Multi-scale pore structure, pore network and pore connectivity of tight shale oil reservoir from Triassic Yanchang Formation, Ordos Basin

Abstract

The accurate evaluation of microscopic pore structure characteristics of shales is of great significance for shale oil development. In order to gain a better understanding of complicated pore structure of lacustrine shales, silty shale samples from the Upper Triassic Yanchang formation in Ordos Basin were investigated using FE-SEM, N2 adsorption, MIP, helium pycnometry and Nano-CT. FE-SEM observations show that five typical pores (interparticle pores, intraparticle pores, dissolution-related pores, organic matter pores and microfractures) are developed in shale samples. The full-scale pore size distributions, determined by combining N2 adsorption, MIP and Nano-CT, show that the pore sizes of the samples range from 2 nm to more than 20 μm, while the dominant pores are in the range of 20–100 nm. A certain difference exits in porosity obtained by Nano-CT, N2 adsorption, MIP, helium pycnometry and the combination methods. The N2 adsorption tends to underestimate the total pore volume and porosity. The MIP may overestimate or underestimate the porosity, due to the matrix compression or limited penetration of mercury, respectively. The combined porosity using multiple methods is reasonably consistent with helium pycnometry. Pore network models of these samples are extracted using the centerline algorithm. From the pore network model, the proportion of connected pores of the studied samples ranges from 15.51% to 37.45%. The coordination numbers obtained from pore network range from 1.34 to 1.84, indicating the relatively poor pore connectivity of the studied samples. Samples rich in organic matter have high proportion of connected pores in the region of interest (ROI) of 3D network. Pore throats show preferential orientation in XY plane (parallel to bedding) and then their two diagonal directions, while the Z axis of the pore structure (perpendicular to bedding) is less connected. These parameters improve the understanding on storage space and transport property of lacustrine shale oil.