Relationship between the fluorescence colour of oil inclusions and thermal maturity in the Dongying Depression, Bohai Bay Basin, China: Part 2. fluorescence evolution of oil in the context of petroleum generation, expulsion and cracking under geological conditions

Abstract

Abstract Abundant measured data from source rocks, crude oils and oil inclusions in the Dongying Depression were used to identify the oil generation, expulsion and cracking stages under geological conditions, so as to build a fluorescence evolutionary model that considers all petroleum generation, expulsion and cracking processes. In the model, oil generation consists of three successive processes: bitumen generation, bitumen decomposition into oil, and oil cracking. These three stages are strongly controlled by thermal stress in the Dongying Depression. Kerogen decomposition into bitumen occurs at around 85 °C (0.42 %Ro), while NSO compounds in the bitumen begin to crack into oil at around 120 °C (0.67 %Ro). Competition between bitumen generation and NSO compound cracking leads to peak bitumen generation occurring at around 130 °C (0.75 %Ro). After that, the main phase of oil generation from intense bitumen decomposition occurs. A transition at approximately 160 °C between bitumen decomposition into oil and the inferred start of oil cracking corresponds to 4000 m (0.98–1.11 %Ro). During the bitumen generation and main oil generation stages, the fluorescence colours of the expelled oils are mainly yellow and orange, and a fluorescence blueshift of the oils only occurs during the main oil generation stage. In the late oil generation stage (∼160 °C), the fluorescence colours of the oils may be close to blue. With increasing burial, once the source rock and reservoir temperatures reach the threshold conditions for oil cracking (160 °C), an intense blueshift of the oil occurs in both source rocks and reservoirs, and even in oil inclusions. This blueshift is shown by a decreased dispersion in the fluorescence colour distribution of oil inclusions with increasing burial, from multiple colours at temperatures 160 °C. Therefore, in oil-prone source rocks containing Type I and II kerogen, a blue fluorescence colour of oil appears to be strongly related to the secondary cracking process. Blue fluorescent oil inclusions generally have higher thermal maturities than oil inclusions with green or yellow fluorescence for oil-prone source rocks. This relationship is based on the assumption that the oil was not from a gas-prone source-rock, and that secondary alteration processes can be excluded, including migration fractionation, gas deasphalting, trapping fractionation, and phase separation. Furthermore, blue fluorescent oil inclusions may be a good indicator for oil-prone source rocks, which is significant for evaluation of source rock quality in deepwater and ultra deepwater areas in offshore basins.