5.2. The changing of carbon isotopes in kerogen, bitumen,
expelled oil and gaseous hydrocarbons during thermal evolution stages
During thermal evolution, kerogen, expelled oil, bitumen, and gaseous
products undergo a series of thermal cracking processes. Among them, the
original kerogen (kerogen0) and original bitumen
(bitumen0) exhibited a reactive nature (Tissot et al.,
1978; Jarvie et al., 2007). When Ts = 335–575 °C, expelled oil,
residual bitumen, and gaseous hydrocarbons of C2+ could
not only be seen as products but also as reactants participating in the
pyrolysis process. From the perspective of pyrolysis, only methane
constantly remains a product during the entire thermal evolution
process. Therefore, ignoring the intermediate reaction during the entire
thermal evolution process, the corresponding reaction formula is as
follows: kerogen0 (original) +
bitumen0 (original) → kerogenr (residual
kerogen) + expelled oil (generated) + bitumenn+r(generated + residual) + C2+ (generated + residual) +
CH4 (generated) . In this reaction formula, the thermal
evolution pathways were analyzed based on the corresponding change
characteristics of carbon isotopes. Based on the principle of chemical
kinetics, with changes in the coal-forming environment, organic matter
type, maturity, etc., the carbon isotope composition of gases could also
be changed. Simultaneously, a series of processes could produce
intermediates in the cracking of kerogen, residual bitumen, or liquid
hydrocarbons, and thus would result in the lighting of
δ13C1~5, i.e., during
thermal evolution of organic matter into hydrocarbons, the12C and 13C were primarily enriched
in the former and latter generated products, respectively (Kinnon et
al., 2010; Papendick et al., 2011; Golding et al., 2013).
In addition, the experimental error range of ±0.5‰ for carbon isotope
analysis was also considered in this analysis (Wu et al., 2018).
Overall, the δ13Ckerogen0 <
δ13C kerogenr and
δ13Cbitumen0 <
δ13C bitumen0+r, and the former degree
of fractionation (δ13C kerogenr-δ13Ckerogen0 = 0.1–0.8, the mean
value = 0.5) was less than the latter degree of fractionation
(δ13C bitumen0+r-δ13Cbitumen0 = 0.1–1.3, the mean
value = 1.11), implying that kerogen and bitumen as reactants in the
thermal evolution process would get heavy at their corresponding carbon
isotope. In contrast, the degree of heaviness determined the strength of
the reaction (Mahlstedt and Horsfield, 2012). Influenced by thermal
action, the carbon isotope of kerogen tended to become heavier in
general. There was no significant difference in the weight gained
between the lower maturity-maturity stage and the
higher-maturity-post-maturity stage, indicating that the reaction
strength of kerogen did not change much during the entire thermal
evolution process. The main sources of bitumen were the residual bitumen
and bitumen generated from kerogen by pyrolysis (Jarvie et al., 2007).
At the lower to higher maturity stage, the carbon isotope of bitumen
became heavier, suggesting that thermal evolution had a greater impact.
Similarly, the original and generated bitumen were constantly parted
during thermal evolution. At the post-maturity stage, the carbon isotope
of the residual bitumen was fundamentally the same as that in the
original bitumen, indicating that the amount of bitumen generated from
kerogen was almost exhausted and reached a certain equilibrium. The
remaining bitumen was that which was generated from kerogen cracking,
which could also be proved by their contents, as shown in Fig. 3.
The main formation path of expelled oil was the cracking of bitumen, and
the carbon isotopic composition of the expelled oil was essentially
unchanged and was lighter than that in kerogen and bitumen. The yield of
expelled oil was lower at the lower maturity stage and was almost in
equilibrium at the higher maturity-post-maturity stage. Therefore, it
can be seen that the expelled oil, as both product and reactant, was in
a state of “supply and demand balance” after the maturity stage;
indirectly, it showed that at the lower maturity stage, the reaction
equation was kerogen → expelled oil + gaseous hydrocarbons, and
dominated by the production of gaseous hydrocarbons (Tissot et al.,
1978; Jarvie et al., 2007). After the maturity stage, it was dominated
by the production of expelled oil. At this stage, the residual bitumen
decreased continuously, indicating that the generated and increased
expelled oil or gaseous hydrocarbons were further cracked into gaseous
hydrocarbons to ensure the equilibrium state of the expelled oil
(Castelli et al.,
1990;
Pepper and Corvi, 1995; Jarvie et al., 2007; Zheng et al., 2011; Qin et
al., 2014; Jiang et al., 2016).
