Structural deformation of shale pores in the fold-thrust belt: The Wufeng-Longmaxi shale in the Anchang Syncline of Central Yangtze Block

Abstract

: The gas-rich Wufeng-Longmaxi shale has been intensely deformed within the fold-thrust belt of the Yangtze Block. To better understand the impact of structural deformation on the shale pore system, this paper systematically investigated the matrix components, porosity and pore structures in core samples from the Wufeng-Longmaxi shale, newly collected from various structural domains in the first commercial shale gas field of the Central Yangtze Block, the Anchang Syncline. The shale porosity generally showed a positive relationship with total organic carbon content. Nevertheless, even at a constant total organic carbon content, the shale porosity decreased from the syncline limb to the syncline hinge zone and with a decreasing interlimb angle in the syncline hinge zone, which aligned with the structural deformation strain during folding. The artificial axial compression of shale samples also confirmed that the decrease in shale porosity was stronger at an elevated axial compression stress and was relatively higher in samples with higher total organic carbon content. The organic pore size decreased with higher structural deformation strain, but the aspect ratio of the pore shape increased. Even quartz failed to resist the effective stress under the intensive structural deformation, changing the correlation between porosity and quartz from positive to negative. In contrast, pore spaces generated by the slipping between clay flakes under intensive deformation accounted for a positive relationship between clay content and bulk porosity. Considering the shale porosity reduction caused by the intensive structural deformation of shale pores, the Wufeng-Longmaxi shale, that is rich in fracture networks between roof and floor layers, may still be an excellent exploration target in the fold-thrust belt of the Yangtze Block.