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

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