川东北下三叠统飞仙关组白云岩成因——来自岩石结构的证据

Lithological characteristics of dolomite in the Lower Triassic Feixianguan Formation of the NE Sichuan Basin,China

以岩石结构分析为基础,结合白云石有序度和Sr含量及Sr同位素等资料,对川东北下三叠统飞仙关组广泛发育的白云岩成因进行研究,从中识别出准同生白云岩和埋藏白云岩两种成因类型。其中具有储集意义的仅为多期次埋藏交代成因的成岩白云岩,包括鲕粒白云岩和晶粒白云岩两种主要结构类型。综合成岩埋藏白云岩的结构与有序度和Sr含量及Sr同位素地球化学特征,可将埋藏白云化过程划分为早、中、晚三个成岩阶段。各阶段储层白云岩的岩石结构虽然有显著的差别,但微量元素Sr含量和Sr同位素地球化学特征具有明显的相似性与同源性,不仅显示了埋藏白云化流体为具备早三叠世海水Sr同位素组成特征的海源地层水而与大气淡水无关,同时反映了多期次的埋藏白云化作用是发生在继承性发展演化的过程中的和相对封闭的体系中,因此,对储层的发育和天然气的聚集成藏都非常有利。

On the basis of analysing the rock structure and combined with the analysis of strontium isotopes and microelement strontium, the authors study the origin of dolomite reservoir in the Lower Triassic Feixianguan Formation of NE Sichuan Basin and recoglize two kinds of origin types： the penecontemporaneous dolostone and burial dolostone. The burial diagenetic dolostone with multiperiodic metasomatic geneses only of reservoir significance, including two main structure types of oolitic dolostone and crystalline dolostone. Intigrativly analyzing the rock structure of the two kinds of dolostone and the geochemical characheristics of strontium isotope and microelement strontium, the authors divide the diagenesis process of the burial dolomitization into early diagenesis, middle diagenesis and late diagenesis. In every diagenesis stage, though the rock structure of reservoir dolostones is notable different. The geochemical characteristics of strontium isotopes and microelement strontium are very similar, but they are congenetic. This not only shows that the burial dolostone diagenetic fluids are sea source formation water which has much strong composition characteristics of strontium isotope in Triassic seawater, but also reflects that the diagenesis process of multiperiodic burial dolomitization happens in the successional evolving and relatively sealing system. So this is much favorable for reservoir development and gas accumulation.