Research status of rock sliding specular reflection and its application in shale gas preservation

Abstract

Objective Specular reflection is a smooth surface formed during the sliding process of rock strata, with a certain metallic and glass luster and reflective properties. In some fault zones, these smooth surfaces are also known as fault mirrors (FMs). Specular reflection can be seen in many rock types, such as mudstone, shale, coal seam, carbonate rock, silicate rock and so on. The mechanism of specular reflection is different in different lithologies, but the formation of specular reflection is related to strata sliding. Therefore, the stress, slip rate, slip distance and temperature related to rock strata sliding have important influence on the formation of specularity. Methods A large amount of specular reflection is developed in the syncline Longmaxi Formation shale in Anchang, northern Guizhou. To find out the factors influencing the formation of specular reflection and the internal relationship between them and the gas content of shale in this paper, the development characteristics and formation mechanism of specular reflection and its effects on fluid flow and shale gas preservation conditions were summarized systematically through a large number of literature reviews. Results The results show that: (1) Stress, slip rate, slip displacement and temperature have important influence on the formation of specular reflection. (2) The formation of specular reflection depends on the combination form of stress and sliding rate. It is easier to form specular reflection under high-speed and high-stress conditions. Under low-speed and low-stress conditions, the specular reflection will not be formed even if the sliding displacement is large. (3) Under the same stress and sliding rate, the overall sliding displacement will promote the increase of specular coverage, and the specular coverage can even reach 100%. (4) The temperature makes the rock particles change from brittle to plastic, which can prevent the brittle fracture of the rock particles to a certain extent, and then sinter the rock particles, prompting the formation of specular reflection. (5) The specular roughness is very low, usually in the micrometer level, the average roughness range is only a few micrometers. (6) Specular cracks and slip distances are formed, which significantly improve the flow of shale gas, accelerated loss of shale gas may lead to poor gas. Conclusion Therefore, in the absence of roof and floor conditions, specular refection may be one of the important reasons for the poor gas content of shale.

Keywords

• specular reflection
• developmental characteristics
• formation mechanism
• shale gas
• seepage capacity
• preservation condition