异常热对流白云岩化研究

Thermal convection dolomitization model had long been proposed and been confirmed by numerical simulation. Nevertheless, this model is not popular due to lacking of reliable geologic cases. In some ancient carbonate platforms, magmation/volcanism was regionally extensive. The induced abnormal thermal sources could have made the Mg-rich fluids of the underlying formations, especially the dolostone sections, convection, moving upward to the shallow limestone formations, and causing dolomitization. This is called here as “abnormal thermal convection dolomitization model”. This model could have been wide spread in the geological history when and where the magmation/volcanism happened. As examples, this research will study the Lower-Middle Ordovician Penglaiba-Yingshan formations from the Tarim Basin and the Middle Permian Qixia-Maokou formations from the Sichuan Basin. Routine petrological, mineralogical and geochemical (major, trace and rare earth elements; C-, O-, and Sr-isotopes; fluid inclusions, etc.) analyses, integrated with the newly emerging Mg-isotope technique, will be used to trace the source of Mg-rich dolomitization fluids. How these abnormal heat sources formed and evolved and the thermal convection pathways will be discussed. Numerical simulation would be conducted to set up the dynamic mechanisms for thermal convection. Then an abnormal thermal convection dolomitization model will be proposed and its contribution in dolostone formation be discussed. Dolostones were formed and evolved during late diagenetic stages. These diageneses help to reveal the distribution of dolostones with different structures. This research is key to understand how these dolostone reservoirs formed and distributed.

“热对流白云岩化”模式，虽早被提出，且数值模拟已证实，但因其缺乏可信的地质实例而并未广泛应用。在古老的、存在区域性岩浆/火山活动的碳酸盐岩台地内，由于有异常热源，可把较深部地层（特别是白云岩层段）中的大量富Mg流体带至较浅部灰岩地层，从而导致“异常热对流白云岩化”，这可能是一种广泛存在的白云岩成因机理。选取具这种地质背景的塔里木盆地中下奥陶统蓬莱坝组-鹰山组和四川盆地中二叠统栖霞组-茅口组的白云岩作为研究对象；结合传统的岩石学、矿物学和地球化学（主微量元素、稀土元素和C、O、Sr同位素，及包裹体等）分析，应用最新的Mg同位素示踪技术，厘清白云岩化所需Mg2+的来源。分析异常热源的形成与演化过程及热对流的流体通道，通过数值模拟建立热对流动力学机制，探讨异常热对流白云岩化机理。根据白云岩的形成及后期演化过程，揭示不同结构白云岩的空间分布规律。这对于理解这类白云岩储层的成因及其空间分布至关重要。

对塔里木盆地中下奥陶统和四川盆地中二叠统白云岩的岩石学、地球化学分析和Mg离子平衡计算。塔里木盆地中下奥陶统白云岩含量总体向上减少，指示白云岩化流体自下而上活动；87Sr/86Sr值(0.7090 to 0.7110)明显高于同期海水(<0.7091)，而与寒武纪时期海水相似；C、O同位素分布也与寒武统白云岩接近；表明中下奥陶统白云岩与下伏寒武统白云岩具成因联系。流体包裹体数据表明这些白云岩具热液特征。Mg离子平衡数值模拟计算表明，白云岩化流体大部分来自下伏寒武系地层，寒武系发育大量准同生白云岩，在压实过程中释放大量富Mg流体，在高温岩浆的热驱动下，沿断裂或渗透性地层向上对流，造成上覆中下奥陶统白云岩自下而上发生白云岩化。与之类似，四川盆地中二叠统发育交代白云岩和粗粒白云石胶结物，部分显示较低δ18OV-PDB值(−12.01‰ to −8.23‰)、正Eu异常、高87Sr/86Sr比值(0.7081–0.7198)、高流体包裹体均一温度(149–255 oC)，指示热液成因。受峨眉山玄武岩活动驱动，热液白云岩化流体沿断裂运动，并沿地层横向扩展。本研究解释了异常热对流白云岩化机制，这有助于理解白云岩的分布，从而为这类可能作为油气储层的分布提供借鉴。