Relationship between the fluorescence color of oil inclusions and thermal maturity in the Dongying Depression, Bohai Bay Basin, China: Part 1. Fluorescence evolution of oil in the context of hydrous pyrolysis experiments with increasing maturity

The fluorescence evolution of oils has been measured by simulation of petroleum generation and expulsion in a half-closed system using hydrous pyrolysis based on four oil-prone source rocks (Type I and II kerogens) from the Dongying Depression, Bohai Bay Basin, China. The results suggest that the fluorescence characteristics of expelled oil are independent of oil-prone kerogen types, but are strongly related to oil generation and expulsion processes which are controlled by thermal maturity of source rocks. The fluorescence spectral parameters (λ_max, Q_650/500 and QF-535) of expelled oils show no obvious variation, or even show gradual redshifts when the highest λ_max occurs at peak bitumen generation, depending on the expulsion process. A blueshift of fluorescence color occurs only during the main oil generation and expulsion stage, which immediately follows the peak bitumen generation stage. However, the expelled oil mainly exhibits green to yellow fluorescence colors (500 nm < λ_max < 590 nm), which suggests that it is impossible for blue fluorescent oil inclusions to be trapped from the oil directly generated in an open or half-closed source rock system for oil-prone source rocks based on the experimental results. This study demonstrates that the relationship between the fluorescence color of oil inclusions and thermal maturity for oil-prone source rocks is valid only when the source rocks reach the main oil generation and expulsion stage, where the source rocks should have consistent expulsion characteristics. Under this situation, the green fluorescent oil inclusions (500 nm < λ_max < 550 nm) generally have a higher thermal maturity than the oil inclusions with yellow fluorescence (580 nm < λ_max < 590 nm). Otherwise, the petroleum expulsion processes may complicate the relationship between the fluorescence color of oil inclusions and thermal maturity, and oil inclusions with yellow fluorescence do not necessarily have a low thermal maturity. Our experiments have established the first boundary conditions for the fluorescence behaviour of oil as a function of thermal maturity, acknowledging that compositional change of the oil is also occurring during migration, and hence before entrapment in reservoirs.