川东南地区龙马溪组底部海相页岩高GR峰沉积成因探讨

上奥陶统五峰组-下志留统龙马溪组是一套黑色富有机质海相页岩,而龙马溪组底部的高伽马（GR）层段为主要产气层段,研究GR峰的沉积成因,对页岩气的勘探开发具有重要意义。利用常规测井和能谱测井、岩芯、岩石薄片、微量元素、矿物成分等资料,对GR峰层段划分五峰组上部页岩A段、五峰组顶部观音桥B段、龙马溪组底部GR峰值C段以及龙马溪组底部次峰D段四个沉积阶段,进而分析了各个阶段的U、TH、K与GR的相关性以及沉积变化特征。研究显示:高GR峰的出现是由于铀元素富集主导的;探讨沉积阶段内多种地质事件引起的沉积环境、有机质、黏土矿物以及含铁、含磷矿物的变化特征及其对铀元素富集的促进作用;高GR峰的出现是一次集合地质事件的地质响应,是由冰期末期的快速海侵、海水分层、陆上火山喷发以及海底岩浆热液等综合因素导致的;根据五峰组顶部观音桥层沉积特征显示,观音桥冰期的成因符合"冰川灾变"和"雪球假说",一定程度上促进了高GR峰的形成。

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

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.