Variation in pore systems with tectonic stress in the overthrust Wufeng-Longmaxi shale of the southern Sichuan Basin, China

Abstract

Pore evolution in organic-rich shale is widely reported to be related to thermodynamic reactions, but its connection with tectonic deformation is overlooked. A systematic comparative analysis was conducted in the Changning area to detect the tectonic deformation of organic-rich shale. The study encompassed mineralogical composition, petrography, and porosity, along with the state of tectonic stress and pore pressure in the Wufeng-Longmaxi shale of a basement-involved fold-thrust belt located in the southern Sichuan Basin. The content of both total organic carbon (TOC) and clay mineral was correlated with bulk porosity, indicating the relevance and impact of both organic and inorganic pores on bulk porosity. However, samples with a constant TOC or clay mineral content exhibited lower bulk porosity in wells located on the forelimb than in wells located on the crest and backlimb of the overthrust anticline. The variably elongated and slot-like morphology as well as the crude alignment of organic pores in samples from wells on the anticline forelimb suggest that the organic pores collapse under tectonic deformation. This observation was different from that of the spherical morphology of organic pores in samples from wells on the backlimb of an overthrust anticline. The ratio of horizontal stress to vertical stress (Shmin/Sv or SHmax/Sv), simulated from well logs, was negatively correlated with the median bulk porosity in the Wufeng-Longmaxi shale within various cores. This suggests that a larger strain, under higher horizontal compression stress, decreased the bulk porosity in the shale of the forelimb areas, despite the content of the mechanically weak component (e.g., TOC or clay minerals) being similar in both. Additionally, the Wufeng-Longmaxi shale generally exhibited a higher pore pressure, but lower bulk porosity, in wells on the forelimb relative to the backlimb, which indicates that the collapsed pore system has, to some extent, inhibited the relief of maturation-induced overpressure during exhumation.