Application of U-Pb dating technique in the study of hydrothermal activities of dolomite reservoir: A case study on 3<sup>rd</sup> member of Yingshan Formation in Gucheng area, Tarim Basin, NW China

Abstract

古城地区是塔东探区的重点勘探区块，下奥陶统鹰三段是塔东探区首要的勘探层系。探寻多期热液作用改造后的优质白云岩储层是扩大研究区勘探成果的关键。本文在储层岩石学特征判识的基础上，应用U-Pb同位素定年技术，结合碳氧锶镁同位素和稀土元素分析测试，分析了热液性质，明确了热液作用发生的确切时期及期次，探讨了热液作用对白云岩储层的影响。研究结果表明，古城地区鹰三段白云岩储层发育准同生-浅埋藏海水成因的粉晶-中晶白云岩及热液作用产物。热液作用可划分为二期：第一期为中-晚奥陶世贫镁热液，表现为鞍状白云石充填缝洞及重结晶次生加大的粗晶白云石，U-Pb测定年龄为464±12Ma到473.9±9.1Ma，氧同位素值比早奥陶世正常海相白云石明显偏负，锶同位素比值明显高于早奥陶世海水值，稀土元素Eu正异常，锶、铁、镁元素含量低；第二期为晚奥陶世-早志留世钙质热液，表现为方解石充填缝洞及对先存白云石的溶蚀，U-Pb测定年龄为448±15Ma到457.4±6.6Ma，氧锶同位素组成严重偏离同时期正常海相白云石范围，轻稀土元素明显富集，Eu正异常，锶、钙元素含量高，锰元素含量低。第二期热液作用是白云岩储层溶蚀的关键，同期NNE向断裂带作为其输导系统，是优质储层发育的有利部位，为有利区带预测提供依据。;The dolomite in 3^rd member of Yingshan Formation in Gucheng area is a significant exploration field in the east of Tarim Basin. High-quality dolomite reservoir reworked by hydrothermal activities is the key to expand exploration. Based on petrographic characteristics, this paper analyzed hydrothermal properties, clarified the precise dating and stages of hydrothermal process, and discussed hydrothermal effects on dolomite reservoir using U-Pb dating technique, combined with analysis of carbon and oxygen, strontium, magnesium isotopes and rare earth elements. The study shows that micrite-medium grained dolomites taken place in paracontemporaneous-shallow buried phase and hydrothermal products in research area. There are two stages of hydrothermal processes. The first stage hydrothermal process is very low in magnesium content in the Middle-Late Ordovician, at the result of saddle dolomite filling in fractures/vugs and coarse dolomite formed by recrystallization aged 464±12Ma~473.9±9.1Ma by U-Pb dating with oxygen isotope negative compared with normal marine dolomite, strontium isotope values higher than Early Ordovican sea values and positive Eu anomalies. The contents of strontium, ferrum and magnesium are relatively low. The second stage hydrothermal process is high in calcium content in the Late Ordovician-Early Silurian, at the result of calcite filling in fractures/vugs and dissolution of existing dolomites, aged 448±15Ma~457.4±6.6Ma by U-Pb dating with significantly negative oxygen isotope and enriched LREE and positive Eu anomalies. The strontium content is high while manganese content is low. Migration pathway of the second stage hydrothermal process as a key of dolomite reservoir dissolution is NNE fracture zones developing high-quality reservoir, which provides a basis for the prediction of favorable zones.