Variations in pore structure of marine shale from the same horizon of the Longmaxi Formation with changing position in a small-scale anticline: Implications for the influence of structural deformation

摘要

Nanopores are well developed in shale, and are influenced by many rock properties, such as mineral composition, TOC (Total Organic Carbon) and thermal maturity. However, the influence of structural deformation on pore properties of shale is still not well understood. In this study, 19 samples from the same horizon of the lower Silurian Longmaxi shale, but from different positions in a small-scale anticline of Wuping Village, Chongqing (China), were collected. Low pressure N-2 and CO2 adsorption experiments were performed to study the influence of structural deformation on the pore structure. The results show that the mineral composition, TOC and thermal maturity of all samples are similar, but the specific surface areas and pore volumes exhibit great differences in different positions of this anticline. The mean N-2-BJH pore volumes, the mean N-2-BET specific surface areas, the mean CO2 D-R pore volumes, as well as the mean CO2 D-R specific surface areas exhibit decreasing trends for the samples from the core part to the sub-core part, and to the limb part of the studied anticline, respectively. Structural deformation has shown a considerable influence on the micropores and mesopores, especially for the pores lower than 10 nm because that tectonic stress increases from the core part to the sub-core part and the limb part of the anticline. Moreover, the structural curving degree characterized using curvature shows good correlation with the pore volume and specific surface area. Compared to the limb part, the core part of the anticline with high curvature is located in the tension environment, and it is beneficial for pore development and gas accumulation. Our study implies that moderate structural deformation can provide more specific surface area for absorbed gas and pore volume for free gas at the core part of the structure on a larger scale.