Abstract: Taking the shale in the eastern Sichuan Basin as examples,we studied the formation and evolutionary patterns of structural fractures in shale through field observation,rock mechanics experiment,physical simulation of photo elastic and finite element simulation in combination with statistical data of development via hydraulic fracturing. It is revealed that the shale has rich deposit rhythm,pre-existing weak surfaces and strong heterogeneity. Shale bedding is the preferred structure surface for fracture development and controls the formation and evolutionary characteristics; Stress will adjust its direction when encountering shale bedding during structural deformation. The compressive stress tends to parallel weak surface,while the tension stress tends to be vertical to weak surface,which leads to weak plane opening and preferred development bedding-parallel fractures. Bedding-crossing fractures would develop when the bedding-parallel fractures are not enough to accommodate tectonic deformation. Characteristics of shale fracture distribution have sharp distinction in different locations of tectonic deformation which can be divided into five areas such as anticlinal top high angle fractures,extension of bedding-crossing fracture to bedding-parallel fracture,slip conversion,bedding-crossing fracture climbing to bedding-parallel fracture and synclinal bottom high angle fractures. It is proposed that areas with multiple sets of cap rocks overlapping is a favorable condition for exploration on structural highs,while the development and expansion of fractures is the key factor for exploration in sweet spots.

Key words: structure deformation, cap rock, fracture, shale rock, Sichuan Basin