Complementary neutron scattering, mercury intrusion and SEM imaging approaches to micro- and nano-pore structure characterization of tight rocks: A case study of the Bakken shale

Small angle neutron scattering (SANS) and ultra-small angle neutron scattering (USANS) techniques have been increasingly utilized to study tight rocks (e.g., mudrocks), due to their capabilities of detecting total pore spaces (both body and throt) across the nm-mu m spectrum. Mercury injection capillary pressure (MICP) is a widely employed technique in the oil and gas industry, used to obtain a variety of petrophysical properties of porous rocks. In this study, we selected six samples from the three (i.e., lower, middle, and upper) members of the Bakken Formation (Late Devonian to Early Mississippian) in the Williston Basin, North Dakota. We utilized the complementary techniques of (U)SANS and MICP to characterize and differentiate their pore systems over a broad measurable range of pore/throat sizes from 1.25 nm to 50 Inn. Detailed processing of (U)SANS scattering data is illustrated to show how the total porosity and pore size distribution are obtained and compared against MICP analyses. The results show that the lower/upper Bakken samples and the middle Bakken samples have distinct mineral compositions and organic matter contents, which could be important factors affecting their pore structure. Assisted with the field emission-scanning electron microscopy (FE-SEM) technique, it was found that organic matter-hosted pores contribute a significant portion of total porosity in the lower/upper Bakken shales, while the middle Bakken samples are mostly composed of mineral pores. Additionally, the porosities measured by the (U)SANS and MICP techniques are related to sample sizes employed for each approach, due to a limited pore accessibility of mudrocks; a larger sample size will possess a higher proportion of isolated pores. In general, the results for the Bakken samples in this study indicate that the combination of (U)SANS, MICP, and FE-SEM approaches gives a more complete picture of the pore structure of tight rocks.