Study on the Sedimentogenesis of the High GR Peak Strata at the Bottom of the Longmaxi Formation in Southeast Sichuan Basin

Key words: 
• southeast Sichuan Basin /
• Wufeng Formation-Longmaxi Formation /
• high GR peak strata /
• enrichment of uranium /
• geological events

Abstract: The Upper Ordovician Wufeng Formation-the Lower Silurian Longmaxi Formation is a set of black organic Marine shale. The high GR interval at the bottom of the Longmaxi Formation is the main gas producing section, which studies the sediment genesis of the GR peak and provides reference for shale gas exploration. Based on the data of well logs, rock cores, petrographic thin sections, trace elements, and mineral composition, the sedimentary stages of the GR peak interval were accurately divided (including upper shale section A of the Wufeng Formation,the top Guanyinqiao shale section B of the Wufeng Formation,section C of the lower GR peak of the Longmaxi Formation, and subpeak section D of the Longmaxi Formation), and characteristics of sedimentary changes for each stage were analyzed. The research shows that, by analyzing the correlation between U, TH, K, and GR, we find that the occurrence of the GR peak is mainly caused by the enrichment of uranium elements. Analysis of the sedimentary environment, organic matter, clay minerals, iron and phosphorous minerals, and their effects on uranium enrichment caused by a variety of geological events at various sedimentary stages were performed, highlighting that the emergence of the high GR peak is a geological response to a collective geological event. It is caused by the comprehensive influence of rapid transgression, stratification of seawater, terrestrial volcanic eruptions, and submarine magmatic hydrothermal events at the end of an ice age. According to the sedimentary facies characteristics of the Guanyinqiao at the top of the Wufeng Formation, the genesis of the Guanyinqiao ice age conforms to the "glacial catastrophe" and "snowball hypothesis" and has a great effect on the formation of the high GR peak.