Various controlling factors of matrix-related pores from differing depositional shales of the Yangtze Block in south China: Insight from organic matter isolation and fractal analysis

Abstract

Abstract In this study, matrix-related pores from differing depositional shales were explored comparatively. Among of them, Lower Cambrian shale (3.83%Ro) and Lower Silurian shale (2.61%Ro) were marine sediments with abundant oil-prone kerogen and rich siliceous minerals, while Upper Permian shale (2.44%Ro) were transitional sediments with redundant gas-prone kerogen and rich clay. The morphology and geometry of pores were investigated via fractal analyses based on N2 adsorption and direct imaging. The effects of organic matter (OM) within different shales were also highlighted through N2 adsorption before and after OM isolation. Lower Cambrian shale possessed the lowest pore volumes (PV) (averaging 0.0109 ml/g) and the lowest pore surface areas (PSA) (averaging 9.09 m2/g) as well as the smallest average pore diameters (APD) (averaging 5.47 nm). Dissolved pore with dead-end openings was the main type. The PV and PSA of isolated OM were only approximately 1 and 2 times higher than that of corresponding samples, respectively. In contrast, Lower Silurian shale possessed the highest PV (averaging 0.0109 ml/g) and the highest PSA (averaging 9.09 m2/g) as well as relatively large APD (averaging 13.43 nm). Organic-hosted pores (OMP) with cellular structure is the main type. The PV and PSA of isolated OM were approximately 8.5 and 3 times higher than that of corresponding samples, respectively. Upper Permian shale with the largest averaging pore diameters (averaging 18.82 nm) presented high PV (averaging 0.0209 ml/g) similar to that of Lower Silurian shale, and a low PSA (averaging 10.85 m2/g) like that of Lower Cambrian shale. Pore associated with clay flakes was the main type. The PV and PSA of isolated OM were only approximately 0.6 times and 1 times higher than that of corresponding samples, respectively. For marine shale, matrix-related pore features are synergy effects of the matrix basis where pre-existing space controls the occurrence of porous OM and functions as the shelter for OMP with an appropriate thermal maturity. However, extensive diagenesis can overprint the effects of matrixes on pore properties, because oil-prone kerogen is sensitive to thermal maturity. The specific material composition of transitional shale limit pore properties, because dominant structured OM is thermally stable with limited migration ability and pore contribution. Hence, diagenetic differences and material diversities may be attributed to the discrepancies of pore properties between marine shale and transitional shale.