The Source-reservoir Assemblage Of Shale Oil In Jimusar Sag And Its Charging Mechanism

Jimusar Sag is the first national shale oil demonstration area in China,and the shale oil exploration potential of the Lucaogou Formation in the sag is huge.The shale oil heterogeneity in the Lucaogou Formation is strong.The research on the source-reservoir assemblages,the evolution of hydrocarbon generation and expulsion of source rocks in the current and increasing maturity stages,and the charging,migration and accumulation process of shale oil in tight reservoirs are still relatively weak.In this paper,a new method of source-reservoir assemblage is proposed based on geological and geochemical data.Using nitrogen adsorption,high-pressure mercury intrusion and micro CT methods,the characteristics of hydrocarbon expulsion channels of source rocks under natural conditions are analyzed.The pyrolysis experiment was used to analyze the evolution of the expulsion pathway of source rocks.By selecting typical reservoir rock samples and conducting charging,migration and accumulation simulation experiments,the mechanism of shale oil accumulation in tight reservoirs is analyzed.The achievements and conclusions are as follows:Combined with the parameters of lithology,thickness,contact relationship and sedimentary cycle,a method for dividing the oil source reservoir structure of the Lucaogou Formation shale is established.The division results show that the upper sweet spot belongs to source-reservoir interbed type,and the lower sweet spot is mainly the reservoir clip source type.The distribution of oil source-reservoir assemblage of Lucaogou Formation shale is controlled by the sedimentary environment and provenance.The sequence of hydrocarbon expulsion efficiency of Lucaogou Formation from high to low is:reservoir intercalated source type>source-reservoir interbed type>source intercalated reservoir type>shale type.The distribution characteristics of hydrocarbon expulsion channels of Lucaogou Formation source rocks are analyzed using micro CT,N₂ adsorption,CO₂ adsorption and high-pressure mercury injection on typical source rock samples.The results show that under the current natural thermal stage,the hydrocarbon expulsion channels of the source rocks of the Lucaogou Formation are pores at 2～100nm and 2～50μm.The pore structure of source rock is mainly controlled by the lamina structure,organic matter abundance and maturity.The shale with lamina has better reservoir space and hydrocarbon expulsion channel than mudstone.Through hydrocarbon generation simulation experiments on natural samples,the evolution of the channel is divided into three stages,and it is proved that the hydrocarbon expulsion channel of source rock in Lucaogou Formation in the oil window is mainly mesopores,and after entering the gas generation stage,it is mainly pore and fractures in the micron scale.The evolution model of the hydrocarbon expulsion pathway of Lucaogou Formation source rock is established.Using the pulse displace method to conduct charging experiments on reservoir rock samples,and using the continuous displace method to conduct migration and accumulation experiments,it is clear that shale oil charging is more difficult in the upper sweet spot than in the lower sweet spot.The coupling of permeability and migration dynamics controls the migration of shale oil in tight reservoirs.The characteristics of both generation and storage of tight reservoirs in the Lucaogou Formation control the accumulation of shale oil.The strong hydrocarbon generation capacity is helpful to improve terminal oil saturation.