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\begin{document}

\title{Evolution mechanism of Pinghu Formation coal-measure source rock from
the Xihu Sag, East China Sea basin: Evidence from carbon isotopic
evolution of kerogen, oil, and gas during hydrous pyrolysis}



\author[1]{Qiang Cao}%
\author[2]{Jiaren Ye}%
\author[1]{Yongchao Lu}%
\author[3]{Yang Tian}%
\author[4]{Jinshui Liu}%
\author[1]{Chenjie Xu}%
\author[1]{Hanwen Yu}%
\author[3]{Lina Sun}%
\affil[1]{China University of Geosciences, Ministry of Education}%
\affil[2]{China University of Geosciences}%
\affil[3]{Yangtze University}%
\affil[4]{CNOOC Shanghai Branch}%


\vspace{-1em}



  
  \date{May 25, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Semi-open hydrous pyrolysis experiments on coal-measure source rocks in
the Xihu Sag were conducted to investigate the carbon isotope evolution
of kerogen, bitumen, generated expelled oil, and gases with increasing
thermal maturity. Seven experiments corresponding were conducted at 335
\selectlanguage{ngerman}°C-575 °C, while other experimental factors, such as the heating time
and rate, lithostatic and hydrodynamic pressures, and columnar original
samples were kept the same. The results showed that the simulated
temperatures were positive for the measured vitrinite reflectance
(\emph{Ro}), with a correlation coefficient
(\emph{R}\textsuperscript{2}) of 0.9861. With increasing \emph{Ro},
lower maturity, maturity, higher maturity, and post-maturity stages
occurred at simulated temperatures (\emph{Ts}) of 335--360 °C, 360--400
°C, 400--480 °C, and 480--575 °C, respectively. Moreover, the carbon
isotopes of kerogen, bitumen, expelled oil, and gases were associated
with an increased \emph{Ro}. Ignoring the intermediate reaction process,
the thermal evolution process can be summarized as
\selectlanguage{english}\emph{kerogen\textsubscript{0} (original) + bitumen\textsubscript{0}
(original) - kerogen\textsubscript{r} (residual kerogen) + expelled oil
(generated) + bitumen\textsubscript{n+r} (generated + residual) +
C\textsubscript{2+} (generated + residual) + CH\textsubscript{4}
(generated)}. Among these, bitumen, expelled oil, and
C\textsubscript{2-5} acted as reactants and products, whereas kerogen
and methane were the reactants and products, respectively. Thus, the
reaction and production mechanisms of carbon isotopes can be obtained
based on their changing degree and yields in kerogen, bitumen, expelled
oil, and gases. Furthermore, combining the analysis of geochemical
characteristics of the Pinghu Formation coal-oil-type gas in actual
strata with these pyrolysis experiments, it was identified that this
area also had substantial development potential.%
\end{abstract}\selectlanguage{ngerman}%



\sloppy


Evolution mechanism of Pinghu Formation coal-measure source rock from
the Xihu Sag, East China Sea basin: Evidence from carbon isotopic
evolution of kerogen, oil, and gas during hydrous pyrolysis

Qiang Cao\textsuperscript{1}, Jiaren Ye*\textsuperscript{, 1}, Yongchao
Lu\textsuperscript{1}, Yang Tian\textsuperscript{2, *}, Jinshui
Liu\textsuperscript{3}, Chenjie Xu\textsuperscript{1}, Hanwen
Yu\textsuperscript{1}, Lina Sun\textsuperscript{2}

\textsuperscript{1}Key Laboratory of Tectonics and Petroleum Resources,
China University of Geosciences, Ministry of Education, Wuhan 430074,
China;

\textsuperscript{2}Hubei Cooperative Innovation Center of Unconventional
Oil and Gas, Yangtze University, Wuhan, Hubei 430100, China;

\textsuperscript{3} CNOOC Shanghai Branch,Shanghai 200030, China.

\textsuperscript{*}Corresponding author (First). Jiaren Ye; Tel. 86-27-
67883295; Email address:

\emph{jrye@cug.edu.cn;}

Corresponding author (Second). Yang Tian; Tel. 86-27- 69111832; Email
address: \emph{519974963@qq.com.}

\textbf{Abstract:} Semi-open hydrous pyrolysis experiments on
coal-measure source rocks in the Xihu Sag were conducted to investigate
the carbon isotope evolution of kerogen, bitumen, generated expelled
oil, and gases with increasing thermal maturity. Seven experiments
corresponding were conducted at 335 \selectlanguage{ngerman}°C, 360 °C, 400 °C, 455 °C, 480 °C,
525 °C, and 575 °C, while other experimental factors, such as the
heating time and rate, lithostatic and hydrodynamic pressures, and
columnar original samples were kept the same. The results showed that
the simulated temperatures were positive for the measured vitrinite
reflectance (\emph{Ro} ), with a correlation coefficient (\emph{R}
\textsuperscript{2}) of 0.9861. With increasing \emph{Ro} , lower
maturity, maturity, higher maturity, and post-maturity stages occurred
at simulated temperatures (\emph{Ts} ) of 335--360 °C, 360--400 °C,
400--480 °C, and 480--575 °C, respectively. The increasing gas
hydrocarbons with increasing \emph{Ro} reflected the higher gas
potential. Moreover, the carbon isotopes of kerogen, bitumen, expelled
oil, and gases were associated with an increased \emph{Ro} ; among
gases, methane was the most sensitive to \emph{Ro} . Ignoring the
intermediate reaction process, the thermal evolution process can be
summarized as\selectlanguage{english}\emph{kerogen\textsubscript{0} (original) +
bitumen\textsubscript{0}(original) - kerogen\textsubscript{r} (residual
kerogen) + expelled oil (generated) + bitumen\textsubscript{n+r}
(generated + residual) + C\textsubscript{2+} (generated + residual) +
CH\textsubscript{4}(generated)} . Among these, bitumen, expelled oil,
and C\textsubscript{2-5} acted as reactants and products, whereas
kerogen and methane were the reactants and products, respectively.
Furthermore, the order of the carbon isotopes during the thermal
evolution process was identified as: \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C
\textsubscript{1} \textless{}\textsuperscript{13}C \textsubscript{2-5}
\textless{} \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C \textsubscript{expelled oil}
\textless{} \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C \textsubscript{bitumen} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C \textsubscript{kerogen}. Thus, the reaction and
production mechanisms of carbon isotopes can be obtained based on their
changing degree and yields in kerogen, bitumen, expelled oil, and gases.
Furthermore, combining the analysis of geochemical characteristics of
the Pinghu Formation coal-oil-type gas in actual strata with these
pyrolysis experiments, it was identified that this area also had
substantial development potential. Therefore, this study provides
theoretical support and guidance for the formation mechanism and
exploration of oil and gas in the Xihu Sag.

