Characterization of kerogens and coals using fluorescence measured in situ at elevated temperatures

Kerogens concentrated from oil-prone source rocks of different depositional environments and macerals isolated from humic coals were characterized using fluorescence microscopy at ambient and elevated temperatures. The initial fluorescence intensities for bulk kerogens (Finitial) under ambient conditions are correlated with their hydrogen index (HI). Four lacustrine kerogens exhibit much higher maximum fluorescence yields (Fmax) during transformation at elevated temperatures than the marine and terrestrial kerogens. The Finitial of these kerogens can be correlated with their Fmax, and may provide some insight into their oil generation potential. Liptinite isolated from humic coals exhibits both higher Finitial and Fmax, consistent with their higher oil generation potential than vitrinite and fusinite (inertinite). Fmax of isolated vitrinites shows a positive correlation with their maturity below 1.2%Ro and a negative correlation at higher maturity, implying the decline of their oil generation potentials at high maturity. Green River kerogen (GR) and marinite exhibit prolonged positive temporal fluorescence alteration (TFA) whereas other kerogens and liptinite display only subtle TFA under ambient conditions. The extents of alteration are enhanced by temperatures. TFAs of the GR kerogen in argon relative to that in air at ambient and elevated temperatures are significantly reduced, confirming oxidation accounts for the TFA. The temperature dependence (thermal quenching) of the kerogen between 25 and 200 °C is comparable to crude oils and exhibits hysteresis in the heating or cooling cycle. The present study has demonstrated that the fluorescence of bulk kerogens at elevated temperatures before or during organic transformation can be used to characterize organic matter.