Abstract: In order to understand the relationship between mobility of the Paleogene shale oil and thermal evolution degree of the shale in Dongying Sag,Bohai Bay Basin,this paper studied three critical geological factors affecting shale oil's recoverability,including the evolution of predominant clay minerals,abnormal fluid pressure,and the evolution of hydrocarbon fluid characteristics.Furthermore,we observed the movable oil existing in fresh shale by using fluorescence microscopy,and measured movable oil quantity by using a physical simulation method.Results show that both mixed-layer illite/smectite and montmorillonite contents decrease continuously with increasing shale burial depth,and reach equilib-rium at about 3 500 m.Therefore,shale below 3 500 m is favorable for fracturing and post-frac maintenance of factures due to the weakening of hydration expansion ability of clay minerals.The continuous overpressure widely developed in shale deeper than 2 800 m provides natural energy for fluid outflow.Shale enters into oil window at a depth over 3 000 m,thus its oil saturation is generally quite high.Oil phase is predominant in shale with depth ranging from 3 000 m to 4 200 m,while condensate gas dominates in shale at a depth larger than 4 300 m.Shale oil viscosity decreases dramatically with increasing burial depth,and reaches a relatively lower value at depth of about 3 500 m,then decreases slightly with increasing depth.Fluorescence microscopy observations and simulation experiments show that movable oil widely exists in shale deeper than 3 000 m,and the ratio of movable oil increases with increasing shale burial depth.Based on a comprehensive analysis of mineral transformation mechanism,fluid pressure,fluid property in shale,and movable oil experiment results,it is concluded that the Paleogene shale below 3 500 m in Dongying sag has good shale oil exploration prospects.

Key words: clay mineral, formation pressure, fluid property, recoverable oil, shale oil, Dongying Sag, Jiyang Depression