Abstract: Conventional logging methods can't be used to effectively identify low-resistivity pays. In this paper, the NMR logging was used to classify and evaluate low-resistivity pays in the Toutunhe Formation on the Eastern Fukang slope of Junggar Basin. Based on the data of NMR, cation exchange capacity, thin section and grain size analysis, the genesis of low-resistivity pays was discussed. Then, the NMR T₂ spectrum was used to qualitatively identify oil/water zones and to calculate porosity, permeability and water saturation. Although the Toutunhe Formation reservoir contains a small amount of mud and clay minerals, its special clay minerals can generate relatively high additional electrical conductivity and high irreducible water content, allowing for the low-resistivity pay (with resistivity lower than 6 Ω·m). According to the shape of NMR T₂ spectrum and the potential existence of spectral peak at 200 ms, oil zone, water-bearing oil zone, oil-bearing water zone and water zone could be effectively classified. Taking 3 ms as the cutoff of T₂ and combined with the porosity-permeability correlation obtained from core experiments, the porosity and permeability of the low-resistivity pays could be calculated. Finally, the cation exchange capacity calculated from NMR logging was substituted into a W-S model to determine the water saturation of the low-resistivity pay. The qualitative identification and quantitative evaluation results of the proposed method for low-resistivity pays well match with the experimental results, showing a good applicability in oil production.

Key words: Junggar Basin, Eastern Fukang slope, Toutunhe Formation, NMR logging, low-resistivity reservoir, logging evaluation