Logging evaluation of shale laminae: A case study from the Wufeng-Longmaxi formations in the southern Sichuan Basin

Abstract

Shale laminae influence shale reservoir quality and fracturing, whereas previous studies are simply based on geological materials. Taking full advantage of geophysical logging data could provide novel insights for shale laminae evaluation in drilled boreholes. Methods Cored boreholes in the Wufeng-Longmaxi formations in the southern Sichuan Basin are utilized in this study. Shale cores, CT scanning based on full-diameter cores, large-sized thin sections, X-ray diffraction, and TOC content are first integrated to clarify shale laminae fabrics. Elemental scanning, microresistivity electrical imaging, multipole array acoustic, conventional and nuclear magnetic resonance logging techniques are interpreted to characterize shale laminae by geological calibration, and the methodology of logging evaluation on shale laminae has been set out. Results It is concluded that the target formation has developed four types of bed, includingthe silicic bed (lamina poorly developed), silicic lamina, calcareous lamina, and argillaceous lamina. Elemental scanning is implemented to determine laminae minerals and TOC content. Electrical imaging and the anisotropy index derived from multipole array acoustic and conventional logs are used to evaluate lamina density or development degree. Besides, electrical imaging is able to reflect the lamina thickness. Nuclear magnetic resonance is performed to analyze the variation in lamina pore structure. Conclusion Although logging resolution and response complexity can significantly affect the accuracy of logging interpretation of shale laminae, logging techniques rooted in petrophysical properties are valuable supplements for the evaluation of shale laminae and lamina associations.

Keywords

• sichuan basin
• wufeng-longmaxi formations
• shale laminae
• geophysical logging
• evaluation methods