\textbf{Keywords:}coal-measure source rock; Xihu Sag; semi-open hydrous
pyrolysis; carbon isotope; maturity; reaction mechanism

\textbf{1. Introduction}

The East China Sea Shelf Basin (ECSSB) is an important petroliferous
basin in the eastern coastal areas of China, and the corresponding
coal-measure strata of Neogene are thick and widely distributed. In
recent years, the increasing intensity of research and development
pertaining to oil and gas exploration, have revealed the crucial role
played by the relevant coal-measure source rock in the formation of oil
and gas resources in this area (Zhu et al., 2012; Ju et al., 2016; Chen
et al., 2017; Hao et al., 2018; Zhu et al., 2019). The Xihu Sag is
located northeast of the ECSS, and the source rocks are primarily
developed in the Pinghu, Huagang, and Longjing formations (Su et al.,
2018; Zhu et al., 2012). In particular, the coal rock of the Pinghu
Formation is the primary source rock in the Xihu Sag and is
characterized by its large thickness and wide distribution (Zhang et
al., 2006; Lin et al., 2017; Xie et al., 2018; Cheng et al.,2019; Wang
et al., 2021). Thus, determining the formation mechanism of thermal
evolution is particularly important for understanding oil and gas
evolution in this area.

However, previous studies on the thermal evolution of organic matter
have concentrated primarily on basic geology (such as petroleum sources,
evolution and origin of paleo-pressure, diagenetic and reservoir-forming
processes, and geochemical characteristics) (Jiang et al., 2016; Hao et
al., 2018; Cheng et al., 2020; Su et al., 2019; Wang et al., 2020; Xu et
al., 2020). These studies primarily utilized geological and geochemical
methods, and the research contents were often limited by static
characteristics such as hydrocarbon generation potential and maturity of
source rocks (Tissot et al., 1974). In addition, during the process of
geological burial in source rocks, the generation of oil and gas, as
well as their occurrence in the ground, were the comprehensive responses
of various physical and chemical actions. Compared to the changeable and
complex geological evolution process and the scale of human life, the
evolution of organic matter was exceedingly slow, such that it was
impossible to observe the thermal evolution process with variable
temperature and pressure, such as hydrocarbon cracking in the system.
Thus, it is extremely challenging to accurately describe the generation
of oil and gas (Lewan et al., 1979; Lewan, 1997; Braun and Bumham, 1990;
Schimmelmann et al.,2001;Hill et al., 2007). With the development and
progress of science and technology and petroleum geology theory,
scientists have gradually realized the importance of investigating
hydrocarbon generation and expulsion of source rocks during the thermal
evolution process (Connan, 1974; Pepper et al., 1995a and 1995 b).
However, there are hurdles such as dissimilar understanding and lack of
accord among researchers on hydrocarbon generation and expulsion
simulations, potential calculation methods, and migration mechanisms;
thus, it became necessary to simulate the organic evolution process in
source rocks through a simulated pyrolysis experiment (Behar et al.,
1992; Leif et al., 2000; Sun et al., 2015; Sun et al., 2019a). The
principle of the hydrocarbon generation and expulsion simulated
experiments was to use short time and high-temperature conditions to
simulate and observe the evolution of organic matter under long-term and
low-temperature conditions in actual geological conditions (Castelli et
al., 1990; Leif et al., 2000; Sun et al., 2019b). Thus, through
simulation experiments of hydrocarbon generation and expulsion in the
laboratory, we could obtain information, such as the amount of
hydrocarbon generation in different evolution stages and the composition
characteristics of gases at different maturity levels, that was
otherwise difficult to obtain in the natural evolution process. However,
there have been few studies on the pyrolysis experiments of coal-measure
source rocks of the Pinghu Formation in the Xihu Sag. In the closed
simulated system, based on the methods of fluid inclusion analysis,
petroleum geochemistry, and rock gold-tube pyrolysis on the condensates
in the Pinghu slope belt of the Xihu Sag, Su (2020) found that coal was
evaluated as source rock and inclined to gas and oil generation at
moderate maturity. In an open simulated system, Zhu (2020) used the
Rock-Eval pyrolysis method to analyze the organic geochemical and
petrographic characteristics of Paleogene coals and organic-rich
mudstones in the Xihu Sag and found that two different regions had fair
to excellent hydrocarbon generative potential but varied in the
hydrocarbon phase. However, almost no pyrolysis experiments have been
conducted on the coals of the Pinghu Formation under the semi-open
system.

