Evolutional Characteristics Of Evaporites-bearing Shale Pore Structure In Dongpu Depression And Its Quantitative Characterization

High hydrocarbon enrichment of shale in saline lacustrine basins has been a hotspot in recent years,among which,the pore structure evolution and controlling factors are significant.In the study,shale samples from the third member of Shahejie Formation(Es₃)in Dongpu Depression,Bohai Bay Basin are objective.Geochemical analysis,analysis of mineral components,Field emission scanning electron microscopy(FE-SEM),high-pressure mercury intrusion,Low-pressure gas adsorption are conducted for the characterization of shale pore structure and its controlling factors,then hydrolysis pyrolysis experiments were utilized to study the pore evolutional characteristics.The geochemical analysis demonstrates that TOC content varies significantly,with an average value of 0.76%.The organic matter types of the samples were mainly type?₂and type?,which are mainly gas-prone types.The maturity of samples are mainly high mature to over-mature.Selected samples are mainly composed of clay minerals,carbonate minerals and brittle minerals,and some of which contain gypsum and halite.Mesopores and macropores contribute most to the pore volume,while micropores contribute most to total SSA.TOC and calcareous minerals are the main controlling factors for the Es3 shale pore structure,while siliceous minerals inhibit the development of pore structure.The impact of clay minerals on the pore structure is ignorable.Evaporate minerals content is positively correlated with PV and SSA when the it's less than 2.8%.However,when the content of evaporate minerals is larger than 2.8%,this positive correlation relationship shifts to negative.For the samples during natural evolution,they are classified by the evaporate minerals content(if it's larger than 2.8%).When the content of evaporate minerals is larger than 2.8%,PV of Macropores and micropores decreases as Ro increases;while for mesopores,PV increases first and then decreases;SSA has the same features.When the content of evaporate minerals is smaller than 2.8%,PV of macropores decreases as Ro increases;while for mesopores and micropores,PV get larger first when Ro increases and then decreases,however,the trends of PV is positive;SSA exhibits the same features.For the 2 samples with similar Ro,the one with fewer evaporate minerals has larger macropores PV and SSA;while for mesopores and micropores,the one with more evaporate minerals has larger PV and SSA.Pore evolutional characteristics are verified by thermal maturation experiment,When the sample enters high maturity stage,pore scale and diameter increased,organic acids were generated and brittle minerals were corroded.The pore volume of micropores increased continuously during the thermal simulation,while the pore volume of mesopores and macropores increased first,then decreased slightly and then increased again.The specific surface area of different pore-scale pores increased with increasing temperature,and the specific surface area of micropores decreased slightly due to pore development and transformation in over-mature stage;the specific surface area of mesopores and macropores increased successively.