Abstract: Free hydrocarbon content is one of the key parameters for resource assessment of shale oil.However, due to the evaporative loss of hydrocarbon, the measured results from lab analysis differ greatly from real value, which leads to the "distortion" of shale oil resource assessment.To investigate the process of the evaporative loss and obtain the hydrocarbon loss amount and correction coefficient, a time-series of analysis has been carried out on fresh oil-bearing shale samples with similar maturity and different lithofacies for both the amount and the composition of the hydrocarbons retained in shale.Results suggest that there are two processes of early rapid and later slow evaporative loss. The volatile components are mainly light hydrocarbons with carbon numbers smaller than C₁₃-C₁₅, while the medium to heavy components are less affected. Both the amount and processes of hydrocarbon evaporative loss are controlled by the physical properties and original oil content of shale samples.The oil-bearing shale samples with higher original oil content and better physical properties appear to have more evaporative loss of hydrocarbons.Results also indicate that more attention should be exercised on original hydrocarbon content when conducting the shale oil resource assessment of "sweet spot" section with high oil content and good physical properties.