Pore Characteristics And Its Evolution Law Of The Late Cretaceous Qingshankou Formation Shale In The Songliao Basin

Shale oil is hosted in nanopores of organic-rich shales,so pore characteristics and the pore evolution of the shales are critical for shale oil accumulation.They are of great significance for shale oil exploration and development.The Songliao Basin is one of the most potential continental shale oil resource basins in China.This thesis is based on cores of Qingshankou Formation in SK-1s drilling well.In this thesis,X-ray diffraction,total organic carbon content,Rock-Eval pyrolysis,field emission-scanning electron microscopy,low-pressure nitrogen adsorption,mercury intrusion capillary pressure,and experiment of the diagenetic physical simulation were conducted to investigate the mineralogical,organic geochemical and pore characteristics,as well as pore evolution law of lacustrine organic-rich shales in the Late Cretaceous Qingshankou Formation in the Songliao Basin.Clay mineral content is high in the Qingshankou Formation shale.As the depth increases,quartz content and illite content increase,while mixed-layer illite/smectite content decreases.The samples of the Qingshankou Formation are classified into seven lithotypes according to mineral content.In the order of mineral brittleness index from high to low,these seven lithotypes are carbonate rock,mixed carbonate mudstone,mixed mudstone,argillaceous/siliceous mudstone,clay-rich siliceous mudstone,mixed argillaceous mudstone,and silica-rich argillaceous mudstone,respectively.This order also represents the fracturability of different lithotypes from strong to weak.Interparticle pores are highly developed in the Qingshankou Formation shale.These shales have mesoporous nature,and slit-shaped pores produced by aggregates of plate-like particles.As the depth increases,the average pore diameter increases,and the specific surface area and micropore volume decrease.As the depth increases,the pore structure and pore surface become simpler.C lay-rich siliceous mudstone is ideal compared to other lithotypes for a shale oil reservoir in the Qingshankou Formation,and it has the highest total organic carbon content and hydrocarbon production potential,good reservoir quality and moderate fracturability.As the thermal maturity increases,the total pore volume and average pore diameter increase,and pore structure becomes simpler.As the thermal maturity increases,pores with pore sizes smaller than 10 nm significantly reduce,while pores with pore sizes larger than 10 nm greatly increase.The pore evolution process of the shale is divided into three stages.The first stage is dominated by mechanical compaction,which leads to larger pores decrease rapidly.Hydrocarbon generation of organic matter is weak at this stage,and it leads to smaller pores increase slightly.As the thermal maturity increases,the second stage is dominated by hydrocarbon generation of organic matter,which leads to organic-matter pores dramatically increase.Carbonate minerals are continuously dissolved by organic acid from hydrocarbon generation,and many larger dissolution pores form in the shale.During this stage,interparticle pores reduce,while intraparticle pores increase.The third stage is the over-mature stage.As the organic matter is continuously consumed,organic-matter pores increase slowly,and porosity is basically stable or increases slowly.The most important controlling factor of pore structure of lacustrine Qingshankou Formation shale is clay minerals,followed by organic matter content.It was the first time to apply multidimensional scaling to analyze correlations between pore structure parameters of shales.The correlations between multiple pore structure parameters can be macroscopically grasped by multidimensional scaling.It was also the first time to fit fractal dimensions with pore structure parameters of shales using stepwise regression.Fitting equations of fractal dimensions were achieved after that.Simpson's paradox can be eliminated by stepwise regression.In this thesis,nanopore characteristics and pore evolution law of lacustrine shales,and effects of lithotypes on shale oil reservoirs have been achieved.The study of the correlation between pore characteristics of shales has also been improved using statistical methods.These studies play an essential role in understanding the pore system of continental shales and the evaluation of continental shale oil reservoirs.