Fig. 4 The δ13C values of kerogen, expelled oil, bitumen and gaseous hydrocarbons at different simulated temperatures
The variation trends of carbon isotopes in gaseous hydrocarbon products with Ro are shown in Fig. 5, including the δ13C values of CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, and nC5H12. When Ro = 0.82–3.24%, δ13CCH4 = -39.7‰, -39.1‰, -37.1‰, -33.1‰, -32.2‰, -30.3‰, and -30.0‰ respectively, becoming heavier continuously in general. The values of δ13CC2H6 were -29.8‰, -28.3‰, -29.0‰, -27.7‰, -27.5‰, -24.3‰, and -19.2‰; there was a tendency for carbon isotope values to become lighter once at Ro = 1.09–1.65%, and all showing a tendency to be heavier at the other stages. The values of δ13C C3H8 ranged from -29.3‰ to -20.7‰ at Ro = 0.82–2.3%, except when the values changed from -27.9‰ to -29.5‰ and all the other stages became heavier. The values of δ13CiC4H10, were -28.5‰, -28.6‰, and -28.7‰ at Ro = 0.82–1.65%, which sharply increased to -22.9‰ when Ro = 1.65–1.93%. The variation tendency of δ13 CnC4H10 was consistent with that of δ13 CiC4H10, which was almost unchanged when Ro = 0.82–1.65%, and changed rapidly from -28.7‰ to -17.1‰ when Ro = 1.65–1.93%. The values of δ13 CiC5H12 and δ13CnC5H12 were only measured whenRo = 0.82–1.65%, and both showed a trend of becoming heavier first and then lighter in a small range. In general, their carbon isotope values showed no obvious changes, which were -28.8‰, -27.2‰, and -28.6‰, and -28.8‰, -27.6‰, and -27.9‰, respectively. Overall, all the carbon isotopes of gaseous hydrocarbons tended to become heavier gradually, and among them, the carbon isotope values of methane were the lightest. Overall, the carbon sequence order was δ13CCH4 < δ13CC2H6 < δ13CC3H8 < δ13CiC4H10 < δ13CnC4H10 at Ro ≥ 1.93% (Ts ≥ 455 °C) and δ13CCH4< δ13CC2H6 < δ13CC3H8 <13CiC4H10 > δ13CnC4H10) < δ13CiC5H12 = δ13CnC5H12 at Ro = 0.82%, which almost followed the normal carbon sequence of δ13C1 < δ13C2 < δ13C3 < δ13C4 < δ13C5 with increasing thermal evolution (Behar et al., 1992;Jarvie et al., 2007). In addition, the carbon sequence in the order of δ13CCH4 < δ13CC2H6 <13CC3H8 > δ13CiC4H10) < δ13CnC4H10 <13C iC5H12 > δ13CnC5H12) , at Ro = 1.09% and δ13CCH4 < δ13CC2H6 > (δ13CC3H8 < δ13CiC4H10 = δ13CnC4H10 <δ13C iC5H12 < δ13CnC5H12 at Ro = 1.65%, which was consistent with the evolutionary trend of δ13C1 < δ13C2 > δ13C3 < δ13C4 < δ13C5.