Sedimentary And Reservoir Feature Study Of Silurian Xiaoheba Formation In Shizhu Synclinoyium Western Hubei-Eastern Chongqing

Western hubei- eastern chongqing is a favourable development blocks for the oil & gas exploration.many gas fields and oil & gas structure has been found in upper assemblage of this area,but few research was finished in the lower assemblage.although there ara some gas-show,silurian stratigraphy were consider as good territorial oil source and cap rocks but a main exploration target,which had caused the sequence stratigraphy and sedimentary system research were much hysteretic,the reservoir feature and the distributing of favourable development blocks is not clear, had restrict the gas exploration severely.Aim at the problem above,this paper not only researed the sequence stratigraphy and sedimentary facies of silurian xiaoheba formation in shizhu synclinoyium with the relative theory guidance,but also forecasted sandstone in this area after researching the seismic response,and analysed the reservoir feature,diagenetic phenomenon and diagenetic evolution, meanwhile discussed they infection to the porosity evolution of the xiaoheba formation.this paper synthetically discussed the distributing of favourable development blocks of the silurian xiaoheba formation,and gave some reciprocal evaluate.According to the analysis of lithology combination featere, sedimentary structures featere , seismic response featere and the grain-size distribution, reckon the silurian xiaoheba formation was a large bed of sediments of delta,there are delta front and fore-delta subfacies in this area,and the sub-channel branch, mouth bar, distal bar and front sheet sand microfacies are the main place of reservoirs for xiaoheba formation. Although xiaoheba sandbody has high compositional maturity,the compositional maturity is between 4-8,few sample is higher than 15 , grain psephicity and selection are preferably, the physical property is not good due to congenital sedimertary condition.the porosity of xiaoheba formation is between 0.27%-13.23%,and permeability(non-crack sample) is between 0.011×10-3um2-1.06×10-3um2,and the average is 0.083×10-3μm2.the result of reservoir evaluate show that there ara no typeⅠand typeⅡreservoir in this area,reservoir is mainly typeⅢ,ⅢA andⅢB are small part of the reservoir(only 11.4% and 24.9%), the reservoir in this area is typical tight sandstone ,if there is no crack to improve the physical property,it`s difficult to form a valuable oil&gas reservoirs.Silurian xiaoheba formation suffered syngenesis diagentic stage, early diagentic stage, middle diagenetic stage, late diagenetic stage and hypergenesis phase ,after such a long and complex diagenetic evolution,the strong compaction and cementation caused the reservoir tight quickly during the early-middle diagentic stage,which restrict the fluidness of acidity liquid,caused form only few secondary porosity during the salad days. Corrosion often carrys through some cracks that can make liquid flow freely.the actual status of reservoir is formed by 3 factors,they are sedimentary, tectonism and diagenesis.sedimentary facies controls the character of lithology and grain size,it`s the foundation for reservoir development. Constructive diagenesis can improve the tight sandstone effectively, it`s the key factor for the reservoir development and tectonism can form a great number of cracks which are the main channels for the liquid,it can hasten the constructive diagenesis happen and development, it`s the vital condition for the reservoir development.Analysed through sedimentary facies, seismic facies and the result of reservoir prediction,there are three favourable development blocks in this area.the best region lies between line sz95-5—line sz94-17,is jiannan structure high. Data and its high point altitude is -3250m, closure is 350m and the close area is 123.36km2,the major axis is 28.511km and the minor axis is 4.866km. During drilling,well jianshen 1 has a favorable gas show,there are three layers of gas logging abnormal,and hold a total thickness of 6m,the total hydrocarbon in these layers reach 63.7%.there are a series of of low frequency medium-strong mutative amplitude imbricated foreset reflections,these foreset reflections are decipher as distal bar and front sheet sand in the delta front. Acoustic impedance inversion show that the sand body is relatively development , the result of relevance cube and variance cube crack forecast also show that the cracks are relatively development. The second favourable development blocks lies between line sz94-13—sz94-7, synthesize condition is a little worse than the best region.this region is fault nose structure in the north of the shizhu synclinoyium , its high point altitude is -2800m and close area is 30.1084km2.this region is delta front subfacies and through the seismic facies tracing there are a series of of low frequency medium-strong mutative amplitude imbricated foreset reflections,these foreset reflections are decipher as distal bar and front sheet sand in the delta front. The sand body hold a thick and widest distributing .acoustic impedance inversion show that the sand body is relatively development , the result of relevance cube and variance cube crack forecast also show that the cracks are relatively development.The third favourable development blocks lies between line sz96-15—sz95-19, synthesize condition is a worse than the two regions.this region is fault nose structure and slope area in the southeast of shizhu synclinoyium , its high point altitude is -2750m and close area is 84.634km2.this region is delta front subfacies and through the seismic facies tracing there are a series of of low frequency medium-strong mutative amplitude imbricated foreset reflections,these foreset reflections are decipher as distal bar and front sheet sand in the delta front. The sand body hold a thick and widest distributing .acoustic impedance inversion show that the sand body is relatively development , the result of relevance cube and variance cube crack forecast also show that the cracks are relatively development.