Hierarchical cluster and principal component analyses of multi-scale pore structure and shale components in the Upper Triassic Chang 7 Member in the Ordos Basin of China

Abstract

An important issue concerning the sustainable production of shale oil is the characterization of pore structure at various scales, but there is an insufficient understanding of the coupled relationship between pore structure and shale components. Integrated analyses using field emission-scanning electron microscopy, mercury intrusion porosimetry, nitrogen physisorption, and small angle neutron scattering were conducted on nine shale samples from the Upper Triassic Chang 7 Member in Ordos Basin to investigate the relationship between multi-scale pore structure and shale minerals and organic matter (OM). The results show that the clays minerals (mainly illite-smectite) are usually combined with OM to form complex OM-hosted pores, and inorganic pores consist of both inter- and intra-particle pores (mostly related to clay platelet clusters). Three groups of shales classified by hierarchical cluster analyses (HCA) vary widely in pore structure, with the highest pore volume and largest pore diameter observed in a decreasing order of Group (1), Group (2), and Group 3. Principal component analysis (PCA) shows that brittle minerals (mainly quartz) have preserved interP pores that contribute to the development of high pore volumes and large pore-throat diameters, and ductile clays minerals and OM tend to fill pores and reduce pore diameter and volume. The complex nature of shale porosity is influenced by multiple factors, and hence accurate and comprehensive characterization requires the use of diverse methods including both experimental and theoretical analyses. An integrated approach to elucidating the factors controlling pore structure in medium-to-low maturity shale will help with efficient shale oil production.