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New Advances in Natural Gas Geology http://www.oilnews.com.cn/bk/images/070821/A012007011.jpg China Petroleum News Center http://www.oilnews.com.cn/bk/images/070821/A012007011.jpg Early natural gas geology was always dependent on petroleum geology and considered part of it. With the discovery of numerous natural gas fields and ongoing research into natural gas, it has become clear that natural gas differs significantly from oil in many aspects, including its formation processes, mechanisms of accumulation, and distribution patterns. The publication of \"Gas Geology\" by H.B. Visotsky in 1979 marked gas geology as an independent discipline. Chinese natural gas geology emerged in the late 1970s to early 1980s; research in this field began to intensify with the establishment, in 1983, of a key scientific and technological project focused on the development and research of coalbed methane, which served as a milestone. Over the past 20-odd years, China has carried out extensive and in-depth research on various issues related to the origin of natural gas, its accumulation, and its distribution patterns, achieving a large number of high-quality research results. 1. The theory of natural gas formation is the core of natural gas geology in China. The theory of coalbed methane, which was introduced to China from Germany in the late 1970s and further developed there, expanded the origin of natural gas from a \"monistic\" view to a \"dualistic\" one, and later evolved into a \"pluralistic\" concept that includes coalbed methane, oil-type gas, and inorganic-origin gas. This theory suggests that both coal and black shales in coal formations are capable of generating gas, with gas formation being the primary process while oil formation is secondary. The theory of coalbed methane has opened up a new field for natural gas exploration in China. The proportion of coalbed methane in the proven natural gas reserves has been increasing year by year; by 2004, coalbed methane accounted for over 70% of the country’s proven natural gas reserves. The development of the \"pluralism\" theory regarding the origin of natural gas has promoted research on the identification and classification of its origins, making it an important component of theories on the origin of natural gas. The carbon isotope composition of alkanes is determined by the type and maturity of the natural gas source rock; the carbon isotope composition of inorganic-origin gas is heavier than that of organically-origin gas, while the carbon isotope composition of coal-derived gas is heavier than that of oil-derived gas. For natural gas formed from the same type of source rock, its carbon isotope composition becomes heavier as its maturity increases. Chinese scholars have developed a relatively comprehensive system for identifying the genetic types of natural gas by utilizing a range of parameters and indicators such as the stable carbon isotope of natural gas, its composition, light hydrocarbons, biomarkers, and noble gas isotopes; this system has then been used to carry out systematic classification of the genetic types of natural gas. 2. Theory of natural gas reservoir formation: The complex process of natural gas reservoir formation has always been a challenge and a focus in the geological study of natural gas. In recent years, with the continuous advancement in natural gas exploration and comprehensive geological research, theories such as the dynamic equilibrium theory of natural gas accumulation, multi-stage natural gas accumulation, and late-stage natural gas accumulation have been proposed ； Formation models for different types of large and medium-sized gas fields have been established ； Preliminary methods for the quantitative evaluation of natural gas aggregation and dispersion during orogeny, as well as methods for determining the stages of orogeny, have been established. Meanwhile, significant progress has also been made in the study of the formation mechanisms of unconventional gas reservoirs (such as deep basin gas reservoirs and coalbed methane reservoirs). 1) Theory of dynamic equilibrium in natural gas migration and accumulation. The model of dynamic equilibrium in natural gas migration and accumulation describes the process of natural gas being generated and released from source rocks, migrating further in the carrier layers, accumulating to form reservoirs, and then dissipating, as a dynamic and continuous process. When the amount of natural gas replenished is greater than the amount lost, natural gas continues to accumulate in the reservoir ； Conversely, the natural gas in the trap continues to decrease, until it is completely lost. The intensity and duration of gas source injection, as well as the storage conditions under sealing, are important factors in the formation of gas reservoirs (Hao Shisheng, 1991, 1994). 2) Mechanism of natural gas sealing: The different physical properties of natural gas compared to oil mean that it requires lower requirements regarding the properties of the reservoir, but higher requirements concerning the seal layer and preservation conditions. Li Guopeng et al. (1996) conducted a comprehensive and systematic study of natural gas cap rocks from various aspects, including cap rock classification and characteristics, the mechanisms of cap rock sealing, laboratory determination of cap rock parameters (breakthrough pressure, specific surface area and pore size distribution, diffusion coefficient, adsorption capacity, laser particle size analysis of shales), logging techniques, and the use of seismic data for cap rock research and prediction. As a result, they developed a set of distinctive research methods for cap rocks that encompass both microscopic parameter testing and macroscopic prediction, as well as qualitative and quantitative analyses. Research suggests that the sealing capacity of overlying layers is related to their lithology; the sealing capacities of rocks with different lithologies vary significantly. Shaly rocks at different diagenetic stages exhibit different sealing capacities, with shaly rocks generally having the strongest sealing capacity around stage A of the late diagenetic stage. Pressure sealing is mainly related to the abnormally low pressure resulting from undercompaction, as well as the abnormally high pressure caused by hydrothermal pressure from the strata, massive hydrocarbon release from source rocks, dehydration of montmorillonite, and tectonic activities; hydrocarbon concentration sealing is primarily associated with whether the overlying rock is a source rock and the maturity level of that source rock. 3) Theory of late formation of natural gas reservoirs: The theory of late formation of natural gas reservoirs is based on the principle of dynamic equilibrium in the formation of natural gas reservoirs as well as the requirements for their preservation. In other words, the late timing of reservoir formation and the continued supply of gas sources to these reservoirs are key factors in the formation of medium- to large-scale gas fields. China’s major oil and gas basins have been frequently altered by tectonic activities. Natural gas reservoirs generally go through multiple formation stages, and those formed in earlier stages are difficult to survive due to the alterations and destruction caused by subsequent tectonic movements; only the reservoirs that formed in the latest stages can remain intact to this day (Zhou Xingxi, 1998) ； Qiu Zhongjian, Kang Zhulin, He Wenyuan, 2002 ； ). The late formation of natural gas reservoirs is determined by three factors: first, the peak period of hydrocarbon generation resulting from the evolution of source rocks that has occurred since the Cenozoic era ； Secondly, neotectonic activities have provided large-scale traps and migration pathways for the accumulation of natural gas, facilitating its accumulation in later stages ； Third, the later natural gas accumulates, the less it dissipates, which is more conducive to the formation of large gas fields (Dai Jinxing, 2003) ； Song Yan, 2003 ； Gong Zaisheng, 2004). 3) Natural gas dynamics theory: The migration, accumulation, and formation of oil and gas occur as a result of the natural flow of hydrocarbons generated from source rocks, under the combined influence of the transport framework and energy fields (including temperature, pressure, and stress fields) during the evolution of sedimentary basins. Therogenic dynamics is an interdisciplinary field that comprehensively utilizes geological, geophysical, and geochemical methods as well as computer simulation techniques. Within the framework of basin evolution history and transport mechanisms, it studies the formation, evolution, migration processes, and accumulation patterns of oil and gas in sedimentary basins through the analysis of energy field evolution and the chemodynamic, hydrodynamic, and kinematic processes governed by it. The basis of the study is the basin evolution history and the fluid transport framework, while the core of the study lies in the evolution of the energy fields (including temperature field, pressure field, and stress field) and the chemodynamic and hydrodynamic processes that govern this evolution.