Genesis and Formation Time of Calcite Veins of Middle-Lower Ordovician Reservoirs in Northern Shuntuoguole Low-Uplift, Tarim Basin

Multiphase calcite veins were developed in the carbonate reservoirs of Middle-Lower Ordovician, which have been an important indicator for both the history of fluid activity and the hydrocarbon accumulation process in the northern Shuntuoguole low-uplift, Tarim basin. Based on the thin section observation, cathodoluminescence, in-situ element analysis, fluid inclusion and strontium isotope analysis of calcite vein, the research divides the phases of calcite vein and determines the origin and formation time of calcite vein in different stages. The results show that there are four phases of calcite vein in the northern Shuntuoguole low-uplift. The Fe/Mn and U/Th values of calcite veins in different periods are different. The cathodoluminescence colors of C1, C2, C3 and C4 calcite are dark red, non-luminous or dark blue, orange yellow and bright yellow, respectively. C1 calcite vein-forming fluid came from deep fluid with high ⁸⁷Sr/⁸⁶Sr. C2 and C3 calcite vein-forming fluids are similar and both derived from diagenetic fluids with seawater properties in the same formation. C1, C2, and C4 calcite veins all formed in the partial reduction environment, and C3 calcite veins formed in the partial oxidation environment. Based on the homogenization temperature of the primary aqueous fluid inclusions developed in the veins in different periods, combined with the burial history and thermal history of the single well, it is determined that the C1 calcite vein was formed at about 445 Ma, corresponding to the Middle Caledonian act Ⅲ, C2 and C3 calcite veins formed at about 430-428 Ma and 418 Ma respectively, corresponding to the Late Caledonian period.