鄂尔多斯盆地太原组灰岩成岩演化及控储作用

Impact of diagenetic on evolution of reservoirs of Taiyuan Formation limestone in Ordos Basin

鄂尔多斯盆地太原组灰岩有良好含气显示,但目前控制灰岩储层形成的关键成岩作用类型尚不明确。针对此问题,利用岩心、薄片、阴极发光、扫描电镜、LA-ICP-MS测试等资料,开展储层特征及成岩作用研究,进行成岩流体示踪,恢复储层成岩演化历程并分析对储层的控制。研究表明:(1)太原组灰岩储层以黏结生屑灰岩为主,共经历海底泥晶化、混合水白云石化、准同生期岩溶、同生-埋藏阶段的5期次胶结以及硅质交代等多类型成岩作用。(2)利用基质灰岩与各期次关键成岩作用产物微区元素地球化学特征对比,识别出浅埋藏期“封存海水”、准同生期暴露“大气淡水”、准同生期-浅埋藏期“混合水”以及埋藏期“残余淡水”等多期次成岩流体环境。(3)储层成岩演化认为准同生期“大气淡水”流体的溶蚀作用是区内储层发育的关键驱动力,而各类胶结以及小规模白云石化作用对储层则是持续充填破坏。太原组灰岩沉积期高频海平面升降控制下的控储成岩作用模型,可为有利区预测提供帮助。

Limestone in the Taiyuan Formation in the Ordos Basin has good gas-bearing properties,but the key types of diagenesis that help the reservoir evolve remain unclear.This study investigates the characteristics of the reservoir and its evolution due to its diagenesis by examining thin-section samples from its core through cathode luminescence,scanning electron microscopy,and LA-ICP-MS.The results showed the following:(1)The limestone reservoir in the Taiyuan Formation was dominated by bioclastic limestone that had undergone many types of diagenesis,including micritization,five-stage cementation,penecontemporaneous karstification,mixed-water dolomitization,and siliceous metasomatism.(2)A comparison of the elemental geochemical characteristics of the limestone matrix and the key diagenetic products of each stage led to the identification of a four-stage environment for the fluid:“sealed seawater”in the period of shallow burial,“atmospheric fresh water”in the penecontemporaneous period,“mixed water”in penecontemporaneous-to-shallow burial period,and“residual fresh water”in the burial period.(3)The dissolution of“atmospheric fresh water”in the penecontemporaneous period was the key force driving the development of the reservoir in the area,while various kinds of cementation and small-scale dolomitization caused continuous damage as the reservoir was filled.A diagenetic model of limestone in the Taiyuan Formation controlled by fourth-and fifth-order high-frequency cycles can help predicted areas favorable for exploration and extraction.