羌塘盆地昂达尔错地区白云岩成岩结构特征及其成因意义

Diagenetic fabrics and genetic significance of the dolostones from the Angdaerco area in the Qiangtang Basin

通过薄片、阴极发光、背散射和扫描电镜等特征分析,根据白云石的晶体颗粒的形态、大小、晶面、接触关系等特征,兼顾白云石的成因信息,将羌塘盆地昂达尔错地区古油藏中白云石划分为粉-细晶曲面它形结构、粉-细晶曲面它形雾心亮边结构、细-中晶直面半自形-自形结构、细-中晶直面半自形-自形雾心亮边结构、细-中晶直面半自形-自形环带结构、中-粗晶曲面它形结构。成岩结构特征反映了成岩期不同成岩流体作用叠加的多成因模式,包括准同生阶段蒸发咸化海水成因、浅埋藏阶段蒸发咸化海水的回流或下渗扩散交代成因、中-深埋藏阶段埋藏白云石化成因以及深埋藏阶段与构造运动相关的热液交代成因。白云岩中常见的残余粒屑结构和藻纹层结构反映出原始的浅滩相、潮坪相及生屑滩相等高能沉积环境更有利于该区白云岩的形成,原始的沉积微相控制着白云岩化作用的发育和白云岩的空间展布。

Six types of diagenetic fabrics have been distinguished for the dolostones from the Angdaerco fossil oil reservoirs in the Qiangtang Basin according to thin section,cathodoluminescence,back scattered electron images and scanning electron microscopy analysis,in integration with crystal size,crystal shape,crystal surface,contact relationship and relevant genetic information. They include power to fine-crystalline nonplanar anhedral fabric;power to fine-crystalline nonplanar fogged center-clear margin fabric; fine-to medium-crystalline planar subhedraleuhedral fabric; fine-to medium-crystalline planar fogged center-clear margin fabric; fine-to medium-crystalline planar subhedral-euhedral zonal fabric, and medium-to coarse-crystalline nonplanar anhedral fabric. The diagenetic fabrics outlined above suggest that the dolomite in this area should be the result of the replacement of calcite by evaporated salted seawater,and be transformed by multistage diagenetic fluids within different genetic models,including evaporated salted seawater genetic model in the penecontemporaneous stage; seepage reflux genetic model in the shallow burial stage; burial dolomitization genetic model in the medium to deep burial stage,and hydrothermal metasomatism genetic model associated with tectonic movement in deep burial stage. Residual granular structure and algal-laminated structure in the dolostones have disclosed that original high-energy sedimentary environments such as the shoal facies,tidal flat facies and bioclastic shoal facies are more conductive to the formation of the dolostones in the study area,and that the development of dolomitization and spatial distribution of dolostones are generally controlled by the original sedimentary microfacies and rock types.