Definition and Occurrence of Shale Gas
Shale gas refers to natural gas extracted from shale formations, with methane being its primary component. Shale is a fine-grained sedimentary rock characterized by high porosity and low permeability, which makes the occurrence of shale gas within it rather complex. Shale gas mainly exists in three forms: free gas, adsorbed gas, and dissolved gas. Free gas resides in the pores and fractures of shale and can be directly extracted; adsorbed gas is attached to the surfaces of kerogen, clay minerals, and other particles, requiring pressure reduction or temperature increase to be released; dissolved gas is a small amount dissolved in kerogen, asphaltene, and petroleum, making its extraction relatively challenging.
Geological Background of Shale Gas Formation
The formation of shale gas begins with the deposition of organic matter in ancient marine or lacustrine environments. In these settings, abundant plant and microbial residues accumulate under anoxic conditions, forming organic-rich mud shale. Over time, these mud shales are buried deeper underground due to geological tectonic movements. As burial depth increases, temperature and pressure rise gradually, causing organic matter to undergo thermal evolution and ultimately generate shale gas.
The Formation Process of Shale Gas
Accumulation and Preservation of Organic Matter
The formation of shale gas starts with the accumulation of organic matter. In ancient marine or lacustrine environments, a large number of plant residues and microorganisms accumulate under anoxic conditions to form organic-rich mud shale. These organic matters mainly include kerogen, asphaltene, and a small amount of petroleum. Kerogen is the main component of organic matter, a complex organic polymer with a high carbon content and low oxygen content. The type and content of kerogen have an important impact on the generation of shale gas. Generally speaking, Type III kerogen is mainly gas-generating, while Type I and Type II kerogen are mainly oil-generating.
Thermal Evolution of Organic Matter
As the burial depth of mud shale continues to increase, temperature and pressure rise gradually, and organic matter begins to undergo thermal evolution. The process of thermal evolution can be divided into several stages:
- Oil Generation Threshold Stage (Ro<0.5%): In this stage, organic matter begins to transform into hydrocarbons, but the main products are liquid hydrocarbons such as petroleum. At this time, the generation of shale gas is relatively small.
- Peak Oil Generation Stage (Ro between 0.5% and 1.2%): As the temperature rises further, kerogen begins to crack massively, generating a large amount of liquid and gaseous hydrocarbons. In this stage, the generation of shale gas gradually increases, but liquid hydrocarbons still dominate.
- Peak Gas Generation Stage (Ro>1.2%): When the maturity of organic matter further increases, liquid hydrocarbons begin to crack into gaseous hydrocarbons, and the generation of shale gas reaches its peak. In this stage, the main component of shale gas is methane, with a content that can exceed 90%.
- Overmature Stage (Ro>2.0%): In the overmature stage, methane in shale gas continues to crack into smaller gas molecules such as hydrogen. This process leads to an increase in the dryness coefficient and reservoir pressure of shale gas reservoirs, but the overall generation of shale gas begins to decrease.
Preservation and Migration of Shale Gas
The preservation and migration of shale gas are key to the formation of commercial gas reservoirs. Due to the low permeability characteristics of shale, shale gas mainly resides in the pores and fractures of shale after its formation and is difficult to migrate on a large scale. However, under the influence of some geological tectonic movements, shale gas may migrate locally through fractures and faults. For example, during late uplift, shale gas may escape due to reduced pressure. Therefore, the stability of geological structures is crucial for the preservation of shale gas.
Factors Affecting the Formation of Shale Gas
The formation of shale gas is a complex geological process, influenced by a variety of factors. Here are some of the main ones:
Depositional Environment
Factors such as the stability of the depositional environment, the productivity of the water body, and the activity of microorganisms affect the preservation and accumulation of organic matter. In stable and anoxic depositional environments, organic matter is more easily accumulated and preserved, thereby providing abundant raw materials for the generation of shale gas.
Type and Content of Organic Matter
Different types of kerogen generate different amounts and types of gases during thermal evolution. Generally speaking, Type III kerogen is mainly gas-generating, while Type I and Type II kerogen are mainly oil-generating. In addition, the content of organic matter also has an important impact on the generation of shale gas. The higher the content of organic matter, the greater the potential for shale gas generation.
Thermal Maturity
Thermal maturity is a key factor in the generation of shale gas. As thermal maturity increases, the generation of shale gas first increases and then decreases. Generally speaking, when the maturity of organic matter reaches above 1.2%, the generation of shale gas reaches its peak.
Geological Structures
Geological tectonic movements affect the preservation and migration of shale gas. During late uplift, shale gas may escape due to reduced pressure. Therefore, the stability of geological structures is crucial for the preservation of shale gas.
Conclusion
The formation of shale gas is a complex geological process involving the accumulation of organic matter, thermal evolution, and the stability of geological structures, among other aspects. By gaining a deep understanding of the formation process of shale gas, we can better recognize the characteristics of this energy source and provide a scientific basis for its exploration and development. With continuous technological progress, shale gas, as an important clean energy source, will play an increasingly important role in the future energy mix.