Coupled formation of fracture assemblages in shale and their influence on permeability

Abstract

Abstract Shale, with its well-developed bedding, exhibits a pronounced anisotropy that commonly leads to the coupled formation of diverse opening-mode fracture types; however, the mechanisms driving this phenomenon remain inadequately understood. Multiple fracture types have been identified in the Permian Lucaogou Formation of the Junggar Basin, NW China, significantly influencing the permeability of these shale reservoirs. Here, we investigate the origin of fluids within these fractures through rare earth element analysis, fluid inclusions, in situ U-Pb dating, and C-O-Nd isotopic compositions of calcite cements. By integrating triaxial experiments with two-dimensional oil and gas migration simulations, we elucidate the mechanisms behind the coupled formation of these fractures and assess their impact on shale permeability. Our findings reveal that fractures in the Lucaogou Formation developed in multiple phases from the Late Permian to the Miocene, encompassing low-angle tensile fractures (5°–15°), oblique shear fractures (35°–50°), high-angle tensile fractures (75°–90°), and three sets of bedding-parallel fractures (types I, II, and III). During the Late Permian, rapid burial in the Jimsar Sag induced compressive stress perpendicular to bedding, resulting in the coupled formation of oblique shear and type I bedding-parallel fractures. In the Late Jurassic, intensified tectonic activity in the Tianshan Mountains caused rapid uplift and generated compressive stress parallel to bedding in the Jimsar Sag, leading to the formation of coupled low-angle tensile and type II bedding-parallel fractures. Continued folding in the Bogda foreland depression during the Early Cretaceous triggered intense activity in the Fukang fault zone, leading to strong thrusting and the formation of high-angle tensile fractures. By the Miocene, hydrocarbon generation in the Lucaogou Formation had led to elevated pore fluid pressures, locally exceeding fracture thresholds along weak bedding planes and resulting in the formation of the type III bedding-parallel fractures. Triaxial experiments and two-dimensional simulations demonstrate that compressive stress perpendicular to bedding significantly enhances shale permeability through the coupled formation of bedding-parallel and oblique shear fractures. Conversely, compressive stress parallel to bedding leads to the coupled formation of bedding-parallel and low-angle tensile fractures, which have limited impact on permeability. These findings challenge conventional models by demonstrating that fractures in bedded rocks often form through coupled mechanisms rather than in successive geological events. The combination of medium- to high-angle fractures with bedding-parallel fractures substantially enhance shale permeability, providing critical insights for resource extraction and reservoir characterization.

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