塔里木盆地西部下寒武统肖尔布拉克组白云岩成因及储集性

Deeply-buried carbonate reservoir has drawn much more attentions than ever before since the progress of petroleum exploration. To better understand the ‘nature and evolution of dolomitizing fluids and their alteration of carbonate reservoir properties’, a key scientific issue of ancient massive dolomitization, this study places the target on the pervasive dolostones in the Lower Cambrian in western Tarim Basin. Based on detailed field outcrop investigations and petrographic studies, different genetic types of dolostones and paragenetic sequence are recognized. The nature and evolution of the dolomitizing fluids are discussed in terms of their isotopic compositions (C, O and Sr) integrated with fluid inclusion microthermometry of these dolostones. Simultaneously, the origin and evolution of reservoir spaces of dolostones are also analyzed. Accordingly, hydrothermal dolomitization model and its significance of reservoir property would further be proposed in combination of the geological setting, regional tectonic evolution and thermal anomalies.

随着油气勘探的不断深入，深层碳酸盐岩优质储层成为研究关注的焦点。为了更好地认识“白云岩成因中流体的性质、演化及其对碳酸盐岩储集性改造”这一科学问题，本项研究以塔里木盆地西部下寒武统肖尔布拉克组地层中广泛发育的白云岩为研究对象，通过对野外典型地质剖面的详细解剖和镜下岩相学观察，查明该地区白云岩的类型及矿物共生序列；运用同位素（碳、氧、锶）地球化学和流体包裹体测温相结合的方法，探讨白云化流体的性质及其运移演化路径，分析白云岩储集空间的形成、演化过程；在此基础上，结合沉积背景、区域构造演化和关键热异常事件，探讨构造-热液白云化作用机理及其改造碳酸盐岩储集性的强度。

塔里木盆地寒武-下奥陶统碳酸盐岩中出现了广泛的白云岩化，为明确白云化流体的性质、演化及其对碳酸盐岩储集性改造，对库鲁克塔格地区下寒武统白云岩成因进行了详细的沉积学与地球化学分析。基于详细的岩相学研究，共识别出3种基质白云石和1种白云石胶结物。基质白云石包括：（1）粉-细晶、直面、自形-半自形漂浮状基质白云石；（2）细-中晶、直面、自形-半自形基质白云石；（3）细-粗晶、曲面、它形基质白云石；胶结白云石包括：（1）粗晶、曲面、鞍形白云石胶结物。漂浮状基质白云石的展布受低振幅缝合线控制，说明其形成的镁离子主要来源于压实、压溶作用驱排释放的残留地层卤水。自形-半自形基质白云石、它形基质白云石和鞍形白云石与宿主微晶灰岩具有相似的同位素组成，表明白云化流体为封存在宿主微晶灰岩中的残留海源卤水，或受到了宿主微晶灰岩的缓冲作用；然而，相对较低的δ18O值和较高的87Sr/86Sr值表明白云化流体在运移过程中与宿主围岩发生了强烈的水-岩反应，浓缩富集了含泥质和/或长石质碎屑沉积物释放的富放射性87Sr和镁离子。挤压环境下次级张性断裂系统是白云化流体运移的重要疏导体，深部热流体可不断地充注汇聚。结合区域构造演化，白云化作用最有可能发生的时限为晚海西期，大气淡水的不断注入最终导致了白云化作用的停止。本项目揭示了汇聚型板块边缘挤压背景下次级张性（或张扭性）断裂系统可汇聚大量的热液流体交代碳酸盐岩形成构造-热液白云岩，但由于流体持续时间短、通量低而限制了构造-热液白云岩的规模。