Predicting oil saturation of shale-oil reservoirs using nuclear magnetic resonance logs

Oil saturation is an important parameter in shale-oil reservoir evaluation. However, due to complex wettability and pore construction, we find that conventional resistivity and nuclear magnetic resonance (NMR) methods do not perform well in calculating oil saturation in shale-oil reservoirs. Hence, we have developed a practical NMR-based method to calculate the oil saturation of the Lucaogou shale-oil Formation, Permian, in Jimusar Sag, Junggar Basin, China. First, we analyze the relationships among the wettability, oil saturation, and T-2 distribution based on the theoretical formula and core analysis data. Results indicate that the ratio of the surface area wetted by water and oil is approximately equal to the ratio of water saturation and oil saturation. So we conclude that oil is mainly stored in relatively bigger pores and the surface relaxivity of the oil-wet surface is lower than that of the water-wet surface, resulting in long relaxation signals, that is, the long relaxation signals of NMR T-2 spectra of shale-oil reservoirs are primarily attributed to oil signals. We have made a series of NMR measurements of as-received samples and confirm this point. Thus, we propose a T-2 cutoff for water and oil to calculate the oil saturation, and we determine 6 ms as the T-2 cutoff based on the oil saturation analysis of cores and NMR logs. Finally, we verify and make application of our method and acquire good results.