Moreover, based on the principle of chemical kinetics, different
zero-point energies between carbon isotopes cause differences in
chemical activity, which would result in the formation of structural
parts or chemical bonds (\textsuperscript{12}C--\textsuperscript{12}C)
rich in light carbon isotopes with a preference to participate in the
reactions. During hydrocarbon generation, it was more difficult for
branched chains with more \textsuperscript{13}C to fall off than those
with less \textsuperscript{13}C. Thus, during the thermal evolution of
organic matter into hydrocarbons, the first products to be generated
were relatively enriched in \textsuperscript{12}C, and subsequent
products were relatively enriched in \textsuperscript{13}C (Krouse et
al., 1988; Waples, 2000; Zhao et al., 2015; Fu et al., 2020). The
contributions of different genesis and origin source rocks to oil and
gas generation may be influenced by the thermal evolution, depositional
environment, and mixing of organic matter from multiple source rocks.
Simultaneously, the differences in organic matter with specific sources
could be corrupted or integrated by the measured compositions of bulk
isotopes and molecules (Xu et al., 2020). Moreover, studying the
characteristics of carbon isotopes can reveal information of parent
material sources and also assess natural gas with different origins and
thermal evolution degrees (Prinzhofer and Huc, 1995; Cramer et al.,
1998; Dai et al., 2009; Takahashi et al., 2014; Wu et al., 2019; Xu et
al., 2020). Thus, it is of great significance to comprehensively study
the carbon isotopic composition of kerogen, bitumen, and gas-liquid
products during thermal evolution.

This study focused on the pyrolysis of coal-measure source rocks in the
Pinghu Formation under a semi-open system with varying simulated
conditions (such as temperature, static pressure, fluid pressure, time,
water addition, and original columnar samples) to determine the main
factors affecting the evolution of organic matter. The advantage of the
semi-open system was that it considered several influencing factors and
was closer to the actual geological conditions. It has also been known
from a large number of pyrolysis experiments domestically and
internationally that the hydrocarbon generation potential of source
rocks can be determined by the content of organic matters in source
rocks. In contrast, the hydrocarbon generation and expulsion
characteristics of source rocks were affected by many factors, including
temperature, pressure, hydrocarbon generation space and porosity, and
presence of water and catalysts (Lewan et al., 1979; Behar et al., 1992;
Schimmelmann et al., 2001; Sun et al., 2015; Sun et al., 2020). Thus,
based on the hydrous pyrolysis of coal-measure source rocks under the
influence of time, temperature, and pressure, we can define the
processes of hydrocarbon generation, thermal characteristics of kerogen,
composition and evolution of gas and liquid products, characteristics of
carbon isotopes, and their response relationships with maturity. This
can in turn facilitate the analysis and evaluation of the developmental
situation, hydrocarbon generation potential, hydrocarbon generation
material, and degree of thermal evolution in coal-measure source rocks
in the Pinghu Formation, and to provide theoretical support and guidance
for oil and gas exploration in the Xihu Sag.

\textbf{2. Geological setting}

The ECSSB was developed on the broad continental shelf of the southeast
Eurasian continental segment; to its west is the Zhemin upwarping folded
zone, and it is separated from the Okinawa Trough, Ryukyu island arc,
and Ryukyu Trench by the Diaoyu Island upwarping folded zone in the
east. The north is bordered by a ray from the Yangtze River to Jizhou
Island, and the north is bordered by a ray from the Guangdong to Taiwan
provinces. The basin is approximately 1,400 km long from north to south
and 250--300 km wide from east to west, with a total area of
approximately 26.7 \selectlanguage{ngerman}× 10\textsuperscript{4} km\textsuperscript{2}.
Influenced by the subduction of the Philippine plate to the Eurasia
plate during the Eocene to Oligocene period, the ECSSB is in a state of
back-arc spreading. Simultaneously, seismic data showed apparent
extension fracture activity near the NE direction, which resulted in the
formation of a structural framework with nearly east-west zoning and a
nearly north-south block in the plane. In addition, the ECSSB can be
divided into three secondary building units: western geotectogene,
central uplift belt, and eastern geotectogene from west to east. Among
them, the eastern geotectogene can be classified into Diaobei Sag, Xihu
Sag, and Fujiang Sag from south to north (Zhu et al., 2012; Ju et al.,
2016; Chen et al., 2017; Hao et al., 2018; Zhu et al., 2019; Su et al.,
2020) (Fig. 1).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Fig. 1 Sketch map showing geographical position of tectonic units and
oil and gas fields in the Xihu Sag, East China Sea Shelf basin

The Xihu Sag is located northeast of the ECSSB and belongs to a
secondary sag of the western geotectogene. The Xihu Sag is approximately
400 km long from south to north and 100 km wide from east to west; it
has a total area of approximately 5.18 \selectlanguage{ngerman}×
10\textsuperscript{4}km\textsuperscript{2}. The sedimentary thickness in
the sag is approximately 9--15 km, making it the largest sedimentary sag
in the ECSSB; in particular, the sedimentary characteristics indicate a
great potential for oil and gas exploration and development as observed
in previous studies.

The sedimentary strata drilled in the Xihu Sag were primarily the
Paleogene and Neogene strata, during which there were many depositional
hiatuses and regional unconformities. The Paleogene Pinghu Formation in
the Xihu Sag underwent multiple tectonic movements and was deposited at
the stage of tectonic thermal subsidence in the post-rift period.
Consequently, the primary sedimentary facies are clastic rocks of the
transitional phase. The Pinghu Formation is divided into three members
from bottom to top, and each member has obvious sedimentary cycles of
fine sediment in the bottom and coarse sediment on the upside. The
lithology primarily comprises of yellow fine sandstone, gray siltstone,
gray mudstone, and coal, with a relatively fine grain size. The distinct
lithostratigraphic characteristics of the Pinghu Formation in the ECSSB
were affected by tectonic and sedimentary patterns, and the overall
lithology is finer from west to east (Zhang et al., 2006; Zhu et al.,
2012; Lin et al., 2017; Cheng et al., 2019; Wang et al., 2021).

