Application of GEM elemental logging in shale reservoir sedimentary environment identification

Element capture logging can be used to determine the elemental contents of rocks in formations. This study focused on the BY2 well, the inaugural parameter well for shale gas in the Cambrian Niutitang Formation of the Baojing Block, located in the Middle Yangtze region. The aim is to investigate the developmental and distributional characteristics of shale gas reservoirs in this formation.By interpreting and processing elemental capture logging data, precise elemental compositions of the shale reservoirs were determined. This analysis led to the creation of a comprehensive geochemical index profile for the Niutitang Formation. Additionally, elemental geochemical indicators were used to identify and reconstruct the paleosedimentary environments. The analysis revealed that the predominant elements in the Niutitang Formation's shale were Si, Al, and Fe, accompanied by lower amounts of K, Ca, Mg, and S. The shale featured relatively high concentrations of Si, Fe, and S, which contributed to its enhanced fracturing ability. The sedimentation process of this shale could be categorized as active continental margin sedimentation. The source material for the sedimentary rocks originated from the Kangdian ancient land, located in the northwest. The sedimentation was primarily normal but was influenced by the presence of hydrothermal fluids in the region's active tectonic zone. The upper section of the Niutitang formation was subject to a dry climate during its depositional period, featuring gentle slope sedimentation at the periphery of a stagnant basin and a lack of oxygen. High water salinity, ample land supply, and low water body paleoproductivity characterized this region. Conversely, the lower section experienced a humid climate and served as a deep-water retention basin. Here, the land supply was limited, but water salinity and paleoproductivity were high, leading to the accumulation of organic matter. The aquatic setting was primarily anaerobic, conditions that were conducive to the preservation of organic matter, and provided an optimal sedimentary environment for the generation and concentration of shale gas.元素俘获测井（Element Capture Logging）可用于测定地层岩石的元素含量。本研究以中扬子地区保靖区块寒武系牛蹄塘组页岩气首批参数井——BY2井为研究对象，旨在探明该组页岩气藏的发育与分布特征。 通过对元素俘获测井数据进行解释与处理，精准测定了页岩储层的元素组成，由此构建了牛蹄塘组完整的地球化学指标剖面。此外，研究还利用元素地球化学指标识别并恢复了古沉积环境。 分析结果显示，牛蹄塘组页岩的主要元素为硅（Si）、铝（Al）及铁（Fe），伴生少量钾（K）、钙（Ca）、镁（Mg）与硫（S）。该页岩硅、铁、硫元素含量较高，这一特性提升了其压裂改造能力。 该页岩的沉积作用可归为活动大陆边缘沉积类型，沉积岩的物源来自西北部的康滇古陆。沉积过程以正常沉积为主，但受区域活动构造带内热液流体的影响。 牛蹄塘组上段沉积时期气候干旱，沉积环境为滞水盆地周缘的缓坡沉积，且处于缺氧状态；该区域水体盐度较高、陆源供给充足，但水体古生产力较低。 反之，下段沉积时期气候湿润，为深水滞留盆地环境。该区域陆源供给有限，但水体盐度与古生产力较高，利于有机质富集；水体环境以厌氧为主，该条件利于有机质保存，为页岩气的生成与富集提供了理想的沉积环境。