Control of medium- to small-scale tectonic deformations on fracture development in Longmaxi Formation shale

Abstract

This study conducted a detailed geological survey and experimental analysis on the area of Golden Buddha Mountain syncline of Nanchuan, Chongqing, to examine the development characteristics and mechanism of fractures in Longmaxi Formation shale. Results showed the following: The width of the microfracture of the Longmaxi Formation shale in the research area ranges from hundreds of nanometers to hundreds of micrometers. The percentage of the shale fracture is high if width is small. The dominant strikes of joints and microfractures are consistent overall, which are the WNW, NW–NWN, NE, and NE–ENE directions. The shale shows that the density and width of the fracture and the permeability in the core of the syncline are larger than those in the wing are. The following conclusions were drawn. (1) Regardless of size, the mechanical origin of fracture is controlled by tectonic deformation. (2) During the Early Yanshanian (J3–K1) Period in the research area, a series of WNW shear fractures, NWN shear fractures, and NW tensile fractures were formed by strong squeezing effect with the NW–SE trending. During the Late Yanshanian–Himalayan Period, the tectonic stress was mainly presented as extrusion with the NE–SW trending and the NW–SE trending stretch, and a series of NE–ENE shear fractures and NE tensile fractures were formed. (3) Medium- to small-scale tectonic deformations exert dominant control effects on the fracture development of Longmaxi Formation shale in the research area. (4) A fracture network consists of joints and microfractures in shale in different scales. The finer the microfractures, the denser they grow.