For the carbon isotope values of expelled oil, the observed values were -27.5‰, -27.5‰, -27.4‰, -27.3‰, -26.9‰, -27.2‰, and -27.0‰, with a mean of -27.3‰. Compared with the variation characteristics of carbon isotopes for kerogen, it tended to be heavier first and then lighter. Moreover, the highest value appeared at Ro = 2.3% (Ts = 480 °C), which reached -26.9‰.
The changed characteristics of carbon isotopic values on the residual oil were characterized by getting heavier first (when Ro = 0.82–1.93%, δ13Cbitumen = -26.3–26.1‰), and then changing sharply to the heaviest value (Ro = 2.3%, δ13Cbitumen = -25.8‰), after which it became lighter sharply to a value of -27.0‰ whenRo = 2.3–2.56%, and further lightened slowly to -27.5‰.