Changes in Nanoscale Pore Structures and Mesopore Connectivity as a Result of Artificial Maturation of a Source Rock from the Shanxi Formation, Ordos Basin

Pore structure properties in shales, such as pore volume, specific surface area, pore size distribution (PSD), and pore connectivity, are important factors to impact petroleum generation, storage, and migration. However, research on changes in pore structure during thermal maturation of shales from transitional deposition environments is still limited. In this work, a series of hydrous pyrolysis experiments at temperatures from 250-550 degrees C were carried out on a source rock sample from the Lower Permian-aged Shanxi Formation in the northern Ordos Basin of China. (Ultra)small-angle neutron scattering [(U)SANS] and lowpressure gas (N-2/CO2) adsorption (LNA/LCA) methods, over a measurable range of 0.35-1000 nm in pore diameters, were used to analyze the pore properties of the resultant solids from the pyrolysis experiments, providing data on the evolution of gas-accessible and gas-inaccessible pores. Both pore volumes and specific surface areas decreased initially, then increased, and finally decreased with increasing maturity, reaching their peaks at 340 and 500 degrees C, respectively. The ratio of gas-inaccessible mesopores (2-50 nm in diameter), defined as (SANS-measured - LNA-measured/SANS-measured mesopores), initially increases, then decreases, and finally increases again. In shale samples subjected to the temperatures of 250-340 and 340-380 degrees C, the abundance of inaccessible pores 2-18 and 2-50 nm in diameter increase due to a possible filling of bitumen and generated oil; in addition, the inaccessible pore percentage at 9-S0 nm is decreased at 380-460 degrees C. A multifractal analysis was used to interpret (U)SANS PSD data to characterize the heterogeneity of pore structures at different thermal evolution stages, and the singularity index alpha(0) and its widths could be considered as major parameters to distinguish nanopore evolution and heterogeneity of PSDs from immature to overmature shales.