\textbf{3. Experimental and analytical methods}

\textbf{3.1. Representative sample selection}

Three core samples of coal-measure dark mudstone, the main type of
source rock, were collected from the Upper Pinghu Formation in Wells K1,
K3, and K4 from 4,204, 3,236.7, and 4,350 m, respectively (Fig. 1).
These samples were used for total organic carbon (TOC) content,
Rock-Eval pyrolysis, and vitrinite reflectance (\emph{Ro} )
measurements. Only the sample from Well K4 was suitable for the
formation thermal simulation experiment. Thus, the dark mudstones used
in this study were collected from the upper Pinghu Formation in the K4
well (4,350--4,352 m). The basic geochemical data were: TOC: 0.68\%;
hydrogen index (HI): 49 mg/g TOC; \emph{Ro} : 0.75\%; and organic matter
was type \selectlanguage{english}III, which also corresponds to the typical dark mudstone in the
Xihu Sag. To ensure sample uniformity, the columnar core used in the
simulation experiment was a cylindrical core column formed by further
pressing after crushing the samples to 40--60 mesh.

\textbf{3.2. Experiments}

The simulation experiment was conducted on the DK-\selectlanguage{english}II type
(third-generation) formation porosity thermocompression simulation
experiment instrument from the Wuxi Research Institute of Petroleum
Geology, Sinopec. The instrument had the advantages of the simulated
experimental conditions approaching the actual geological conditions,
retaining the original columnar and pore space, and obtaining accurate
experimental data (Zheng et al., 2009; Zheng et al., 2011; Qin et al.,
2014; Ma et al., 2017).

Therefore, to investigate the hydrocarbon generation potential of coal
source rocks in the Pinghu Formation, we designed an experimental
process based on the parameters of sedimentary and burial history,
thermal evolution history, depth, and lithostatic pressure. In addition,
combined with the simulated temperature, time, and pressure of the
instrument, the experimental process has been detailed subsequently. 1)
Experimental parameter setting: Different simulation depths were set to
realize the results of simulating the different thermal evolution
stages. As a result, the simulated depths were 2.8, 3.9, 4.45, 5.2, 5.9,
6.2, and 6.4 km, and the corresponding simulated temperatures were 335
degC, 360 degC, 400 degC, 455 degC, 480 degC, 525 degC, and 575 degC, respectively.
The heating rate of the experiment was 1 degC/min, and after reaching the
target temperature, the temperature was retained at the target
temperature for 48 h. 2) Operation of the experimental instrument: After
pressing the start button, the experiment commenced. 3) Acquisition and
quantification of the pyrolysis products: The gaseous hydrocarbons were
collected first, followed by separation and quantification of the
expelled oil and residual water. Finally, the solid residues were
extracted using chloroform to obtain the content of chloroform asphalt
``A,'' which was defined as residual bitumen (Ma et al., 2017).

Stable carbon isotopes of kerogen, expelled oil, residual bitumen, and
gaseous hydrocarbons at every simulated temperature point were
determined using a stable isotope ratio mass spectrometer (Delta Plus V
and Mat-253; Thermo Fisher Scientific, Waltham, MA, USA). The carbon
isotope values were reported as \selectlanguage{greek}δ-\selectlanguage{english}values, which are expressed as
\selectlanguage{greek}δ\selectlanguage{english}\textsubscript{sample}=
(R\textsubscript{sample}/R\textsubscript{standard}-1) \selectlanguage{ngerman}× 1,000 (\selectlanguage{english}%0) (where
R denotes the ratio of \textsuperscript{13}C/\textsuperscript{12}C). The
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C values were normalized to the Pee Dee Belemnite
(PDB) (Sun et al., 2015; Wu et al., 2019).

\textbf{4. Results}

\textbf{4.1. Pyrolysis products}

The correlation between the simulated temperature and measured vitrinite
reflectance (\emph{Ro} ) is shown in Fig. 2. The corresponding
\emph{Ro}to these simulated temperatures of 335 \selectlanguage{ngerman}°C, 360 °C, 400 °C, 455
°C, 480 °C, 525 °C, and 575 °C were 0.82\%, 1.09\%, 1.65\%, 1.93\%,
2.3\%, 2.56\%, and 3.24\%, respectively. A clear linear positive
correlation between them with a correlation coefficient (\emph{R}
\textsuperscript{2}) of 0.9861 was observed. Thus, this pyrolysis
experiment could be seen as a continuous thermal evolution process with
increasing maturity. Next, we used the value of the measured\emph{Ro}to
replace the simulated temperatures (Wu et al., 2018; Sun et al., 2019a).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

Fig. 2 The correlation between the simulated temperature and measured
vitrinite reflectance (\emph{Ro} )

The changes in gaseous hydrocarbons, expelled oil, and residual bitumen
with increasing \emph{Ro} are shown in Fig. 3. With an increase
in\emph{Ro} , the gaseous hydrocarbon yield tended to increase
gradually. When \emph{Ro} = 0.82--1.09\%, the yield of gaseous
hydrocarbons increased slowly with a smaller change in amplitude. When
\emph{Ro} = 1.65--2.56\%, the yield of gaseous hydrocarbons increased
rapidly at a constantly increasing rate, which had a larger increase in
amplitude. When \emph{Ro} = 2.56--3.24\%, the yield of gaseous
hydrocarbons increased sharply with an increase in amplitude = 124.74\%.

Overall, the yield of expelled oil first reached a specific value and
remained constant at the corresponding value. The yield of expelled oil
at \emph{Ro} = 0.82--1.09\% and corresponding simulated temperature
(\emph{Ts} ) = 335--360  increased rapidly from 19.36 to 46.76 mg/g
TOC, with an increased amplitude of 141.53\%. When \emph{Ro} =
0.82--1.09\% (\emph{Ts} = 335--360 ), the high amplitude of expelled
oil increased slowly from 46.76 to 50.16 mg/g TOC compared with the
previous thermal evolution stage. At the stage where \emph{Ro}
\textgreater{} 1.65\%, i.e., after the simulated temperature was higher
than 400 , the yield of expelled oil was 50.16, 50.07, 50.25, 50.18,
and 50.22 mg/g TOC, respectively. Therefore, the yield of the expelled
oil was constant.

