Study On Shale Gas Enrichment Conditions In The Strong Reconstruction Zone

Recently,as the main exploration and development places of shale gas,Chongqing attracts high attention at home and abroad.It has more profound significance to begin the research of shale gas enrichment conditions in strong reconstruction zone and prediction of favorable zones,especially in Chongqing northeast area,such a rare strong reconstruction zone.Taking lower Cambrian Shuijingtuo/Lujiaping formation(synchronous strata in different facies area)in Chengkou area as the research object in this paper,based on 6 section planes in the field,rock cores of 7 wells,especially the whole set data of 2 exploration wells recently drilled,the test data of the rock samples and results of well logging and seismic interpretation,through the analysis of geological characteristics in the strong reconstruction zone and the main controlling factors for shale gas enrichment,especially on the influencing factors and evaluation of shale gas preservation conditions,we generally studied on shale gas enrichment conditions and its main controlling factors,established a system of evaluation indicators for shale gas preservation in strong reconstruction zone,and predicted the favorable areas in Lower Cambrian Shuijingtuo/Lujiaping Formation.Based on the situation of Exposion and preservation of the formation and hydrocarbon source rock depositted in basin stage,and also the development of folds and faults in China,reserchers have primry devided reconstruction zones into 5 orders,i.e.tiny,feeble,medium,strong and bally strong stages.This paper defined northeast Chongqing as strong reconstruction zone because of its peculiar significant uplift and erosion,strong deformation which caused by multicycle and multistage structural movement.As reserch verified,the northeast Chongqing strong reconstruction zone underwent intensive reformation during multiple structural movement include Caledonian,Hercynian,Indosinian,Yanshan and Himalayan.In this area,structure shows high complexity,numerous faults appeared as thrust,overthrust and strike-slip;attitude of stratums altered in large range upto 60°?75°;outcrop visible folds include angular fold,plunging vertical fold and overturned fold,etc.major faults in this area are:Cheng-Ba fault,Pingba fault,Jimingsi fault and Shashi buried fault,which all streched out parallely in an arc.Cheng-Ba fault divided Chengkou area to northern Dabashan thrust nappe belt and South Dabashan Foreland Fold thrust belt;the second one is further devided by Pingba fault and Jimingsi fault to imbricate fault zone,Fault fold zone and detachment fold zone.The reconstruction strength decreased gradually from northeast to southwest.According to the analysis on different types of rock association and sedimentary facies,it is concluded that the shuijingtuo/lujiaping formation in northeast Chongqing area were mainly developed from shallow to deep continental shelf,further divided into deep water shelf,shallow shelf and the inner edge slope of shelf.In view of the main control factors of the shale gas enrichment in northeast Chongqingstrong reconstruction zone,the following static indexes were analyzed:hydrocarbon generation condition,shale gas reservoir condition,preservation condition and shale gas-bearing properties.The results show that the type of organic matter in the study area is of the humic type I,which indicates that the source rock of the lower Cambrian shale has good kerogen type and with high potential of hydrocarbon generatio.TOC presents gradually decreasing trend from deep to shallow in the section,in the plane,Lujiaping formation is higher than Shuijingtuo formation,increasing gradually from south to north.Ro is generally too large,both of them are in the over mature phase,generally the northern part is in the late over mature phase,and the southern area is in the early over mature phase.Through the research on field samples and core data,the shales of lower Cambrian Shuijingtuo/Lujiaping formation are dominantly composed by quartz,brittle minerals amounted to more than 40%.Shale samples porosity is low,generally less than 2%,dominantly less than 1%;permeability is less than 0.01md.6 kinds of pore types are mainly developed in the shale,dominated by nano-pore.Through the field core analysis of Well Chengye-1,the total gas content of the lower Cambrian black shale is 0.887 m3/t?1.229 m3/t,the average 1.102m3/t,which indicates that the target layer of the study area has good shale gas bearing property.By analyzing the strength and results of reconstruction in Chengkou area,it is believed that shale gas preservation conditions improve with the decrease of the reconstruction intensity from northeast to southwest.Based on the analysis of the structural styles in different secondary structural belts,the corresponding relation between the structural styles and the preservation conditions of shale gas is proposed.Based on the analysis of stratigraphy,structure and sedimentary characteristics,and analysis of enrichment conditions of shale gas in Chengkou area,the results show that:the organic-rich shales formed by special environment is the substance basis of shale gas enrichment,which was very abundant;the degree of thermal evolution of shale is the time window of shale gas enrichment,which is higher also later;the combination of many organic matter pores and inner layer cracks formed sufficient reservoir space for shale gas,which is quite good;structural pattern is basis of shale gas enrichment,which are in varied forms;strong reconstruction is the main factor to destroy shale gas enrichment,but which is too strong.On the basis of the previous evaluation index to predict the favorable area of shale gas exploration,According to the characteristics of northeast Chongqing strong reconstruction zone,selected out 14 affecting factors to the shale gas preservation,that is,thickness of shale section,depth of organic-rich shale,distribution area,regional caprock condition,age of exposed strata(spudding strata),eroded thickness since Yanshan movement,formation dip,angle of fold,density of faults,displacement of the main fault,fracture feature,distance to faults that extend to the surface,distance to outcrup,structural style,established a system of evaluation indicators for shale gas preservation conditions of strong reconstruction zone,and then,predicted favorable areas of shale gas in the northeast of Chongqing.The results show that,the favorable areas of shale gas in the northeast of Chongqing strong reconstruction zone are:the Jianzhuba synclinorium in the north of Cheng-Ba fault,an area of 282.17 km2;Baishahe anticline and Tianba anticline located between Cheng-Ba fault and Shashi buried fault,separately with area of 169.42 km2 and 175.87km2.