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 (12C–12C)
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 13C to fall off than those
with less 13C. Thus, during the thermal evolution of
organic matter into hydrocarbons, the first products to be generated
were relatively enriched in 12C, and subsequent
products were relatively enriched in 13C (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.