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 <(δ13CiC4H10 >
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 <(δ13CC3H8 >
δ13CiC4H10) <
δ13CnC4H10 <(δ13C 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.