For residual bitumen, i.e., the yield of residual oil, a continuously
decreasing tendency was observed. When \emph{Ro} = 0.82--1.09\%
(\emph{Ts} = 335--360 \selectlanguage{ngerman}°C, the medium maturity stage), the residual
bitumen decreased from 17.78 to 17.01 mg/g TOC with increasing\emph{Ro}
; however, the change was not apparent. When \emph{Ro} = 1.09--1.65\%,
the residual bitumen suddenly decreased from 17.01 to 5.7 mg/g TOC with
a decreased amplitude of 66.49\%. When \emph{Ro} = 1.65--2.3\%, a
continuously declining trend slowly occurred in the yield of residual
bitumen, with values of 5.70, 3.41, and 3.55 mg/g TOC. Thereafter, when
\emph{Ro} \textgreater{} 2.56\%, the residual bitumen decreased from
1.35 to 0.47 mg/g TOC, close to 0.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

Fig. 3 The changing of gaseous hydrocarbons, expelled oil and residual
bitumen with the increasing \emph{Ro}

\textbf{4.2. Stable carbon isotope composition}

The \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C values of kerogen, expelled oil, bitumen, and
gaseous hydrocarbons are listed in Table 1 and Fig. 4.

The \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C values of kerogen and bitumen in the original
sample (\emph{Ro} = 0.75) were -26%0 and -27.6%0, respectively. With an
increase in \emph{Ro} , the \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C values of kerogen
were -25.5%0, -25.3%0, -25.4%0, -26.1%0, -25.6%0, -25.4%0, and -25.2%0, with a
mean of -25.5%0. In general, there was a tendency first to become lighter
and then heavier. When \emph{Ro} = 1.93\%, the carbon isotope of kerogen
was the lightest, with a value of -26.1%0.

Table 1 The \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C values of kerogen, expelled oil,
bitumen and gaseous hydrocarbons at different simulated temperatures\selectlanguage{english}
\begin{longtable}[]{@{}llllllllllll@{}}
\toprule
Temperature & Ro & Kerogen & Expelled oil & Bitumen &
CH\textsubscript{4} & C\textsubscript{2}H\textsubscript{6} &
C\textsubscript{3}H\textsubscript{8} &
iC\textsubscript{4}H\textsubscript{10} &
nC\textsubscript{4}H\textsubscript{10} &
iC\textsubscript{5}H\textsubscript{12} &
nC\textsubscript{5}H\textsubscript{12}\tabularnewline
\midrule
\endhead
() & (\%) & \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}CPDB(%0) &
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}CPDB(%0) & \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}CPDB(%0) &
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}CPDB(%0) & \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}CPDB(%0) &
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}CPDB(%0) & \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}CPDB(%0) &
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}CPDB(%0) & \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}CPDB(%0) &
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}CPDB(%0)\tabularnewline
Unheated & 0.75 & -26 & & -27.6 & & & & & & &\tabularnewline
335 & 0.82 & -25.5 & -27.5 & -26.3 & -39.7 & -29.8 & -29.3 & -28.5 &
-28.9 & -28.8 & -28.8\tabularnewline
360 & 1.09 & -25.3 & -27.5 & -26.2 & -39.1 & -28.3 & -27.9 & -28.6 &
-28.2 & -27.2 & -27.6\tabularnewline
400 & 1.65 & -25.4 & -27.4 & -26.5 & -37.1 & -29.0 & -29.5 & -28.7 &
-28.7 & -28.6 & -27.9\tabularnewline
455 & 1.93 & -26.1 & -27.3 & -26.1 & -33.1 & -27.7 & -24.9 & -22.9 &
-17.1 & &\tabularnewline
480 & 2.30 & -25.6 & -26.9 & -25.8 & -32.2 & -27.5 & -20.7 & & &
&\tabularnewline
525 & 2.56 & -25.4 & -27.2 & -27.0 & -30.3 & -24.3 & & & &
&\tabularnewline
575 & 3.24 & -25.2 & -27.0 & -27.5 & -30.0 & -19.2 & & & &
&\tabularnewline
\bottomrule
\end{longtable}

For the carbon isotope values of expelled oil, the observed values were
-27.5%0, -27.5%0, -27.4%0, -27.3%0, -26.9%0, -27.2%0, and -27.0%0, with a mean
of -27.3%0. 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 \emph{Ro} = 2.3\% (\emph{Ts} =
480 \selectlanguage{ngerman}°C), which reached -26.9\selectlanguage{english}%0.

The changed characteristics of carbon isotopic values on the residual
oil were characterized by getting heavier first (when \emph{Ro} =
0.82--1.93\%, \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{bitumen} =
-26.3--26.1%0), and then changing sharply to the heaviest value
(\emph{Ro} = 2.3\%, \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{bitumen} =
-25.8%0), after which it became lighter sharply to a value of -27.0%0
when\emph{Ro} = 2.3--2.56\%, and further lightened slowly to -27.5%0.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

Fig. 4 The \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C values of kerogen, expelled oil,
bitumen and gaseous hydrocarbons at different simulated temperatures

