Temperature-dependent evolution of shale oil occurrence states: NMR characterization and quantitative evaluation

Characterization of shale oil occurrence states is critical for accurate resource evaluation and efficient development. However, existing methods are unable to directly reveal the in situ occurrence states and quantitative characteristics of shale oil under reservoir conditions. In this study, sealed-coring shale samples from the Dongying Sag, Bohai Bay Basin, were investigated by high-temperature and high-pressure NMR combined with variable-temperature NMR experiments. The effects of temperature on the NMR responses of hydrogen-bearing components, signal calibration conversion relationships, and shale oil occurrence states were systematically analyzed. A temperature-dependent quantitative evaluation method for adsorbed oil and free oil was established. The results show that increasing temperature significanty alters the relaxation characteristics and occurrence states of shale oil. Adsorbed or weakly bound oil gradually transforms into free oil with increasing temperature. The T 1 and T 2 peak positions of pore water and free oil increase with temperature, whereas the calibration conversion relationships obtained by mass compensation decrease with increasing temperature. Based on the temperature-dependent dynamic spectral partitioning scheme and calibration conversion relationships, the adsorbed and free oil contents were quantitatively evaluated. When temperature increased from 25 °C to 100 °C, the average adsorbed oil content decreased from 4.80 mg/g to 2.36 mg/g, and the adsorbed oil proportion decreased from 35% to 18%. This study establishes an NMR-based evaluation method for shale oil occurrence states under high-temperature conditions, providing a methodological basis for the fine characterization of shale fluids under reservoir conditions and the temperature correction of NMR logging interpretation parameters.