*2.3. Determination of Optimal T2cut-off*

In this study, the core NMR experiment of water saturation and centrifugation was conducted on 29 cores to determine the optimal *T*2*cut-off* value in the study area. The specific parameters are shown in Table 2. According to the cut-off value, the *T*<sup>2</sup> spectrum of NMR logging was divided into macropores and small pores. The *T*<sup>2</sup> spectrum of NMR logging less than the *T*<sup>2</sup> cut-off value represents the small pores and retains its original shape; the *T*<sup>2</sup> spectrum of NMR logging greater than the *T*<sup>2</sup> cut-off value represents the macropore part, which needs morphological correction. The size of the *T*2*cut-off* value was related to the density of the core. With increasing porosity and permeability, the *T*2*cut-off* value also tended to increase, yet the fitting degree was relatively low (determination coefficient R2 = 0.38). Although the *T*2*cut-off* value had a good correlation with the geometric mean value of the *T*<sup>2</sup> spectrum (determination coefficient R2 = 0.68), the geometric mean value of the *T*<sup>2</sup> spectrum of the oil-based mud well was quite different from the geometric mean value of the *T*<sup>2</sup> spectrum of saturated water, so it cannot be applied to actual logging interpretation. Therefore, the average value of the experimental *T*2*cut-off* value of 29 cores of 17.48 ms was finally selected as the optimal *T*2*cut-off* value.


**Table 2.** Statistical table of physical property parameters of 29 sandstone core samples.
