The heterogeneity of pore structure in lacustrine shales: Insights from multifractal analysis using N2 adsorption and mercury intrusion

A series of parameters from a variety of techniques, such as pore size distribution, surface area, pore volume, etc., have been used to characterize the pore structure of unconventional shale reservoirs. However, the investigation on the quantification of pore structure heterogeneity is still scarce. In this aspect, a multifractal theory combined with N-2 adsorption and mercury intrusion was used to obtain pore structure characteristics and quantify the heterogeneity of pore size distribution of nine lacustrine shale samples from the second member of Kongdian Formation (Ek(2)) in Cangdong Sag, Bohai Bay Basin, China. The results show that the Ek(2) shale contains complex pore network, and pores with small pore size predominate in pore system. Both N-2 adsorption branches and pore throat size distributions from mercury intrusion demonstrate multifractal nature. The width of singularity spectra from multifractal analysis is used to quantitatively characterize the heterogeneity degree of pore size distribution and shows good consistency with pore size distributions derived from N-2 adsorption and mercury intrusion. The pore structure from mercury intrusion is much heterogeneous than that from N-2 adsorption, indicating that an increased number of large pores will increase the heterogeneity of pore size distributions. Compared with other rock compositions, TOC has an obvious impact on the heterogeneity of pore structure in small pore size range.