Quantitatively predicting the thermal maturity of oil trapped in fluid inclusions based on fluorescence and molecular geochemical data of oil inclusions in the Dongying depression, Bohai Bay Basin, China

The fluorescence characteristics of individual oil inclusions and molecular composition analysis of oil inclusions in 20 Eocene sandstone samples from the Dongying depression were used to assess the relationship between the fluorescence colors of the oil inclusions and their thermal maturity. The oil inclusions from the Shahejie Formation member III middle submember, member III lower submember, and member IV upper submember samples primarily have yellow fluorescence colors (500 nm < wavelength of the maximum intensity (lambda(max)) < 590 nm), whereas the oil inclusions in the Shahejie Formation member IV lower submember reservoir sample only have blue fluorescence (440 nm < lambda(max) < 500 nm). The molecular maturity parameters from steranes, hopanes, and aromatic hydrocarbons showed that the oil inclusions with yellow fluorescence colors have relatively low thermal maturities within a narrow range (0.75%-0.82% vitrinite reflectance equivalent [R-equ]), which is significantly lower than that of the sample containing oil inclusions with blue fluorescence colors (126% R-equ). The yellow fluorescent oil was interpreted to be related to the main oil generation and expulsion process, whereas the blue fluorescent oil was mainly controlled by the thermal cracking process of C-14(+) aromatic hydrocarbons in source rocks, reservoirs, and oil inclusions. This study demonstrates that the fluorescence colors of oil indusions may be good indicators of thermal maturity in oil systems charged from oil-prone source rocks and, for the first time, makes it possible to quantitatively predict the thermal maturity of oil inclusions based on their fluorescence characteristics. However, biodegradation, water washing, phase separation, and trapping fractionation should be considered before using this correlation because these processes can also influence fluorescence colors.