Shear deformation of fold structures in coal measure strata and coal-gas outbursts: Constraint and mechanism

Abstract

The coal measure strata formed in the late Paleozoic era underwent tectonic movements on several occasions, leading to the development of a range of fold structures in the strata as a result of highly interlayered gliding and shearing deformation. In addition, brittle ductile tectonic coals were widely distributed in the reservoir, constituting one of the primary causes of coal-gas outbursts in Chinese mines. This indicates a strong connection between shear deformation and such outbursts. In this study, structural geology, gas geology, and rock mechanics are all taken into consideration to investigate the controlling effects of the fold structure formation process on coal thickness and tectonic coal formation. Numerical simulation, based on the stress test data, was deployed to identify the stress distribution law adjacent to fold structures under the modern stress field. Additionally, the mechanical mechanism underpinning coal-gas outbursts near the fold structure was determined by making a comparison with the distribution law relating to such outbursts. The results demonstrate that the formation and evolution of the fold structure not only form the material basis of outbursts but also control their power source. During the fold formation process, interbed sliding and shearing between strong and weak rock strata were caused by differences in the mechanical properties of the coal bed and rock layer, resulting not only in a change to the local thickness of the coal seam but also in its deformation and structural alteration. Interbed shearing and local stress concentration, caused by the coal gangue, led to coal damage and the development of layered tectonic coal of consistent thickness, simultaneously improving its ability to adsorb gas and providing the material basis for coal-gas outbursts. This reduced the seam’s capacity to resist such events. The conditions for these outbursts are created by the sudden desorption of excess gas as a result of formation pressure release during coal mining and the widespread distribution of tectonic coal. Under the modern tectonic stress field, the stress distribution characteristic is controlled by the fold structure shape; and because of the aforementioned differences in the mechanical properties of the coal bed and rock layer, the interlayer deformation is asynchronous. This causes shear stress concentration within a specific range of the anticline’s two wings. This concentration zone happens to be exactly aligned with that of coal-gas outbursts, meaning that shear stress concentration is considered to be both the power source for and main cause of the region’s outbursts.