The variation trends of carbon isotopes in gaseous hydrocarbon products
with \emph{Ro} are shown in Fig. 5, including the \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C
values of CH\textsubscript{4}, C\textsubscript{2}H\textsubscript{6},
C\textsubscript{3}H\textsubscript{8},
iC\textsubscript{4}H\textsubscript{10},
nC\textsubscript{4}H\textsubscript{10},
iC\textsubscript{5}H\textsubscript{12}, and
nC\textsubscript{5}H\textsubscript{12}. When \emph{Ro} = 0.82--3.24\%,
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{CH4} = -39.7%0, -39.1%0, -37.1%0,
-33.1%0, -32.2%0, -30.3%0, and -30.0%0 respectively, becoming heavier
continuously in general. The values of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{C2H6} were -29.8%0, -28.3%0, -29.0%0,
-27.7%0, -27.5%0, -24.3%0, and -19.2%0; there was a tendency for carbon
isotope values to become lighter once at \emph{Ro} = 1.09--1.65\%, and
all showing a tendency to be heavier at the other stages. The values of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C \textsubscript{C3H8} ranged from -29.3%0 to -20.7%0
at \emph{Ro} = 0.82--2.3\%, except when the values changed from -27.9%0
to -29.5%0 and all the other stages became heavier. The values of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{iC4H10}, were -28.5%0, -28.6%0, and
-28.7%0 at \emph{Ro} = 0.82--1.65\%, which sharply increased to -22.9%0
when \emph{Ro} = 1.65--1.93\%. The variation tendency of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13} C\textsubscript{nC4H10} was consistent with that
of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13} C\textsubscript{iC4H10}, which was almost
unchanged when \emph{Ro} = 0.82--1.65\%, and changed rapidly from -28.7%0
to -17.1%0 when \emph{Ro} = 1.65--1.93\%. The values of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13} C\textsubscript{iC5H12} and
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{nC5H12} were only measured
when\emph{Ro} = 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%0,
-27.2%0, and -28.6%0, and -28.8%0, -27.6%0, and -27.9%0, 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
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{CH4} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{C2H6} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{C3H8} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{iC4H10} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{nC4H10} at \emph{Ro} [?] 1.93\%
(\emph{Ts} [?] 455 \selectlanguage{ngerman}°C) and
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{CH4}\textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{C2H6} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{C3H8}
\textless{}\selectlanguage{greek}\textbf{(δ\textsuperscript{13}C\textsubscript{iC4H10}
\textgreater{} δ\textsuperscript{13}C\textsubscript{nC4H10})}\selectlanguage{english}
\textless{} \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{iC5H12} =
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}Cn\textsubscript{C5H12} at \emph{Ro} = 0.82\%,
which almost followed the normal carbon sequence of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{1} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{2} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{3} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{4} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{5} with increasing thermal
evolution (Behar et al., 1992;Jarvie et al., 2007). In addition, the
carbon sequence in the order of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{CH4} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{C2H6}
\textless{}\selectlanguage{greek}\textbf{(δ\textsuperscript{13}C\textsubscript{C3H8}
\textgreater{} δ\textsuperscript{13}C\textsubscript{iC4H10})}\selectlanguage{english}
\textless{} \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{nC4H10}
\textless{}\selectlanguage{greek}\textbf{(δ\textsuperscript{13}C \textsubscript{iC5H12}
\textgreater{} δ\textsuperscript{13}C\textsubscript{nC5H12})}\selectlanguage{english} , at
\emph{Ro} = 1.09\% and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{CH4}
\textless{} \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{C2H6} \textgreater{}
(\selectlanguage{greek}\textbf{δ\textsuperscript{13}C\textsubscript{C3H8} \textless{}
δ\textsuperscript{13}C\textsubscript{iC4H10})} =
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{nC4H10}
\textless{}\selectlanguage{greek}\textbf{δ\textsuperscript{13}C \textsubscript{iC5H12}
\textless{} δ\textsuperscript{13}C\textsubscript{nC5H12}}\selectlanguage{english} at \emph{Ro} =
1.65\%, which was consistent with the evolutionary trend of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{1} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{2} \selectlanguage{greek}\textbf{\textgreater{}
δ\textsuperscript{13}C\textsubscript{3}}\selectlanguage{english} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{4} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{5}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

Fig. 5 The variation trend of carbon isotopes on gaseous hydrocarbon
products with \emph{Ro}

\textbf{5. Discussion}

\textbf{5.1. Division of thermal evolution stages based on the pyrolysis
products}

The pyrolysis process can be divided into four stages based on the
variation characteristics of the yield of expelled oil, residual
bitumen, and gaseous hydrocarbons with increasing maturity (Fig. 6):
1)\emph{Ro} = 0.82--1.09\% (\emph{Ts} = 335--360 \selectlanguage{ngerman}°C), which was the
lower maturity stage dominated by the generation of liquid hydrocarbons.
This stage is characterized by rapid oil and lower gas generation
(Tissot et al., 1974; Lewan, 1997; Hill et al., 2007; Sun et al.,
2019b). The slope was used to measure the inclination of the steep
curve. In mathematics, the slope of a line is the same for any point,
and it is a measure of the direction and steepness of the line. The
slope of a line can be calculated using principles of algebra and
geometry. The slope of the curve at a point reflects how quickly the
variables of the curve change at that point. When the slope (\emph{k} )
is positive, a greater value indicates a more rapid increase. When
\emph{k} is negative, a smaller value indicates a more rapid decrease
(Matuszewski, 2006; Taylor et al., 2019). The slope of the curve for
increased hydrocarbon yield (\emph{k\textsubscript{n}} ) was equal to
(\emph{Yield\textsubscript{n+1}} - \emph{Yield\textsubscript{n}} ) /
(\emph{Ro} \textsubscript{n+1} - \emph{Ro} \textsubscript{n}), as shown
in Table 2. At this stage, the curve of the slope for increased expelled
oil (\emph{k\textsubscript{e1}} = (\emph{Yield\textsubscript{2}}
--\emph{Yield\textsubscript{1}} ) / (\emph{Ro} \textsubscript{2}
--\emph{Ro} \textsubscript{1})) reached 101.48, indicating the
characteristics of rapid expelled oil generation. 2) \emph{Ro} =
1.09--1.65\% (\emph{Ts} = 360--400 \selectlanguage{english}), which was the maturity stage
dominated by the generation of lighter liquid hydrocarbons and wet
gaseous hydrocarbons. At this stage, the yield of residual bitumen oil
decreased sharply (\emph{k\textsubscript{b}} = -20.20), and the yield of
expelled oil did not change significantly (\emph{k\textsubscript{e}} =
6.07, which was significantly lower than that in the first stage). The
yield of hydrocarbon gases began to show an increasing trend, and the
corresponding value of \emph{k} also increased rapidly from 14.52 to
39.39. 3) \emph{Ro} = 1.65--2.3\% (\emph{Ts} = 400--480 degC), the yields
of gaseous hydrocarbons increased continuously, expelled oil almost did
not change, and the residual oil decreased to a lower value, which could
be seen as the stage of thermal cracking to wet gases. 4) \emph{Ro} =
2.3--3.24\% (\emph{Ts} = 480--575 degC), which was the post-maturity
stage. A large amount of methane was produced, and the oil and gas
further cracked into methane. The increase in the amplitude of gaseous
hydrocarbon was larger, and the corresponding
\emph{k\textsubscript{g}}reached 172.89, providing proof of higher
methane production. However, the yield of expelled oil was further
reduced when the negative\emph{k\textsubscript{e}} and residual bitumen
were in equilibrium with\emph{k\textsubscript{b}} near zero.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

