The Resource Potential And Enrichment Mechanism Of Shale Oil In The Lower Sub-member Of The Shahejie Formation Of Dongpu Depression

The shale of the lower sub-member of the Shahejie Formation(Es₃^L)in the Dongpu Depression has revealed excellent hydrocarbon generation potential and characterized by abundant shale oil resources.It is of great theoretical and practical significance for the exploration and exploitation of Es₃^L shales of the Dongpu Depression to clarify the shale oil enrichment mechanism and the potential of retained hydrocarbon.In this study,the geological and geochemical characteristics of Es₃^L shale from different sedimentary environments in Dongpu Depression,including that in saline,transitional,freshwater environments,were comprehensively evaluated.Saline rock,gypsiferous mudstone and gypsiferous shale were mainly developed in the saline area and transitional area,revealed a high abundance of organic matter and good hydrocarbon generation potential.The organic matter of the kerogen is mainly typed II₁and II₂.The lithology in the freshwater area is mostly sandstone interbedded with mudstone,with low abundance of organic matter and moderate hydrocarbon generation potential and primarily type III organic matter.Based on the geological analysis of source rocks,the oil shale resources was predicted by numerical simulation method.Firstly,two parameters of chloroform bitumen“A”and hydrocarbon pyrolysis S₁ were used to represent the hydrocarbon retention of the shale.The hydrocarbon content of shale simulated by chloroform asphalt“A”method in saline area,transitional area and freshwater areas are 1.24×10⁸ t,5.34×10⁸ t and 0.23×10⁸ t,respectively,with a total hydrocarbon content of 6.81×10⁸ t.The hydrocarbon amount of shale obtained by pyrolysis S₁ method in saline area,transitional and freshwater area are 0.63×10⁸ t,4.95×10⁸ t and 0.51×10⁸ t,respectively,with a total hydrocarbon content of 6.09×10⁸ t.Hydrocarbon generation potential method was used to determine the amount of hydrocarbon generation,expulsion and retention under different thermal evolution degrees,and it was found that the hydrocarbon retention content in the oil generation window was relatively high.The physical simulation was used to simulate the process of hydrocarbon generation and expulsion under geological conditions of the saline area.X-ray,nitrogen adsorption and FE-SEM were used to analyze the mineral content,pore types evolution and pore size distribution characteristics at different simulated temperatures.The results showed that(1)the content of evaporite is positively correlated with the content of retained oil;(2)the mesopores developed in the study area are approximately 15-21 nm.During the oil window,the retained oil of shale mainly existed in the relatively large mesopores in the adsorption state,while during the main gas generation stage,the retained oil occurred in the micropores and relatively small mesopores with pore-filling state.