Characteristics Of Pore Development And Evolution Of Lacustrine Shales In Typical Areas In Bohai Bay Basin

With the successful exploration and development of shale oil in the North America,exploration of shale oil in the terrestrial basin receives increasing attention.One of the key scientific issues in studying the retention and accumulation mechanism of shale oil is the regularities of development and evolution of nanopores in shale oil.In this paper,it uses the immature-low mature shale from Ek2 of Cangdong Sag in Bohai Bay Basin as examples to study the development and evolution of shale pores during their thermal evolution.It could provide a theoretical basis for studying the mechanism of retention and accumulation of shale oil and predicting the“sweet pot”.The study of shale pore natural evolution profile shows that there are primary pores in the immature stage(Ro<0.5%),which has an average pore size of 6.19nm and an average surface area of 7.46 m~2/g and the total pore volume of 0.0215cm~3/g.In the low-mature stage(0.5%<Ro<0.7%),organic matters begin to generate hydrocarbons.The average pore size is 11.3nm,average surface area is 1.14 m~2/g and the total pore volume is 0.0057cm~3/g.In the mature stage(0.7%>Ro>1.2%),a large amount of hydrocarbons are generated in organic matters,and nanopores are widely developed.The average pore size is 8.56nm,average surface area is 20.24m~2/g and the total pore volume is 0.0514cm~3/g.Through hydrocarbon-generating thermal simulation experiments,the developmental evolution of shale pores in the mature-high mature stage of thermal evolution was further studied:in the mature stage(room temperature~375°C),the shale sample develops intergranular pores,intercrystalline pores,organic pores and microfractures developed.The average pore size is 11.09nm,average surface area is 2.71m~2/g and the total pore volume is 0.0382cm~3/g.In the high mature stage(375°C~425°C),the shale samples mainly develope organic pores with a small amount of inorganic pores and microfractures.The average pore size is 8.76nm,average surface area is 2.68m~2/g and the total pore volume is 0.0378cm~3/g.In the over mature stage(>425°C),the shale sample mainly developes inorganic pores and microfractures with a little organic pores.The average pore size is 18.52nm,average surface area is 1.83m~2/g and the total pore volume is 0.0189cm~3/g.Based on the characteristics of shale pores development and evolution in both the natural evolution and the evolution of hydrocarbon generation thermal simulation,a complete model of pore development and evolution of Ek2 shale in the study area is established.It systematically reveals the characteristics of shale pores development and evolution.With the increase of the thermal evolution of organic matters,organic matters begin to generate hydrocarbons,and organic pores gradually replace inorganic pores to become the main body of the pore,which accompanied by the development of microfractures.The average pore size shows the trend of increasing first,then decreasing,then increasing again.The total pore volume and the specific surface area have a similar feature,which shows the first decreasing,then increasing,and then decreasing.The organic maturity is the main factor that influences the shale pore development and evolution.The increase of thermal evolution promotes the decomposition of organic matters and the generation of oil and gas,resulting in the nanopores development.Simultaneously,organic types and inorganic minerals also play a role in the development of pores.