Fig. 6 The yield of expelled oil, residual bitumen and gaseous
hydrocarbons with the increasing maturity

Table 2 The slope to the curve of increased hydrocarbons yield
(\emph{k\textsubscript{n}} )\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
The range of Ts (\%) & The range of Ro (\%) & stage & Linear slope &
Linear slope of gaseous hydrocrabons (kg) & Linear slope of expelled oil
(ke) & Linear slope of residual bitumen (kb)\tabularnewline
\midrule
\endhead
335-360 & 0.82-1.09 & Lower maturity stage & k1 & 14.52 & 101.48 &
-2.85\tabularnewline
360-400 & 1.09-1.65 & Maturity stage & k2 & 39.39 & 6.07 &
-20.20\tabularnewline
400-480 & 1.65-2.30 & Higher maturity stage & k3 & 82.29 & 0.14 &
-3.31\tabularnewline
480-575 & 2.30-3.24 & Post-maturity stage & k4 & 172.89 & -0.03 &
-3.28\tabularnewline
\bottomrule
\end{longtable}

\textbf{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 (kerogen\textsubscript{0}) and original bitumen
(bitumen\textsubscript{0}) exhibited a reactive nature (Tissot et al.,
1978; Jarvie et al., 2007). When \emph{Ts} = 335--575 \selectlanguage{ngerman}°C, expelled oil,
residual bitumen, and gaseous hydrocarbons of C\textsubscript{2+} 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: \selectlanguage{english}\emph{kerogen\textsubscript{0} (original) +
bitumen\textsubscript{0} (original) - kerogen\textsubscript{r} (residual
kerogen) + expelled oil (generated) +
bitumen\textsubscript{n+r}(generated + residual) + C\textsubscript{2+}
(generated + residual) + CH\textsubscript{4} (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 \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{1\textasciitilde{}5},
i.e., during thermal evolution of organic matter into hydrocarbons,
the\textsuperscript{12}C and \textsuperscript{13}C 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 \selectlanguage{ngerman}±0.5\selectlanguage{english}%0 for carbon isotope
analysis was also considered in this analysis (Wu et al., 2018).
Overall, the \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{kerogen0} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C \textsubscript{kerogenr} and
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{bitumen0} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C \textsubscript{bitumen0+r}, and the former degree
of fractionation (\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C
\textsubscript{kerogenr}-\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{kerogen0}
= 0.1--0.8, the mean value = 0.5) was less than the latter degree of
fractionation (\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C
\textsubscript{bitumen0+r}-\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{bitumen0}
= 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
C\textsubscript{2+}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 \emph{Ro} , and the correlation equation was
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{1} = 4.632 \emph{Ro} - 43.493 with
a correlation coefficient (\emph{R} \textsuperscript{2}) of 0.9142,
indicating that the carbon isotope of \textsuperscript{13}C was enriched
continuously with thermal evolution. The corresponding enrichment degree
of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{1} reached 9.7%0. 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
at\emph{T\textsubscript{S}} [?]400, i.e., the low maturity-maturity
stage, the enrichment degree of \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{1}
was lower at 2.6%0. However, at \emph{T\textsubscript{S}} [?]400 , namely
the high-post-maturity stage, the enrichment degree of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{1} increased rapidly to 9.7%0.
Altogether, the results indicate the lower maturity stage was primarily
controlled by kerogen and bitumen, and the main controlling factor was
expelled oil and C\textsubscript{2+} gases after the maturity stage. It
was further proved that the formation mechanism of methane significantly
differed before and after 400 \selectlanguage{ngerman}°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
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{n}-\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{n-1}(n
[?] 2) with increasing \emph{Ro} (Table 3), it can also be seen that the
different corresponding difference values first decreased and then
increased. The \emph{Ro} corresponding to the lowest point was
successively advanced; the larger the value of \emph{n} , the smaller
was the \emph{Ro} at the low point of the difference value. For example,
the values of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{4}--\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{3}demonstrated
little change at \emph{Ro} = 1.09\% and 1.65\%, those of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{3}--\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{2}showed
a significant change between \emph{Ro} = 1.65\% and 1.93\%, while those
of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{2}--\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{1}showed
a significant change after \emph{Ro} = 2.3\%. In the lower evolution
stage, \emph{Ro} \textless{} 1.65\%, the cracking of gaseous
hydrocarbons with higher carbon numbers was not significant. In the
higher evolution stage, after \emph{Ro} \textgreater{} 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
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{n}--\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{n-1}(\emph{n}
[?] 2) with increasing \emph{Ro}\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
Simulated temperature() &
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{2}-\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{1}
&
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{3}-\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{2}
&
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{i4}-\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{3}
&
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{n4}-\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{3}
&
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{i5}-\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{i4}
&
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{n5}-\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{n4}\tabularnewline
\midrule
\endhead
335 & 9.9 & 0.5 & 0.8 & 0.4 & -0.3 & 0.1\tabularnewline
360 & 10.8 & 0.4 & -0.7 & -0.3 & 1.4 & 0.6\tabularnewline
400 & 8.1 & -0.5 & 0.8 & 0.8 & 0.1 & 0.8\tabularnewline
455 & 5.4 & 2.8 & 2.0 & 7.8 & &\tabularnewline
480 & 4.7 & 6.8 & & & &\tabularnewline
525 & 6.0 & & & & &\tabularnewline
575 & 10.8 & & & & &\tabularnewline
\bottomrule
\end{longtable}

