentary subfacies,eight sedimentary microfacies: fan delta plain subfacies(including gluten-based debris flow microfacies,sandy fine conglomerate dominated river microfacies,mudstone dominated river diffuse microfacies),fan delta front subfacies(including The sedimentary channel microfacies dominated by sandy fine conglomerate and coarse sandstone containing fine gravel,the diversion bay microfacies dominated by medium-fine-grained lithic sandstone,and the far-sand dam microfacies dominated by fine sandstone)and the former delta subfacies(including the microfacies of the shallow lakes dominated by silty mudstones,and the microscopic facies of the coarse sandstones containing fine gravel).The submarine distributary channel microfacies of the leading edge sub-phase is superimposed and contiguous,which is the key phase of high-quality reservoir development;the main flow mechanism of traction flow is strong,the content of heterogeneous is small,and the primary pores are developed.The reservoir has good physical properties.In addition,the thin mudstone and the lacustrine mudstone between the rivers form a top and bottom slab,which together with the leading edge channel sand body constitute a good reservoir-cap combination.(3)The main reservoir types of Baikouquan Formation are the dissolution pores(42.49%),the residual intergranular pores(38.01%),the shale shrinkage joints(8.74%)and the micro-cracks(8.11%).The capillary pressure curve is characterized by fineness,and has the characteristics of small pores and fine throat.The pore throat structure in the study area is complex,and the overall performance are medium-low porosity and micro-throat;the porosity is mostly distributed between 4-16%,the average is 9.23%,the permeability is distributed in the range of 0.01-94.8m D,and the average is 2.3m D.The overall deviation of the percolation conditions is poor.The main types of diagenesis in the study area are compaction,cementation and dissolution.In the former two cases,the physical properties of the reservoir will be deteriorated,but the chlorite cementation and siliceous cementation in the early stage of diagenesis can protect the primary pores of the reservoir to a certain extent,which has positive significance.Reservoir physical properties generally play a positive role,but the diagenetic environment is closed or not,fluid properties,the nature of debris particles,and the level of shale content together cause differences in dissolution.(4)Tectonic effect controls the macroscopic layout of the reservoir by controlling the development of the stratum and the development of the fault.The sedimentation further determines the material composition and pore throat structure of the sand body.The diagenesis adjusts the material distribution and contact mode in the reservoir and transforms reservoir properties.