The Lower Silurian Longmaxi Formation in the southern Sichuan Basin is composed of a series of dark carbonaceous shales deposited in a hydrostatic shelf reduction environment. In this study, the ratio of uranium to thorium (U/Th), the total organic carbon (TOC), and the biological silicon content (Si_Bio) were selected as the characteristic parameters to precisely analyze the sedimentary environment and its impact on reservoir quality. The results show that the Weiyuan area in the Early Silurian Longmaxi period experienced two transgression-regression cycles, forming two third-class sequences, SSQ1 and SSQ2, which can be divided into six sedimentary microfacies: organic-rich siliceous argillaceous shelf, organic-rich silicon-containing argillaceous shelf, organic-rich silty argillaceous shelf, deep-water silty argillaceous shelf, shallow-water silty argillaceous shelf, and shallow-water argillaceous silty shelf microfacies. The organic-rich siliceous argillaceous shelf and organic-rich silicon-containing argillaceous shelf microfacies developed in the deepest transgressive system tract (TST1), with high U/Th, high TOC, and high Si_Bio, which are identified as the main control facies for reservoir development. These two microfacies are located in the middle of the study area, while a transition occurs in the east affected by the Neijiang Uplift. According to the classification criteria proposed in this article, the favourable shale gas reservoirs in Weiyuan area are characterized with high U/Th (>1.25), high TOC (>3%), and high Si_Bio (>15%). This paper proposed an evaluation method for shale sedimentary facies based on elemental and electrical logging characteristics, avoiding the limitations of core samples, which makes the quantitative division of shale sediments and the efficient recognition of high-quality reservoirs available. It is of great significance for delineating the potential production areas in the study area and beneficial for the scaled development of shale gas reservoirs.

中文翻译：

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基于元素特征的威远地区龙马溪组页岩气藏沉积相

四川盆地南部下志留统龙马溪组由一系列沉积在静水陆架还原环境中的深色碳质页岩组成。在本研究中，铀与钍的比例 (U/Th)、总有机碳 (TOC) 和生物硅含量 (Si _Bio)作为特征参数来精确分析沉积环境及其对储层质量的影响。结果表明，早志留世龙马溪威远地区经历了两次海进-海退旋回，形成了SSQ1和SSQ2两个三级层序，可划分为6个沉积微相：富含有机质硅质泥质陆棚、富含有机质含硅泥质陆棚、富含有机质粉质泥质陆棚、深水粉质泥质陆棚、浅水粉质泥质陆棚、浅水泥质粉质陆棚微相。最深海侵体系域(TST1)发育高U/Th、高TOC、高Si的富有机硅泥质陆棚和富有机硅泥质陆棚微相_Bio，被确定为储层开发的主要控制相。这两个微相位于研究区中部，而受内江隆起影响的东部发生转变。根据本文提出的分类标准，威远地区页岩气有利储层具有高U/Th（>1.25）、高TOC（>3%）、高Si _Bio(>15%)。本文提出了一种基于元素和电测井特征的页岩沉积相评价方法，避免了岩心样品的局限性，使得页岩沉积物的定量划分和优质储层的有效识别成为可能。对圈定研究区潜在产区具有重要意义，有利于页岩气藏规模化开发。