Abstract: Fractures in organic-rich shale control the desorption, occurrence and preservation of shale gas, and are closely related to the gas-bearing property of shale. This paper studies the organic-rich shale in Longmaxi Formation in the Changning area of southern Sichuan by observing the outcrops, cores and fracture development characteristics of the rock slices under the microscope. In combination with rock mechanics experiments and logging data, this study focuses on the causes and dominant factors of different types of fractures, and probes into the relationships between fracture development characteristics and shale gas-bearing property. The results show that:(1) The macroscopic fractures in outcrops are dominated by high-angle and upright shear fractures, characterized with short extension distances, small spacing, and large density. The downhole presents grid fractures similar to checkerboard grids, formed by mutual intersecting of shear fractures in multiple periods, as well as multiple forms of fracture assemblages formed through vertical cutting of high-angle and upright fractures and horizontal bedding fractures. A variety of micro-fractures formed by tectonism, diagenesis, and hydrocarbon generation and expulsion of organic matter were observed under the microscope. The intricate fracture system inside rock may be the result of the combined effects of early diagenesis and late tectonic movement. (2) This multi-directional structural shear fracture is controlled by at least three stages of tectonic movement. The organic-rich siliceous shale facies is the main lithofacies type that controls the development of fractures in shale in this area. The main types of fractures include bedding fracture, reticulated fracture, high-angle fracture and oblique-crossing fracture. The brittleness of shale is at a medium level and is greatly affected by Poisson's ratio, and its burial depth plays a major controlling role. (3) The target horizon is mainly located in the brittle-ductile transition zone. The rocks mostly present a failure mode of 1-2 major fractures and several micro-fractures. The integrity of the rock can be effectively guaranteed. The vertical opening degree of fractures is low and the sealing ability is good. The risk of longitudinal dissipation is low, corresponding to high pressure coefficient and gas content. The fracture development section of the first sub-member of the first Member of Longmaxi Formation corresponds to the high content of organic matter and brittle minerals, which together promote the development of fractures in the shale, thereby forming a space for good gas occurrence, enhancing the gas-bearing capacity of shale to a certain extent.

Key words: Changning area, Longmaxi Formation, fracture characteristics, dominant factors, brittle-ductile transition zone, failure pattern, gas-bearing properties

摘要： 富有机质页岩的裂缝控制页岩气的解吸、赋存及保存，与页岩的含气性密切相关。以川南长宁地区龙马溪组富有机质页岩为研究对象，对露头、岩心以及镜下岩石薄片的裂缝发育特征进行观测，并结合岩石力学实验、测井资料，重点研究了不同类型裂缝的成因、主控因素，并深入探讨了裂缝发育特征与页岩含气性的关系。研究结果表明：①露头的宏观裂缝以高角度—直立的剪切缝为主，延伸距离短、间距小且密集；井下呈现由多期剪切缝相互交切形成的类似于"棋盘格子"的网格缝，以及高角度—直立缝与水平层理缝垂直切割的多种形态的裂缝组合；镜下观察到由构造作用、成岩作用以及有机质生烃和排烃作用所形成的多种微裂缝，岩石内部错综复杂的裂缝系统可能为早期成岩作用和晚期构造运动综合作用的结果。②这种多向的构造剪切缝受控于至少3期构造运动；富有机质硅质页岩岩相是控制该地区页岩裂缝发育的主要岩相类型，裂缝主要类型包括层理缝、网状缝、高角度缝以及斜交缝。页岩脆性处于中等水平，受泊松比影响程度较大，其埋深起主要的控制作用。③目的层主要处于脆性—延性转换带，岩石多呈现1~2条主裂缝和数条伴生微裂缝的破坏模式，其岩石的完整性能够得到有效的保证；裂缝垂向开启程度低、封堵性较好，纵向逸散风险程度低，对应较高的压力系数和含气量；龙马溪组一段Ⅰ亚段中裂缝发育段对应较高含量的有机质和脆性矿物，两者共同促进了裂缝在页岩内部的发育，由此形成良好的气体赋存空间，在一定程度上增强了页岩的含气性。

关键词: 长宁地区, 龙马溪组, 裂缝特征, 主控因素, 脆性—延性转换带, 破裂模式, 含气性