Research On Development Of Fractures And Their Structural Control In The Longmaxi Shale,Nanchuan Region,Chongqing

As the shale of Longmaxi formation in Lower Silurian, which located in the area of Nanchuan, C hongqing, being the research object of this thesis, using the method of field investigation, the collection of data in research area, test of indoor analysis and systematic theoretical analysis, this thesis studies systematically the charac teristics of fracture development and tectonic controls of the shale of Longmaxi formation in study area, and analyzes palaeostress fields in research area and restores its structural evolution.Research shows that the structure in Nanchuan area can be divided into 3 structural layers, i.e., I(the Caledonian period), II(the Hercynian-Indosinian period), and III(the Yanshanian period), and the main directions of structural traces are NNE, nearly NS and NNW. The Yanshanian and the Himalayan periods are the main tectonic deformation period in Nanchuan area : in the early and middle period of Yanshanian period, being affected by the stress field of SSE-NNW, the principal stresses from SE to NW made the formation of the NNW, NE conjugate joints and the NNW shear structural plane; in the late Yanshanian period, being affected by the stress field of NWW-SEE, it formed the NW, SEE conjugate joints and the SN, NNW shear structural plane; in Himalayan period, being affected by extension of NW-SE which resulted in transforming the products produced in Yanshanian movement, it formed a series of large-scale NNE normal faults in the anticlinal flanks, which composed graben and horst faults.The shale is mainly developed with dominant joints in the high angle of NE, NEE, NWW and NNW. The layer developed the same joint system under the same condition of tectonic deformation while varied slightly in different geological age. The joint is mainly oblique with a few strike joints, dip joints and bedding joint and mainly in growth layer. The vertical extension of the joint is between 0 and 100 cm, and the opening is between 0 and 1mm with higher joints’ density in the core of fold and lower density in the two wings. The micro fracture is mainly developed in the direction of NE, NW, NNE and NWW, basically the same as the development direction of macro fracture, mainly developing the unfilled fracture, in which the density in the core of fold higher than that in wings.Tectonism has control effect, to some extent, on the density of joint, the dominant orientation of joint and the opening of joint. The density of and opening of the joint corresponds to the tectonic setting and the strength of tectonic deformation. The joints’ density in active zone and the opening degree of the joint are developed. The anisotropy of the fault and the fold complicates the development of joint and variation of the density and opening degree of the joint, resulting in the closer to the tectonic zone the higher the density of joint and the degree of opening, and vice versa. The northern and northwestern parts of the structure is stable in which the development of the joints and the opening degree is weak. Tectonics evolution and tectonic deformation has control effect on the development of micro fracture. The earlier structure has control effect on the development of micro fracture as on the development of macro fracture. In the post-diagenetic stage, after experiencing several complicated and poly-tectonic compressional deformation, the micro fracture is tend to develop in the shale reservoirs. The effect of the fold structure on development of micro fracture mainly reflected in three aspects: the opening degree of micro fracture, the development direction of fracture and the density of fracture surface. The effect of the fault structure on development of fracture mainly reflected in two aspects: the opening degree of micro fracture and the density of developmental line of fracture.