Analysis Of Mesozoic Negative Inversion Tectonics And Prospects Of Lower Silurian Shale Gas In Southern Jiangsu

Lower Yangtze basion is superimposed basin which has experienced multiphase tectonic movements in the Mesozoic and Cenozoic,so the structure of the basin is complicated.Jurong basin located in Nanjing depression of Lower Yangtze area is a rift basin which formed in Yanshanian.Based on previous work,the study of Jurong basin shows oil and gas potential very well.The upper Paleozoic source rocks of Jurong basin entered the mature-high mature phase in the later of late cretaceous.It is very important to recognize the structure of Jurong basin and to exploration and development of oil and gas that analyzing the scope of influence detailedly.The black shales in the lower Silurian and the upper Ordovician series are very important source rocks and reservoirs of shale gas.The activities of Jurong basin greatly influenced the development of shale fracture.However,the development of shale fracture is an important influence factor to oil and gas storage and hydraulic fracturing.The study of fracture has vital significance in the oil and gas exploration and development.In the thesis,first of all,the research of paleomagnetism and the 3D seismic profiles interpretation was used to study the tectonic setting of Jurong basin.Second,in order to divide the Silurian and Ordovician strata,we carried out geological field work,the study of tuff zircon chronology and shallow well drilling of Jurong basin,thus we got the core of shales and limestones.Finally,by scanning the core using industrial computed tomography,we studied the major existence forms of fracture in core and the main influence factors when industrial computed tomography scan different lithologic cores.In the thesis,by studying the interpretation of 3d seismic profile,Jurong basin exist negative inversion structure which is typical structure in Mesozoic,then we clarified the evolution process of Mesozoic tectonic.Combined with the previous work,constrained geological outcrops and drilling data,the negative inversion structure having forward modeling helped by the software of trishear is found in the Jurong basin,which has a series of characteristics:firstly,analyzing the 3D seismic section of the basin,reflecting the compressive tectonic environment during Indosinian event and early of Yanshan event,the expansion of the tectonic environment which formed a series of expansive basins during the end of Yansan event.Secondly,the sedimentary formation of Cretaceous-Quaternary was controlled by the negative inversion structure which formed a basin cut of in north and covered in south.The faults around the negative inversion structure don't be inverted because of expansive stress absorbed by the negative inversion structure inverted in middle of Cretaceous.Thirdly,the expression of Yanshan event isn't entirely destructive to the traps of oil and gas,because of the good shows of oil and gas around the negative inversion structure.In order to study the characters of fractures in shales,we used industrial CT to scan the shale cores in the Jurong basin,and analyzed the CT scanning data of shales by the digital processing technology.We established the model of CT scanning data and extracted the fractures by VGStudio MAX software.In this way,we could study the difference between the structure of fractures in the shales composed of different mineral composition and the various CT images.The results show that there are two groups of different strike and dip of fracture.One group with high angle is approximately 70°-90°;the other group with low angle is approximately 30°-40°.Jurong basin may have experienced at least two tectonic events after the Rhuddanian.Shales composed of different composition,or greater density of mineral component,whose industrial CT scanning image is clearer.And the larger size of mineral grain,the more obvious effect of scanning.