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Accurate gas saturation calculations are critical to evaluating the production of marine shale gas reservoirs. As a high-resolution exploration method, geophysical resistivity well-logging technology has been widely applied in almost all types of oil/gas reservoirs to evaluate formation fluid saturation. Although the calculated saturations are accurate for conventional reservoirs, it is a challenging task to acquire the gas saturation of shale gas reservoirs due to the presence of complex rock compositions and fluid types. It is necessary to analyze different influencing factors on electrical properties to establish a more applicable gas saturation model for marine shales. In this work, we make full use of geological data, well logging data, and rock experiment data to analyze the influencing factors of electrical properties in the Wufeng-Longmaxi Formation in the Sichuan Basin, China. Six conductive factors are studied, including stratigraphic structures, clay minerals, pyrite, organic matter, pore structures, and formation fluids. Then, a shale conductivity model is developed, in which high- and low-resistivity layers are connected in parallel. Based on the conductivity model, a new method for influencing factors of stepwise stripping conductivity is proposed to calculate shale gas saturation. Finally, by interpreting the well logging data of two shale gas wells, we compared the saturation calculation results of different methods to demonstrate the accuracy of the new method. The results show that thin, low-resistivity layers, clay minerals, pyrite and overmature carbonized organic matter reduce the resistivity of shale and weaken the contribution of fluids to the measured shale resistivity. Moreover, the calculation accuracy of this new method is better than that of Archie’s equation, Simandoux’s equation, and the neutron-density porosity overlay method. The findings of this paper will help gain insight into the mechanism of resistivity responses for marine shale reservoirs and improve the accuracy of the estimated gas saturation.

The oil-based mud filtrate will invade the formation under the overbalanced pressure during drilling operations. As a result, alterations will occur to the nuclear magnetic resonance (NMR) response characteristics of the original formation, causing the relaxation time of the NMR T2 spectrum of the free fluid part to move towards a slower relaxation time. Consequently, the subsequent interpretation and petrophysical evaluation will be heavily impacted. Therefore, the actual measured T2 spectrum needs to be corrected for invasion. For this reason, considering the low-porosity and low-permeability of sandstone gas formations in the East China Sea as the research object, a new method to correct the incorrect shape of the NMR logging T2 spectrum was proposed in three main steps. First, the differences in the morphology of the NMR logging T2 spectrum between oil-based mud wells and water-based mud wells in adjacent wells were analyzed based on the NMR relaxation mechanism. Second, rocks were divided into four categories according to the pore structure, and the NMR logging T2 spectrum was extracted using the multidimensional matrix method to establish the T2 spectrum of water-based mud wells and oil-based mud wells. Finally, the correctness of the method was verified by two T2 spectrum correction examples of oil-based mud wells in the study area. The results show that the corrected NMR T2 spectrum eliminates the influence of oil-based mud filtrate and improves the accuracy of NMR logging for calculating permeability.