Petroleum-charge history of Paleozoic carbonates in the Tahe oil field, Tarim Basin, northwestern China: Insights from oil geochemistry, fluid inclusion, and U-Pb dating

The Tahe oil field is the largest Paleozoic petroleum accumulation in China, with a multistage petroleum-charge history. Ordovician carbonate reservoirs in this field are investigated in this work based on oil geochemistry, fluid inclusion analyses, and in situ U-Pb dating of calcite cements. Four charge episodes are identified based on the following: (1) the similar origin but different maturation histories of sampled crude oils; (2) the four distinct fluorescence-color and fluorescence-spectra parameters revealed by oil inclusions; and (3) the distinct in situ U-Pb ages obtained, which indicate that three stages of calcite cements associated with primary oil inclusions with different fluorescence colors occurred at 459.4 ± 7.5 to 457.6 ± 6.2 Ma, 310.0 ± 3.6 to 304.6 ± 5.5 Ma, and 220.5 ± 7.3 to 215.7 ± 2.6 Ma, with uncertainties of calculated ages reported at the 2σ level. In addition, there are different fluorescence-color and color-spectra parameters between primary and secondary oil inclusions when considering the three stages of calcite cements. In addition, four oil-charge episodes are identified at ca. 445–452.1 Ma, 295.6–311.5 Ma, 215–224.8 Ma, and 108.5–116.6 Ma based on the lower homogenization temperatures (Th) of aqueous inclusions coeval with the oil inclusions. These four episodes relate to the Caledonian, Hercynian, Indosinian, and Yanshanian Orogenies, respectively. This work is important because it suggests that fault reactivation during the main tectonic episodes generated the main pathways for oil in the Ordovician carbonate reservoirs of northwestern China. It reveals that the lower Th of aqueous inclusions coeval with oil inclusions, when combined with the U-Pb dating of calcite cements, can be used to determine the timings of oil charge in the Tahe oil field, not only in China but also in the deeply buried carbonate reservoirs hydrocarbon systems worldwide.