It is difficult to determine flow pathways of paleofluids in sedimentary basins. Petrography, fluid inclusion, and isotopic compositions from different phases of calcite veins in the Ordovician in the Tazhong area were studied. Our results reveal a complex diagenesis process with two importantly crossformational fluid movements, coupled with three phases of calcite veins (C1, C2, and C3) precipitation and thermochemical sulfate reduction by hydrocarbons. Fluids that precipitated C2 and C3 calcites were saline and enriched in 18O and thus may have originated from underlying evaporates, whereas fluid that precipitated C3 had an additional contribution from 87Sr-rich, Ba-rich, and Zn-rich Precambrian basinal clastic units. Fluid inclusion homogenization temperatures, salinities, and δ18Owater values of C2 veins show decreasing trends with increasing distances from strike-slip faults. This may indicate that C2 fluid might migrate into Ordovician carbonate reservoirs through strike-slip faults within 6 km during late Hercynian orogeny (late Permian). The similarly decreasing trends are observed in C3 veins, indicating that the eastern part of No. 1 fault seemed to be the main fluid-flow pathway for C3 calcite precipitation during Himalayan orogeny. Hydrocarbon may have migrated along the similar pathways as supported by (1) abundant primary hydrocarbon-bearing fluid inclusions in the C2 and C3 calcites and (2) well-documented hydrocarbon migration and mixing. This is an ideal case showing how spatial variation in geochemical data from fracture fillings can be used to trace paleowater and likely hydrocarbon flows in sedimentary basins; it thus may be useful in predicting fluid flow and distribution of hydrocarbons in the Tarim Basin and other basins.

中文翻译：

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塔里木盆地塔中地区奥陶系碳酸盐岩方解石脉系及其在示踪古水流和油气运移中的重要性

沉积盆地中古流体的流动路径难以确定。研究了塔中地区奥陶系方解石脉不同相的岩石学、流体包裹体和同位素组成。我们的研究结果揭示了一个复杂的成岩过程，具有两个重要的跨地层流体运动，再加上方解石脉（C1、C2 和 C3）三相沉淀和碳氢化合物对硫酸盐的热化学还原。沉淀 C2 和 C3 方解石的流体是含盐的并富含 18O，因此可能源自下伏蒸发，而沉淀 C3 的流体则额外贡献了富含 87Sr、Ba 和 Zn 的前寒武纪海盆碎屑岩单元。流体包裹体均质温度、盐度、C2 脉的δ18Owater 值和δ18Owater 值随着离走滑断层距离的增加呈递减趋势。这可能表明，C2 流体可能在晚海西造山运动（晚二叠世）通过 6 km 以内的走滑断层运移到奥陶系碳酸盐岩储层。在 C3 脉中观察到类似的下降趋势，表明 1 号断层的东部似乎是喜马拉雅造山运动期间 C3 方解石沉淀的主要流体流动通道。碳氢化合物可能沿着类似的路径迁移，这得到了 (1) C2 和 C3 方解石中丰富的原生含烃流体包裹体和 (2) 有充分记录的碳氢化合物迁移和混合的支持。这是一个理想的案例，展示了如何使用来自裂缝填充物的地球化学数据的空间变化来追踪沉积盆地中的古水和可能的碳氢化合物流；因此，它可能有助于预测塔里木盆地和其他盆地的流体流动和油气分布。