Fractal characterization of pores in shales using NMR: A case study from the Lower Cambrian Niutitang Formation in the Middle Yangtze Platform, Southwest China

- Fractal dimensions of shales were measured using low field NMR.
- The correlations between NMR fractal dimensions and pore structure were performed.
- Effect of organics and inorganics on NMR fractal dimensions was analyzed.
- Micropores and meso-/macropores contributed to permeability and porosity, respectively.

The fractal dimensions of the Lower Cambrian Niutitang Formation shales in the Middle Yangtze Platform, Southwest China were investigated for pore structure and fractal characteristics using low-field nuclear magnetic resonance (NMR) and N2 adsorption. The field emission scanning electron microscopy (FE-SEM), N2 and CO2 adsorption, NMR, porosity and permeability analysis were performed on six shale samples collected from outcrops. The NMR T2 distributions are consistent with the combination of pore size distributions from N2 and CO2 adsorption. The discrepancy of fractal dimensions between NMR and N2 adsorption resulting from the pore size distribution indicates that NMR fractal dimensions provide more accurate insight into the fractal characteristics of pore structure in shales than N2 adsorption fractal dimensions. The results show that the min(DNMR), with respect to the shorter relaxation times, reflects the roughness of the micropore surface area and the max(DNMR), with respect to the longer relaxation times, reflects pore volume for larger pores. The surface fractal dimension min(DNMR) has a positive correlation with TOC contents and thermal maturity, attributable to the conversion of kerogen and occurrence of organic matter pores. Min(DNMR) increases with increasing quartz content, while max(DNMR) increases with increasing clay content. The porosity from helium gas injection ranges from 2.8% to 6.7%, and permeability is between 3.55 and 6.62 × 10−2 mD. The min(DNMR) has a positive correlation with permeability, and the max(DNMR) exhibits a positive correlation with porosity, indicating that the volume fractal dimension max(DNMR) and surface fractal dimension min(DNMR) provide new indicators to measure porosity and permeability, respectively.