坎波斯盆地南部下白垩统硅化碳酸盐岩储层成因及演化

Genesis and evolution of Lower Cretaceous silicified carbonate reservoirs in southern Campos Basin

通常认为,碳酸盐岩储层中一旦出现硅化现象,储层物性就会遭到破坏,难以存储油气。但有学者发现,由碳酸盐岩完全蚀变而成的硅质岩同样发育溶蚀孔洞,并且有油气发现。坎波斯盆地下白垩统湖相碳酸盐岩中硅质来源、硅质与储集空间的关系及储集空间演化规律等问题尚无系统研究,碳酸盐岩发生硅化后,储层孔隙度如何变化很难回答。基于前人研究成果,结合岩石镜下微观特征、地球化学特征分析,对坎波斯盆地下白垩统硅化碳酸盐岩储集层的岩石学特征、储集空间类型、硅质来源及储集空间演化规律等开展研究,研究结果表明发生溶蚀的硅质岩可以作为储集层。碳酸盐岩中的硅质主要为交代成因,富硅质的热液从基底通过大断裂进入碳酸盐岩地层,对原始碳酸盐岩进行溶蚀,产生硅质胶结、交代作用以及对碳酸盐岩蚀变而来的硅质岩进一步溶蚀;总体上,硅化碳酸盐岩的储集空间演化可划分为(1)碳酸盐岩高孔阶段、(2)硅质碳酸盐岩中孔阶段、(3)碳酸盐质硅质岩低孔阶段、(4)硅质岩中孔阶段、(5)硅质岩高孔阶段5个阶段;热液活动存在多期性,在沿大断裂分布的硅质岩中可以找到物性较好的裂缝-孔洞型储集层。

It is generally considered that the reservoir properties will be destroied once silicification occurs in carbonate reservoirs,leading to difficulty in the storage of oil and gas.However,some scholars have found that siliceous rocks originated from completely corroded carbonates can also develop dissolved pores,where oil and gas have also been discovered.There still lack systematic studies on the genesis of silica in the Lower Cretaceous lacustrine carbonate rocks of Campos Basin,relationship between silica and reservoir space,and evolution law of the reservoir space.After the silicification of carbonate rocks,it is difficult to grasp the changes in the evolution of reservoir porosity.Based on previous studies and the analyses of microscopic rock characteristics under microscope and geochemical characteristics,the paper investigates the petrologic features,reservoir space types,silica source and storage space evolution law of Lower Cretaceous silicified carbonate reservoirs in Campos Basin.The results suggest that dissolved siliceous rocks can act as reservoirs;silica in carbonates was formed due to metasomatism,and silica-rich hydrothermal fluid flowed into the carbonate formations through major faults,producing a cementation and metasomatism on the carbonate rocks;there were multi-stage hydrothermal fluid activities,including the dissolution of original carbonate rocks,cementation and metasomatism of silica,and dissolution of the siliceous rocks from completely corroded carbonate rocks.In general,the reservoir space evolution of the silicified carbonate rocks can be divided into five stages,including carbonate rocks with high porosity,siliceous carbonate rocks with medium porosity,carbonate siliceous rocks with low porosity,siliceous rocks with medium porosity,and siliceous rocks with high porosity.In addition,fractured-cavity type reservoirs with better physical properties can be found in siliceous rocks distributed along major faults.