## 5 Genetic Mechanism of Low-Resistivity Shale Reservoir
### The Impact of Reservoir Microscopic Characteristics on Resistivity  
Diagenesis refers to the physical, chemical, and biological processes that occur during the long geological history of sediments from post-deposition to pre-metamorphism. Changes in formation conditions and fluid properties result in varying impacts on the pore structure, material composition, and physical properties of shale reservoirs. As the burial depth increases, various minerals within the reservoir will undergo chemical reactions with formation water, causing changes in the shale reservoir. These processes may lead to significant changes in shale resistivity. In this study, shale composition and surface structure information were acquired at the same location using secondary electron imaging (SE2) and backscattered electron imaging (BSE). By integrating these imaging techniques, a series of high-precision (10 nm resolution), small-field-of-view (1000 \(\times\) 1000 pixels), arranged in a matrix with overlaps, were obtained. Finally, these images were merged using Atlas software (version number is CSTR:17081.11.atlas), resulting in a high-precision, large-field-of-view MAPS image [25]. Shale samples from both normal- and low-resistivity wells were analyzed through large-field-of-view MAPS imaging, exhibiting the characteristics detailed below. Normal-resistivity well: Particles and boundaries are distinct, each mineral exhibits a clear outline, and there is a lower clay mineral content, primarily distributed in organic matter in needle-like or fine-grained forms. The organic matter exhibits high surface porosity, distributed over a large area, with well-developed organic pores. Local areas showcase strawberry-shaped and single-crystal pyrite, though overall content is low (Figure 8a). Figure 7: The relationship between resistivity and varying water content in shale gas wells within the Luzhou area. (**a**) Comparison diagram of Logging and core resistivity of typical shale gas wells in different states; (**b**) Variation law of core resistivity of typical shale gas wells with different porosity and water saturation. Low-resistivity well: The microscopic mineral distribution characteristics in low-resistivity wells are starkly contrasting to those in normal-resistivity wells. There is a high level of consolidation between particles, with individual mineral grains being substantial, measuring up to tens of microns in size. Clay minerals are distributed in a flowing pattern around the edges of larger mineral grains, while organic matter is fragmented into small, irregular shapes. Furthermore, along the mineral grain edges, numerous pyrite crystals have formed within the interstices of the striped clay minerals and organic matter, accompanied by localized clusters resembling strawberries (Figure 8b). A comparative analysis reveals substantial disparities in the microscopic distribution of minerals between normal- and low-resistivity wells, potentially attributed to the intensity of diagenesis under high water saturation conditions. The formation of shale reservoirs involves intricate diagenetic processes, with the transformation between feldspar, clay, quartz, and other minerals exerting a significant influence on mineral composition. The chemical equation for the specific conversion is as follows:  
\[\text{Al}_{2}\text{SiO}_{5}\text{(OH)}_{4}\text{ (kaolinite) + KAlSi}_{3}\text{O}_{8}\text{ (Potassium feldspar) = KAl}_{3}\text{Si}_{3}\text{O}_{10}\text{(OH)}_{2}\text{ (Illite)+ 2 SiO}_{2}+\text{H}_{2}\text{O} \tag{2}\]  
\[\text{3 KAlSi}_{3}\text{O}_{8}\text{ (Potassium feldspar) + 2H}^{+}+\text{H}_{2}\text{O}=\text{KAl}_{3}\text{Si}_{3}\text{O}_{10}\text{(OH)}_{2}\text{ (Illite) + 6 SiO}_{2}+2\text{K}^{+}+\text{H}_{2}\text{O} \tag{3}\]  
The low-resistivity shale reservoir exhibits high water saturation (as shown in Table 1) and abundant diagenetic fluids. This impedes the reaction described by chemical Equation (2) while enhancing the reaction described by chemical Equation (3).