Investigation of the Origin of Low Resistivity and Methods for the Calculation of Gas Saturation in Shale Gas Reservoirs in the Fuling Area

Abstract

To address the challenge of inaccurate gas saturation values calculated by using Archie-type resistivity model as a result of the low resistivity of shale gas reservoirs in the Wufeng–Longmaxi Formation in the Fuling area, two nonresistivity gas saturation models were established, in this study, based on the relationships between the gas saturation and dipole array acoustic/conventional logging parameters as well as gas saturation and total organic carbon (TOC). The feasibility and accuracy of the proposed methods were verified with example wells. The results indicate that the main cause of the low resistivity in shale gas reservoirs in the Fuling area is shale graphitization; however, multiple factors such as the development of pyrite and the additional conductivity of clay also contribute. The S–P wave slowness ratio (RMSC) of shale gas reservoirs in the study area decreases significantly with increasing gas saturation and has a good correlation with the density (DEN). Thus, a saturation calculation model could be established based on the RMSC and DEN and the gas saturation from core analysis. The gas saturation in shale gas reservoirs increases logarithmically with increasing TOC with a good correlation. Therefore, a saturation calculation model could be established based on measured TOC values. The calculation accuracy of the two models is higher than that of the Archie-type resistivity model. This study shows that the saturation model established by fitting the RMSC and DEN is suitable for the case of core analysis with fair regional applicability. The TOC-based saturation model has good universality. Therefore, these two new models are suitable for gas saturation calculations in low-resistivity shale gas reservoirs and have the potential for wider applications.

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