Genetic Mechanisms and Multiparameter Logging Identification of Low-Resistivity Oil Pay: A Case Study of the Triassic Chang 6 Member, Zhidan Area, Ordos Basin, China

Abstract

Low-resistivity pay has been found throughout the world. The causes and logging responses of low-resistivity reservoirs are complex and variable. The weak variations in resistivity between the oil pay and the adjacent water pay makes it difficult to identify fluids by resistivity log analysis, which reduces the overall exploration benefit of the oil field. Therefore, it is very important to study the genesis and logging identification technology of the low-resistivity oil pay. In this paper, we first analyzed the core results such as X-ray diffraction, scanning electron microscopy, mercury intrusion, phase permeability, nuclear magnetic resonance, physical properties, electric petrophysical experiment, micro-CT, rock wettability, etc. The results show that: ① the development of low-resistivity oil pays in the studied area is mainly controlled by irreducible water saturation. The complicated pore structure, high gamma ray sandstone, and rock hydrophilicity are the factors that lead to the increase of irreducible water saturation. ② The salinity of formation water and the invasion of drilling fluid also have a certain influence on the variation of reservoir resistivity. ③ In order to magnify the difference between oil and water, sensitive parameters of logging response are extracted according to the controlling factors of low- resistivity reservoir. Then, AC-R ILD, SP-PSP, GR*GR*ΔSP-RILD, and (R ILM-R ILD)/R ILDR ILD cross-plots, some overlap method, and movable water analysis are used to identify low-resistivity oil pays synthetically. In the case study, the comprehensive application of the above identification method can effectively improve the accuracy of fluid recognition step by step. It provides reference for identifying more low-resistivity reservoirs with similar geological conditions.