Gaseous hydrocarbons can be generated either by kerogen or by further
pyrolysis of intermediate bitumen or expelled oil. Therefore, the
difference in the carbon isotopes of gaseous hydrocarbons was restricted
by the dynamic fractionation effect of the above two reactions. In other
words, their respective isotope dynamic fractionation was closely
related to the hydrocarbon formation mechanism of organic matter at
different thermal evolution stages. For example, methane was the
lightest carbon isotope in this study. Its generation pathway can be
divided into 1) direct generation from kerogen, 2) generation from the
cracking of bitumen, 3) generation from the cracking of expelled oil,
and 4) generation from the secondary cracking of C2+gases (Pepper and Corvi, 1995; Hill et al., 2003; Jarvie et al., 2007;
Behar et al., 2008).
In general, the carbon isotope of methane showed a particularly good
linear correlation with Ro , and the correlation equation was
δ13C1 = 4.632 Ro - 43.493 with
a correlation coefficient (R 2) of 0.9142,
indicating that the carbon isotope of 13C was enriched
continuously with thermal evolution. The corresponding enrichment degree
of δ13C1 reached 9.7‰. The
considerable carbon isotope dynamic fractionation also indicated that it
was the most sensitive to maturity; therefore, it could be used for gas
source correlation due to its reliability. We observed that atTS ≤400℃, i.e., the low maturity-maturity stage,
the enrichment degree of δ13C1 was
lower at 2.6‰. However, at TS ≥400 ℃, namely the
high-post-maturity stage, the enrichment degree of
δ13C1 increased rapidly to 9.7‰.
Altogether, the results indicate the lower maturity stage was primarily
controlled by kerogen and bitumen, and the main controlling factor was
expelled oil and C2+ gases after the maturity stage. It
was further proved that the formation mechanism of methane significantly
differed before and after 400 °C. Moreover, according to previous
studies, the secondary cracking of gaseous hydrocarbons was generally in
the order of higher carbon number to lower carbon number (Behar et al.,
1992; Jarvie et al., 2007; Sun et al., 2019b). When gaseous hydrocarbons
with higher carbon numbers are cracked to gaseous hydrocarbons with
lower carbon numbers, the carbon isotope of gaseous hydrocarbons with
this carbon number deviates from the normal evolutionary track.
According to the variation in the values of
δ13Cn-δ13Cn-1(n ≥ 2) with increasing Ro (Table 3), it can also be seen that
the different corresponding difference values first decreased and then
increased. The Ro corresponding to the lowest point was
successively advanced; the larger the value of n , the smaller was
the Ro at the low point of the difference value. For example, the
values of
δ13C4–δ13C3demonstrated little change at Ro = 1.09% and 1.65%, those of
δ13C3–δ13C2showed a significant change between Ro = 1.65% and 1.93%, while
those of
δ13C2–δ13C1showed a significant change after Ro = 2.3%. In the lower
evolution stage, Ro < 1.65%, the cracking of gaseous
hydrocarbons with higher carbon numbers was not significant. In the
higher evolution stage, after Ro > 1.65%, the
gaseous hydrocarbons with a higher carbon number would crack into
gaseous hydrocarbons with a lower carbon number successively, resulting
in an increase in the difference value in the carbon isotope. Therefore,
it was further explained that the formation mechanism of methane was
different at different thermal evolution stages. Among them, the
reactions corresponded to reaction processes 1 and 2 at the lower
maturity stage, and reaction 2 was dominant. Reaction processes 1, 2,
and 3 occurred at the maturity-higher maturity stage, and reaction 3 was
dominant. At the post-maturity stage, reaction processes 1, 2, 3, and 4
all occurred, with reaction 4 being dominant.
Table 3 shows the variation in the values of
δ13Cn–δ13Cn-1(n ≥ 2) with increasing Ro