Effects Of Volcanic Ash On Organic Matter Enrichment In The Wufeng-Longmaxi Formation Shales,Sichuan Basin

Most major pay zones of shale oil and gas in North America and China are accompanied with volcanic ash deposition,indicating that volcanic ash deposition has a good spatiotemporal coupling relationship with high-quality source rocks.This paper mainly focuses on K-bentonite（product of volcanic ash alteration）sandwiched in the shales of Wufeng-Longmaxi Formation in the Sichuan Basin to clarify the relationship between volcanic ash and organic matter enrichment in geological history.Geochemical methods constrained by petrological evidences are used to clarify petrological and geochemical characteristics,and distribution pattern of K-bentonites.The study focuses on impact of volcanic ash on the production and preservation of organic matter,and finally establishes an organic matter enrichment model involving volcanic ash.The study is based on mineralogical and geochemical analysis of samples from two outcrop sections（the Guanyinqiao and Tianba sections）and four coring Wells.The results show that the K-bentonites are composed of non-clay minerals and clay minerals.Non-clay minerals include quartz,gypsum,albite,pyrite,calcite and a small amount of anatase.Clay minerals are the main components of K-bentonites,mainly include illite-smectite mixed layer,illite,and a minor amount of chlorite and kaolinite.XRF data indicate that K-bentonites are rich of Al,K,S,Ti,Co and V.CI chondrite-normalized multi-element diagrams indicate that K-bentonites are rich of U,Pr,Ta and depleted of Nb,Pb,Sr and Eu.Generally,the K-bentonites are characterized by an Eu negative anomaly and enrichment of light rare earth elements.Discrimination diagrams of trace elements suggest that the protolith of K-bentonites are rhyolite,rhyodacite,andesite and trachyandensite.The protolith mainly formed in a volcanic arc setting.Volcanic ashes were originated from Qinling volcanic arc belt and the convergence belt between Yangze Block and Cathaysia Block.Comparison results of paleoproductivity and redox proxies before and after volcanic ash deposition indicate that the effects of volcanic ash on paleoproductivity,redox condition and organic matter enrichment are only limited in the range of 1 to 2 centimeters after the deposition of volcanic ash.In this range,volcanic ash can release nutrient elements such as P,N,and Fe to promote primary producer bloom,thus increase paleoproductivity.Reductant contained in volcanic ash such as Fe²⁺can consume oxygen thus increase anoxic degree of water body,meanwhile sulfate contained in volcanic ash can trigger bacteria sulfate reduction and increase sulfuration degree of water body.The degradation of organic matter at the beginning of BSR and the dilution of organic matter by volcanic ash lead to the decrease of TOC content in the affected area of volcanic ash.Comprehensive analysis indicates that volcanic ash deposition was not a key controlling factor on organic matter enrichment of the marine shales in the Wufeng-Longmaxi Formation in the Sichuan Basin.