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.