基于硫元素化学种等指标的页岩氧化还原条件判识

氧化还原条件是控制页岩岩性特征和有机质保存的重要因素之一，可以使用多种地球化学方法对其进行评估。为了探讨相关地球化学指标对海相页岩沉积环境氧化还原条件的适用性，对四川盆地长宁地区N208井筇竹寺组和龙马溪组20个岩心样品进行的总有机碳含量（TOC）、硫元素化学种和主微量元素含量测试结果显示:筇竹寺组和龙马溪组底部层段的有机碳含量总体较高，平均值分别为3.73%和3.50%；向上有机碳含量普遍变低，平均值分别为0.410%和1.70%。筇竹寺组页岩的硫元素化学种以还原态硫为主，揭示其沉积水体还原程度高；相较而言，龙马溪组页岩的还原态硫含量相对较低，表明其沉积水体的还原性比筇竹寺组弱，为相对的弱还原环境。结合微量元素富集系数构建的S-Mo_EF-TOC体系判识结果显示:筇竹寺组页岩整体上属于贫硫酸盐的厌氧环境，其底部为厌氧环境，向上还原程度变低；龙马溪组页岩总体为含硫酸盐的厌氧—贫氧环境，向上还原程度变低，表现为贫氧环境。这些分析结果可以为加深认识筇竹寺组和龙马溪组页岩层系的地球化学特征及页岩气资源潜力评价提供新的参考。

Multiple Proxies for Redox Condition Indentification Based on Sulfur Species: A case study of the cored Qiongzhusi and Longmaxi Formations of well N208

Redox conditions play a vital role in the lithology and preservation of organic matter in shale rocks， which various geochemical methods can be used to identify. Many studies have showed that geochemical indicators have their own scopes， and their practical application needs to suit measurements according to local conditions. To study the applicability of geochemical indicators and obtain the redox conditions of marine sedimentary shale， the contents of total organic carbon （TOC）， sulfur species， and main and trace elements of 20 core samples from the Qiongzhusi and Longmaxi Formations of well N208 were performed. The results showed that TOC in the bottom section of the Qiongzhusi and Longmaxi Formations was generally high， with averages of 3.73% and 3.50%， respectively； TOC of the upper and middle parts of the Qiongzhusi and Longmaxi Formations was generally low， with average values of 0.41% and 1.70% respectively. The sulfur species in the shales of the Qiongzhusi Formation were pimarily reduced， indicating a high degree of reduction in the sedimentary water body. In contrast， the reduced sulfur of the Longmaxi Formation shales was relatively low， which suggests that the reduction degree of the sedimentary water body of the Longmaxi Formation was lower than that of the Qiongzhusi Formation. Based on the enrichment coefficient of trace elements， the S-Mo_EF-TOC system indicates that the Qiongzhusi Formation shale was dominated by sulfate-poor suboxic and anoxic conditions， with an anoxic environment in the bottom and a suboxic in the middle and upper sections. The shales of the Longmaxi Formation are formed in suboxic conditions containing sulfate， with an anoxic environment in the bottom and a suboxic in the middle and upper sections. These results have a good corresponding relationship with the petrological characteristics of the Qiongzhusi and Longmaxi Formation shales and can provide a new reference for deepening the geochemical understanding of these two formations and evaluating their shale gas resource potential.