Integrated pore size and grain size characterization of geomaterials from Ewekoro, Eastern Dahomey Basin

• PSD via regularized least-squares inversion of NMR relaxation curves. • Grain size analysis using sieve method based on univariate statistical parameters. • Micro- and meso-pores in all samples, and macropores only in limestone samples. • All samples manifest multiple peak pore systems. • Mean and sorting values reveal intermediate to high energy depositional environment. The pore and grain size of rocks and geomaterials are very crucial factors in evaluating their usefulness in industrial productions, geological and petrophysical assessments, geotechnical engineering projects and environmental studies. The study techniques include low field NMR measurements via CPMG pulse sequence on fully saturated disc-shaped samples and grain size analysis on sample fragments. NMR Relaxation curves were inverted via conventional regularized least-squares inversion routine to generate pore size distribution (PSD). Grain size analysis using sieve method was based on grain size distribution (GSD) and statistical parameters (mean ( Mz ), sorting ( σ i ), skewness ( SK i ), and kurtosis ( K G )). PSD depicts the presence of micropore and mesopores in all samples, with macropores detected only in limestone. All samples manifest well connected multiple peak pore systems. Sandstone and glauconite samples exhibit bimodal PSD. PSD for Limestone samples is trimodal which is a manifestation of a more complex pore system common in carbonate rocks mainly due to the composition of grains, matrix and cement. M z values reveal more of medium/coarse grain composition across samples, implying an intermediate to high energy depositional environment. σ i values indicate poor to medium sorting which reveals that the rock sediments were likely deposited in fluvial-continental environments. K G values are a mix of mesokurtic, leptokurtic and platykurtic, indicating a combination of depositional environments. Observed spread in skewness values also corroborates the diverse nature of sediment transport for rock formation and existence of mixed or transitional environment.