多重流体介质下塔里木盆地古城寒武系碳酸盐岩储层形成与保存

Formation and protection of the carbonate reservoir of Cambrian under multiple fluid medium,Gucheng,Tarim basin

以塔里木盆地古城地区寒武系为例,探讨多重流体介质下碳酸盐岩储层形成与保存问题。古城地区寒武纪发育台缘带,上寒武统共发育4期礁滩体。根据礁滩体储层的形态特征、岩石学特征和地球化学特征,可知存在大气淡水、白云石化流体、热液流体与硅化流体等4种重要流体参与碳酸盐岩的成岩演化过程。其中大气淡水为准同生期;白云石化流体主要作用于准同生的早成岩阶段;热液流体存在2期,分别为海西早期的盆地卤水和二叠纪的中酸性岩浆热液;早期组构选择性硅化流体主要为寒武纪海水,晚期非组构选择性硅化流体包括地层水和两期热液。分析多重流体介质对储层形成和保存的作用,大气淡水导致的层间岩溶促进溶蚀孔洞的形成,具有较高水岩比例的盆地卤水也具有较强的溶蚀能力。白云石化作用对孔隙形成贡献并不大,但有利于已有孔隙的保存和继承。热液流体和Ⅱ、Ⅲ期硅化流体为同一流体的2种不同表现形式,岩浆热液对硅化作用的贡献更大。硅化作用整体表现为较强的硅质胶结,破坏已有孔隙空间,对储层的保存不利。

Taking the Cambrian carbonate of Gucheng area of Tarim basin as the study argett,the formation and protection of the reservoir under multiple fluid medium was studied.Platform margin with four reef-bank complexs were developed in Gucheng area during the late Cambrian.Based on the morphology,petrology,and geochemistry characteristics of carbonate reservoir,four important fluids participating in the diagenesis and evolution of carbonate reservoir were inferred.They are meteoric water,dolomitization fluid,hydrothermal fluid and silicification fluid.The meteoric water is syngenetic;the dolomitization fluid is parasyngenetic or militated during early diagenesis stage;the hydrothermal fluids are basinal brine activated during early Hercynian and water associated with intermediate-acidic magmas in the Permian;the fluid resulting in the early stage fabric selective silicification is Cambrian sea water while late stage silicification fluids are formation water and hydrothermal fluid.Meteoric water and basinal brine had great effect on the formation of reservoir.The local karst caused by meteoric water resulted in the formation of dissolved pores and basinal brine with relative high water/rock ratio also had dissolution capacity.The dolomitization fluid rarely promoted the formation of reservoir,but it is important for the protection and succession of pore spaces.The Ⅱ and Ⅲ stage silicifications may also be attributed to hydrothermal according to comparative analysis.Silicification showed strong cementation and was negative for the protection of reservoir.