Natural evidence of organic nanostructure transformation of shale during bedding-parallel slip

Abstract There are considerable variabilities in the macroscopic structures, fabric development, and petrophysical properties of shale-involved detachment systems, which also alter the nature of organic materials in the weak or “easy-slip” zones on the nanoscale. Bedding-parallel slip is one of the main drivers of organic matter deformation and graphitization and therefore can constrain the evolution of organic nanostructure and porosity. However, the mechanisms and details of such processes are not yet well understood. Deformation in a suite of organic-rich shales (Lower Silurian–Lower Cambrian) selected from six geological sections around the Sichuan Basin, China, is dominated by bedding-parallel slip and tectonic compression that occur at varying scales across the various sections. The deformation results in changes in the shale fabric and its organic thermal maturity. The Raman-estimated organic maturity increases with the grade of deformation. The highest maturity was estimated on the mirror-like slip surface, which indicates the structurally and thermally altered organic material with a slight graphitization. We used transmission electron microscopy to observe an abundance of better-stacked basic structural units (BSUs) with the minimum lattice spacing of ~0.38 nm within the organic component, which is indicative of high aromaticity and partial graphitization of the organic molecular structures in the slip zones. Scanning electron microscope images reveal that the organic porosity is intimately linked to organic types and associated with detachment deformation. As tectonic compression and thermal metamorphism increase, the relative abundance of isolated coarse macropores (pore size 200 nm) and microcracks increases, whereas the relative abundance of organic spongy pores and mesopores to micropores decreases. Associated with this evolution of organic pore networks with increasing deformation/maturity grade, there is a decrease in the gas adsorption capacity, pore surface area, and pore volume. This decrease is caused by a transition to better-stacked BSUs and smaller lattice spacing that correspond to changes in the type and size of organic pores. The reduction in organic micropore and mesopore volumes is attributed to the adjustment of BSUs due to compressive tectonic stress and graphitization of the organic materials, which could emerge as key primary and secondary controls on organic nanoporosity, particularly in overmature shale. We suggest that predicting shale-related bedding-parallel slip events in areas of shale gas exploration could be significant for estimating of total gas resources.