Hydrothermal alterations of the Ordovician carbonate reservoir rocks in the Ordos Basin

The interaction between deep-seated hydrothermal fluids and hydrocarbon reservoirs has garnered significant a global oil and gas exploration shifts towards deeper and ultra-deep strata. However, the complex chemical properties and unpredictable migration patterns of hydrothermal fluids pose substantial challenges for deep oil and gas exploration. This study focuses on the Ordovician hydrothermally altered carbonate rocks in the Ordos Basin to investigate chemical properties, sources, and migration modes of hydrothermal fluids in deep-seated strata. Multiple approaches, such as core and thin section observations, laser Raman spectroscopy, fluid inclusion microthermometry, inductively coupled plasma mass spectrometry (ICP-MS), and laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) were employed to conduct this investigation. Petrological observations reveal that hydrothermal alterations in the carbonate rocks are characterized by the presence of widespread saddle dolomite, irregular dissolution pores, and hydrothermal mineral assemblages, including quartz, calcite, celestite, barite, and pyrite. Elemental geochemistry and fluid inclusion microthermometry indicate that the hydrothermal fluids are a complex mixture, enriched in Si, Li, Zn, and heavy rare earth elements (REEs), and are associated with high-salinity brines. The episodic changes in situ elemental concentrations of hydrothermal quartz are attributed to the migration of hydrothermal fluids, driven by episodic tectonism, which aligns with the known episodic hydrocarbon expulsion. The findings of this study suggest that the episodic migration and low-temperature alterations of hydrothermal fluids are likely prevalent in deep-seated anhydrite-salt-bearing carbonate rocks.