Geological Conditions
Depositional Environment
The formation of shale gas is closely related to the depositional environment. Suitable paleogeographic environments (such as temperature, salinity, and water depth) are conducive to the prosperity of aquatic organisms, thereby increasing the efficiency of organic matter production. Meanwhile, reducing and anoxic conditions are favorable for the preservation of organic matter, while high-energy and oxic environments are not. For example, the New Albany Shale, deposited in a deep-water environment, has certain layers with organic matter content as high as 20%, which shows that the depositional environment plays an important role in the accumulation and preservation of organic matter.
Stratigraphic Depth and Temperature-Pressure Conditions
As the stratigraphic burial depth increases, temperature and pressure will also rise accordingly, thereby promoting the thermal cracking of organic matter and the generation of shale gas. Generally speaking, the main generation stage of shale gas usually occurs when the formation temperature reaches above 60℃, and when the temperature exceeds 120℃, the organic matter enters the high-maturity stage to produce natural gas. However, too high a temperature may lead to the over-cracking of organic matter, generating non-hydrocarbon substances and reducing resource utilization efficiency.
Characteristics of Organic Matter
Type and Abundance of Organic Matter
The type and abundance of organic matter are the basis for the formation of shale gas. According to different sources, organic matter can be divided into sapropel, humus, and mixed types. Sapropel organic matter, mainly derived from aquatic organisms, has a higher maturity and is more likely to generate petroleum; humus organic matter, mainly from terrestrial plants, has a lower maturity and usually generates natural gas. In addition, high organic matter abundance (such as total organic carbon content TOC greater than 2%) is an important condition for the formation of shale gas.
Organic Matter Maturity
Organic matter maturity is a key indicator of the degree to which organic matter is converted into petroleum or natural gas under thermal action. Maturity is mainly affected by factors such as temperature, time, biochemical action, catalytic action, and radioactive action. For example, for every 10℃ increase in temperature, the thermal cracking rate of organic matter increases by about 2 to 3 times. In actual exploration, it is necessary to reasonably evaluate the maturity of organic matter in combination with formation temperature data.
Thermal Evolution Degree
The thermal evolution degree directly affects the generation and type of shale gas. According to Tissot's classification scheme, different maturity stages correspond to different hydrocarbon generation situations. For thermogenic gas-bearing shale, entering the gas generation window is a necessary condition for shale gas enrichment. As maturity increases, the organic matter in shale gradually converts into gas, thereby increasing the content of shale gas. However, too high a thermal evolution degree may lead to changes in the physical properties of shale, thereby affecting its gas-bearing capacity.
Geological Structures
Geological structures have an important impact on the formation and preservation of shale gas. Tectonic activity may cause deformation, fracturing, and fracturing of shale layers, thereby improving the porosity and permeability of shale layers and enhancing gas-bearing capacity. For example, large faults and associated large-scale fractures may lead to the escape of shale gas, severely destroying gas-bearing capacity. In addition, regional tectonic movements may also affect the preservation conditions of shale layers, thereby affecting the generation and preservation of shale gas.
Preservation Conditions
Preservation conditions are one of the important factors affecting the formation and evolution of shale gas. Good preservation conditions can reduce the escape of shale gas and increase its enrichment degree. For example, in stable geological conditions, the preservation conditions of shale gas are better, and the maturity is lower. On the contrary, in active geological conditions, such as tectonic movements and volcanic activities, although the maturation process of organic matter is accelerated, the risk of shale gas escape also increases.
Conclusion
The formation and evolution of shale gas is a complex geological process, influenced by a variety of factors such as depositional environment, characteristics of organic matter, thermal evolution degree, geological structures, and preservation conditions. In the actual exploration and development process, it is necessary to comprehensively consider these factors to achieve the effective development and utilization of shale gas. Future research should further explore the generation mechanism of shale gas, the storage laws, and the quantitative relationships of influencing factors to provide a more scientific theoretical basis and technical support for shale gas development.