Origin of giant Ordovician cavern reservoirs in the Halahatang oil field in the Tarim Basin, northwestern China
Zhanfeng Qiao, Anjiang Shen, Shaonan Zhang, Anping Hu, Feng Liang, Xianying Luo, Guanming Shao, Yongsheng Wang, Jian‐xin Zhao, Peng Cao, Lei Jiang
Abstract
Abstract Cavern and vug porosities in the Ordovician carbonates in the Halahatang oil field are currently hot targets for hydrocarbon exploration in the Tarim Basin of northwestern China. However, crucially, the formation of cavern and vuggy porosities have not been fully understood due to multiphase tectonic-driven (e.g., faults, unconformities) diagenetic alterations. Here, we integrated petrology observations and geochemical analyses (C, O, Sr isotopes, trace and rare earth elements [REE], clumped isotopes, and laser-based U-Pb dating), aiming at determining the timing for tectonic-driven calcite precipitation, which is closely tied to karstic reservoir development. Vug-filling calcites were emplaced in the late Caledonian (460.8 ± 3.4 and 448.6 ± 5.3 Ma) and early Hercynian (335 ± 19 Ma), and the U-Pb ages for fracture-filling (ca. 364 ± 53 and 282.9 ± 5.4 Ma) and megacrystalline (from 324 ± 23 to 300.9 ± 4.8 Ma and 244.13 ± 13 to 240.5 ± 4.1 Ma) calcites predominantly fell within the Hercynian. In addition, the latest fracture-filling calcites show a slightly younger U-Pb age of ca. 158 ± 17 Ma. The combination of other geochemical analysis, including clumped isotopes, δ13C, δ18O, and 87Sr/86Sr isotopes, REE, and fluid δ18O calculation, suggest that these calcites were precipitated from formation fluids substantially influenced by meteoric water along with alternative hydrothermal fluid. Hence, this study emphasizes that fracture-related cavern reservoirs of the Halahatang oil field were well correlated with the early Hercynian, late Hercynian, and Yanshanian tectonic events and their associated fluids mixing activity. The methodologies and outcomes from the present study may be able to guide future hydrocarbon exploration in the Halahatang oil field and be applied to other oil fields with similar tectonic backgrounds.