Discussion of the Relationship between Volcanic Ash Layers and Organic Enrichment of Black Shale: A case study of the Wufeng-Long-maxi gas shales in the Sichuan Basin

Key words: 
• unconventional petroleum sedimentology /
• organic matter enrichment /
• bentonite /
• shale gas /
• Sichuan Basin

Abstract: Previous studies have shown that the deposition of volcanic ash from modern volcanic activity into seawater can improve marine surface productivity,promote the flourishing of plankton,and facilitate the formation and enrichment of organic matter. However,there are few studies on the relationship between volcanic ash deposition and organic matter enrichment in geological history,which has always been controversial. A large number of volcanic ash layers developed in the gas shales of the Wufeng-Longmaxi Formation during the Ordovician-Silurian transition in South China,providing a typical strata system for this research. Total organic carbon(TOC) content,paleo-productivity, and redox conditions of the shale horizons, including volcanic ash layers and no volcanic ash layers, were comparatively analyzed. The results show that the volcanic ash deposition during the Ordovician-Silurian transition had a relatively weak effect on marine productivity and did not significantly promote the accumulation of organic matter. The redox condition has a close relationship with TOC content,suggesting it could be the main controlling factor. The influence of volcanic activity on the sedimentary environment is a very complicated process. Therefore,in the future, comprehensive studies with higher resolution(such as centimeter-scale and millimeter-scale) should be carried out by using geological,geochemical,and other testing methods to finely analyze the impact of volcanic activity on the formation and enrichment of organic matter. Sediments related to volcanic activity are one of the research targets of unconventional petroleum sedimentology,and the determination of the relationship between them and organic enrichment in shale would have an important role in the exploration and development of unconventional petroleum,such as shale oil and gas.