Abstract:

By the macro and micro comprehensive research methods of seismic-geological interpretation profile, integrated core, imaging log interpretation（FMI）, thin sections authentication, and argon ion polishing scanning electron microscope, the characteristics, main controlling factors of the properties of the shale of Wufeng Formation-Longmaxi Formation in the Yangchungou structural belt of southeast Chongqing and their influence on shale gas-bearing properties are analyzed. The study shows that there are many fractures such as shear fractures, bedding fractures, slip fractures, cleavage fractures, and shrinkage fractures developing in the shale of Wufeng Formation-Longmaxi Formation in the Yangchungou structural belt. The integrated analysis shows that shale fractures and interlayer sliding fractures there are vertical with middle-high-angle, whereas shear fractures are horizontal with low-angle. Most of the fractures are formed in multiple stages resulting in fracture nets, creases, micro faults and other phenomena. The development and distribution of these fractures are controlled by the tectonic activity, shale mineral composition and mechanical properties, and development of bentonite. Tectonic geology is the external cause of fracture development. The Yangchungou structural belt presents a fault-folding fold structure and is affected by multi-stage tectonic movements. Shear fractures, reticulate fractures, complex fracture network belts, and crumple belts are very developed. Shale mineral composition and mechanical properties are the internal causes of fracture development, controlling the development of micro-fractures and bedding fractures, such as cleavage fractures, intergranular fractures and shrinkage fractures. The Poisson ratio in 1st Longshan member of Yangchungou area is relatively small, the Young’s modulus is large, and the brittleness index is high. All those are good for the formation of various cracks. The more frequently the porphyry in the shale layer is, the closer it is to the main slip surface between the Wufeng Formation and the Linxiang Formation, the more obvious the slip phenomenon is, and the more developed the interlayer slip fractures are. Although the natural fractures in the Yangchungou structural belt are very developed, the preservation conditions have not suffered serious damage, so the shale gas exploration potential is still good in the study area. Drilling revealed that the total gas content of the shale is equivalent to that of the adjacent area, and the total hydrocarbon measured by gas is better than that of the adjacent area. The proportion of free gas is also higher than that of the adjacent area. It indicates that the formation of local structural fractures has expanded the storage space of free gas in shale gas, which is beneficial to the rock gas accumulation.

Key words: shale, fracture, Wufeng-Longmaxi Formation, Southeastern Chongqing, Yangchungou structural belt, controlling factors