U-Pb Dating of Calcite Veins Developed in the Middle-Lower Ordovician Reservoirs in Tahe Oilfield and Its Petroleum Geologic Significance in Tahe Oilfield

Multiphase calcite veins are developed due to tectonic activities in the carbonate reservoirs of Middle-Lower Ordovician in the Tahe oilfield, which is important for studying the evolution of reservoir fluids and the process of hydrocarbon accumulation. Three phases fracture-filled calcite veins were identified in the carbonate reservoirs of Middle-Lower Ordovician in the Tahe oilfield by core observation, thin section, cathodoluminescence and in-situ element. The three-phase calcite veins have differences in cathodoluminescence color, rare earth elements, strontium, carbon, oxide isotopes and timing of calcite veins generation. The in-situ U-Pb dating by laser ablation yields the absolute ages of the three-phase calcite vein generation of 466±11 Ma to 458.6±8.8 Ma, 329.7±1.6 Ma to 308.1±4.1 Ma and 249.3±2.6 Ma to 220.5±7.3 Ma, respectively. The fluids for second and third phases of calcite vein generation are mainly derived from deep-seated diagenetic fluids with the influence of hydrocarbon-fluids. The fluid of the third phase of calcite vein generation may also be affected by crust-derived strontium, resulting in a higher ⁸⁷Sr/⁸⁶Sr ratio than that of the second phase of calcite veins. The absolute ages of calcite veins generation can be used to constrain the fault activity and hydrocarbon charge histories in multi-cycle superimposed basins. The timing of the three phases of calcite veins generation corresponds to the periods of fault activity caused by the Middle Caledonian episode I, the Middle Hercynian episode Ⅰ-Ⅱ, and the Late Hercynian-Indosinian period. The symbiotic relationship between calcite veins and hydrocarbon fluid inclusions indicates that the Middle-Lower Ordovician reservoirs experienced one episode of oil charge episode of the Middle Hercynian episode and two episodes of oil charge after the Indosinian period.