\textbf{5.3. Ascribing meaning to the formation and exploration of oil
and gas}

Combined with the actual geological evolutionary conditions, they were
not a definitive form of thermal evolution. Using the corresponding
pyrolysis experiments, we identified the evolutionary model of different
thermal evolutional stages, which helped us analyze the formation
mechanisms of oil and gas in the Pinghu Formation in actual geology. The
variation of methane carbon isotopes in shale gas ranged from -39.7%0 to
-30.0%0 in our experiments. Cao et al. (2015) studied the composition,
light hydrocarbons, and carbon isotopes of natural gases from the Xihu
Sag and found that the gases were a mixture of coal type, accounting for
the major proportion and oil-type gases with high maturities. In
addition, they also found that the
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{1} Xihu Sag had delta values
between -47.74%0 and -28.2%0. As a result, the values of methane carbon
isotopes in the process of artificial simulation fell in the range of
methane carbon isotopes during the actual geological evolution process
in the Xihu Depression. Thus, it can be inferred that there is great
potential for coal-oil-type gases. In addition, the natural gas carbon
was seen as a performance for the series of carbon isotope changes and
presented the order of
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{1}\textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{2} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C\textsubscript{1}. There was no carbon isotope
reversal, which also proved the larger evolutional space. Therefore, the
carbon isotope in this area can indicate the maturity of oil-gas and
provide a theoretical basis and guidance for production evaluation.
Combined with the analysis of geochemical characteristics of the Pinghu
Formation coal-oil-type gas in actual strata, it can be considered that
this area also has development potential.

\textbf{6. Conclusions}

This study focused on the carbon isotopic evolution of kerogen, bitumen,
expelled oil, and gases during the pyrolysis of coal-measure source
rocks in the Xihu Sag under semi-open conditions. In addition, we
investigated the relationships between their carbon isotopes and
maturity. As a result, the following conclusions were drawn:

Four thermal evolution stages occurred in the pyrolysis samples. 1)
Lower maturity stage at \emph{Ro} = 0.82--1.09\% (\emph{Ts} = 335--360
), characterized by rapid oil and lower gas generation. 2) Maturity
stage at \emph{Ro} = 1.09--1.65\% (\emph{Ts} = 360--400 ),
characterized by the generation of lighter liquid hydrocarbons and wet
gaseous hydrocarbons. 3) Higher maturity stage at \emph{Ro} =
1.65--2.3\% (\emph{Ts} = 400--480 ), characterized by the thermal
cracking of oil to wet gases. 4) Post-maturity stage at \emph{Ro} =
2.3--3.24\% (\emph{Ts} = 480--575 ), characterized by the second
cracking of oil-gas to methane.

Combining the carbon isotopes of kerogen, bitumen, expelled oil, and
gases during thermal evolution, it was found that there was an obvious
relationship between them. Ignoring the intermediate reaction process,
the thermal evolution process can be summarized
as\emph{kerogen\textsubscript{0} (original) +
bitumen\textsubscript{0}(original) - kerogen\textsubscript{r} (residual
kerogen) + expelled oil (generated) + bitumen\textsubscript{n+r}
(generated + residual) + C\textsubscript{2+} (generated + residual) +
CH\textsubscript{4}(generated)} . The process contained both reactants
and products, and only methane remained the product in each case.
Therefore, the carbon isotope of methane was the lightest, and the
general order was \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C \textsubscript{1}
\textless{}\textsuperscript{13}C \textsubscript{2-5} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C \textsubscript{expelled oil} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C \textsubscript{bitumen} \textless{}
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C \textsubscript{kerogen}. During the thermal
evolution of organic matter into hydrocarbons, \textsuperscript{12}C
and\textsuperscript{13}C were primarily enriched in the former and
latter generated products, respectively. Among the products, the
correlation between the methane carbon isotope and \emph{Ro} was the
most obvious, indicating that the methane carbon isotope can better
characterize the corresponding maturity. Thus, based on the pyrolysis
products and the carbon isotopes of oil and gases, we can clearly define
the different formation mechanisms at different thermal evolution
stages.

Finally, by combining the analysis of geochemical characteristics of the
Pinghu Formation coal-oil-type gas in actual strata with these pyrolysis
experiments, it was concluded that this area also had substantial
development potential.

\textbf{Acknowledgments:}

We thank CNOOC Shanghai Branch for providing valuable opportunity for
sampling and geological data. We thank Sinopec Wuxi Institute of
Petroleum Geology for providing valuable opportunity for experimental
conditions and technical support. In addition, this study was supported
by National Science and Technology Major Project of China
(No.2016ZX05027001-005),National Natural Science Foundation of China
(No. 40172051).

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\end{enumerate}

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