2D NMR-based quantitative characterization of oil contents in shales of the Paleogene Hetaoyuan Formation, Nanyang Sag, Nanxiang Basin

Shale oil development is facing challenges in accurately quantifying oil-bearing properties and determining key parameters. In this study, focusing on frozen core samples from the Paleogene Hetaoyuan Formation in the Nanyang Sag, Nanxiang Basin, we improve the experimental method for two-dimensional nuclear magnetic resonance (2D NMR). Based on systematic oil-water calibration experiments, we conduct 2D NMR experiments, Rock-Eval pyrolysis, multi-step temperature-programmed pyrolysis, and azeotropic distillation on the samples. The experimental results reveal that the crude oil and pore water with different occurrence states in shales can be quantitatively characterized by determining the 2D NMR T₁-T₂ spectrum-based evaluation chart for shale oil, combined with calibration experiments. The results indicate that the contents of free and adsorbed oil calculated using 2D NMR slightly exceed those of free and adsorbed hydrocarbons determined by temperature-programmed pyrolysis, with strong positive correlations observed between the counterparts. These findings demonstrate that 2D NMR enables more effective hydrocarbon retention. In contrast, the effective porosity and oil saturation calculated using 2D NMR exhibit relatively weak correlations with gas logging-derived porosity and azeotropic distillation-derived oil saturation, respectively, reflecting the influence of sample properties and experimental methods. 2D NMR technology offers the effective retention of light hydrocarbons in shale oil. Meanwhile, this technology can yield oil-water saturation and multiple key parameters for quantitative shale oil evaluation, including the contents of free oil, adsorbed oil, and pore water. These advantages highlight the significant technical advantages and great application potential of 2D NMR in the geological assessment of shale oil.