Study On Shale Oil Reservoir Characteristics Of Shahejie Formation In Jiyang Depression,Bohai Bay Basin

The Es₃^x and Es₄^s layers of Shahejie Formation of the Paleogene in Jiyang Depression of Bohai Bay Basin,is a high quality source rock with wide distribution area and large thickness,and many wells have obtained industrial oil flow,showing good shale oil reservoir development potential.The shale oil reservoir is characterized by heterogeneity,multiple rock types,complex structures,various types of reservoir space,nano-micro aperture.Systematic research of shale oil reservoir characteristics has a great significance to shale oil development.In this paper,the research object is the Es₃^x and Es₄^s shale oil reservoir of four coring wells of Shahejie Formationin Jiyang Depression.With sedimentary petrology,petroleum geology,reservoir geology and unconventional oil and gas geology as guiding theories,from mineralogy of shale oil,the reservoir space characteristics,physical characteristics and fluid occurrence characteristics of shale oil reservoir are represented by a variety of technical means.The shale oil reservoir characteristics of Shahejie Formationin Jiyang Depression of Bohai Bay Basin are evaluated comprehensively.Through core observation,section authentication and X-ray diffraction whole rock analysis,based on the analysis of mineral petrology characteristics and the mud,calcite and dolomite content,the comprehensive petrology classification schemeis presented,which is suitable for shale oil fine-grained sedimentary rocks in the study area,dividedrocks of shale oil layer into three categories and twenty-six rock types.The classification results show that shale oil reservoir lithologies in the study area are dominated by limy mudstone,argillaceous limestone,lime-bearing mudstoneand mud-bearing limestone,with minor dolomite thin layer.From Es₄^s to Es₃^x,the calcite content decreased and the mud gradually increased.As a whole,mud-bearing limestone and argillaceous limestone changed to limy mudstone and lime-bearing mudstone.Various lithologic staggered appearances,the attitude have a gradual transition from lamellar to stratiform.Based on petrologic feature and lithological association analysis,the reservoir space micro characteristics research is carried out by high resolution scanning electron microscope,backscattered electron and X-ray energy spectrum.The reservoir space of shale oil reservoir is divided into two types:pores and fractures.The pores include intergranular pores,dissolved pores,intercrystalline pores and intracrystalline pores.The fractures include tectonic fissures,bedding fissures,abnormal overpressure fissures and argillaceous shrinkage joints,etc.Through the high resolution backscattered images analysis,the micro pore characteristics of shale oil reservoirs are quantitatively characterized,including pore types,pore diameter and its distribution and so on.The main pore types of shale oil reservoir are intergranular pores and dissolved pores,followed by intercrystalline pores.The shale oil reservoir is dominated by nano-micro pores.With the pore diameter increasing,the number of pores is decrease rapidly.The number of nano-pores accounted for 90%to 98%,but the reservoir surface porosity is mainly provided by micro-scale poresaccounted for 2%to 10%in number.The surface porosity of dolomite is about 10%,followed by the mudstone,is mainly 6%to 9%.The surface porosity of limestone is 0.5%⁵%.The pore size distribution and its structure characteristics of nanoscale pore of shale oil reservoir are characterized by nitrogen isothermal adsorption experiments.Research and classify four nitrogen adsorption isotherm types,by calculation the parameters of specific surface area,pore volume and fractal dimension of different lithology samples,the degree of development and pore structure complexity level are analyzed.The dolomite has clintheriform pore structure,high pore volume and good pore connectivity.The pore development of mudstone is better and the pore volume is slightly larger than that of limestone.The pore structure of mudstone and limestone are mainly thin neck-wide body and clintheriform.The mudstone has the most complex structure,strong heterogeneity and large specific surface area.The physical properties of shale oil reservoir data show that,in these five major lithologies,the physical properties of argillaceous dolomite are the best,followed by limy mudstone and lime bearing mudstone.The limestone has the worst physical properties.Thephysical properties of shale oil reservoir are mainly affected by mineral composition,organic matter,diagenesis and burial evolution.Tectonism and hydrothermal process also have a certain impact.The field emission environment scanning electron microscopy and X-ray energy spectrum elemental analysis show that the shale oil is mainly free liquid hydrocarbons,and the occurrence spaces mainly includes intergranular pores,dissolved pores,intercrystalline pores and bedding fissures.The oiliness classification standard of shale oil is based on light hydrocarbon compensation and heavy hydrocarbon correction of pyrolysis S₁ and chloroform asphalt"A".Combined with the scanning electron microscope oiliness observation,X-ray energy spectrum microcosmic fluid reserves research,the oiliness classification of shale oil and log data?LogR model,the oiliness evaluation methodology of shale oil in the study area is established,the fluid occurrence characteristics in the vertical of shale oil reservoir are analyzed.The main lithology of high oil-bearing layer in Shahejie Formation is limy mudstone,followed by limy mudstone and argillaceous limestone alterationin.