DISTRIBUTION CHARACTERISTICS OF BENTONITE IN THE WUFENG-LONGMAXI SHALES AND ITS EFFECT ON SHALE RESERVOIR QUALITY

This study focuses on the characteristics of the bentonite distribution within the Wufeng-Longmaxi Formation marine black shale in the Fuling area, Sichuan Basin, hence to explore the interrelation between the bentonite layers and the shale reservoir quality. Analysis regarding the development of petrophysical characteristics, shale reservoir quality and development mechanism of heterogeneous reservoir quality was carried out based on detailed core description, whole rock / clay X-ray diffraction analysis, argon ion polishing scanning electron microscopy and etc. The result shows that there are totally 42 sets of bentonite interlayers within the Wufeng Formation and lower Longmaxi Formation in the study area. We classified the strata containing bentonite layers into dense interval, sparse interval and scarce interval according to the thickness of the bentonite layers and the thickness of interlayers. By comparing the characteristics of the shale reservoir quality in different intervals, it is realized that the sparse interval is of high brittle mineral, high organic carbon content and high gas content, thus being the most favorable shale section within the reservoir. Further analysis on paleo-redox conditons and primary productivity suggest that discrete volcanic eruptions might conduce to the changes of the redox conditions during the deposition. And the moderate volcanic activity is beneficial to the enrichment of organic matter and siliceous minerals, which also favors the formation of advantageous shales.