Quantitative pore characterization and the relationship between pore distributions and organic matter in shale based on Nano-CT image analysis: A case study for a lacustrine shale reservoir in the Triassic Chang 7 member, Ordos Basin, China

Abstract

The microstructure of shale has always been of interest in studies associated with shale gas reservoirs. In addition, the influence of the presence of organic matter on the pore distribution is not well-understood. Although considerable attention has been given to the total organic carbon (TOC) content together with the total porosity, concerns regarding investigating the pores in rocks separately according to the different media where the pores are distributed are still rare. This paper presents a comparative study of the pore size distribution (PSD) in organic matter (OM) and the mineral matrix (MM) within the same sample with the OM-hosted pores and MM-hosted pores accurately distinguished and quantitatively analysed. Four shale samples from a shale play in the Ordos Basin are scanned using nano-CT scanning with a resolution of 65 nm. The calculated total organic carbon (TOC) of the samples are 3.48%, 4.47%, 2.76%, and 3.01% with corresponding porosities of 4.30%, 4.97%, 4.50%, and 4.74%, respectively. The results show that more of the large pores exist in the OM with better connectivity and irregular shapes, while the pores in the MM are smaller on average, have less connectivity, and primarily consist of tiny spheres. The pore-medium-ratio (PMR) was introduced to illustrate the pore volume distribution characteristics within each specific medium. The PMR is a ratio between the volume of pores in a specific medium (i.e., MM or OM in this study) and the volume of that medium. According to the PMR curves, the small pores are distributed homogeneously, while the large pores have a heterogeneous distribution and are medium-dependent. It is concluded that the OM affects the pore distribution in three aspects: 1. the OM has a higher PMR; 2. most large pores occur in the OM; and 3. the small pores in the MM tend to appear around the OM. Insights into the various distributions of the pores have been developed. Pores in either medium exhibit an exponential distribution in number. Three sections of the pore volume distribution and surface area distribution are found. Furthermore, the concept is verified that small pores have a large effect on the surface area, while large pores have a large effect on the pore volume.