Abstract
In recent years, the Ordovician carbonate reservoirs in the Tarim Basin have received attention due to continuous advancements in of deep strata exploration for oil and gas. The Ordovician carbonates in the Tahe Oilfield have experienced multiple phases of tectonic movement and ancient karst action to form reservoirs consisting of multi-scale spaces such as large caves, fractures, and dissolution pores. This study identifies indicators of atmospheric freshwater karst and hydrothermal karst in the Ordovician carbonate rocks of the Tahe Oilfield by comparing and analyzing lithological observation, geochemical data, fluid inclusions, logging interpretation, and seismic data. The spatial and temporal distribution of karst reservoirs are summarized, and the results show that the stage of tectonic movement and pulsating uplift of strata occurred in the early Caledonian and Hercynian orogenies, and the carbonate rocks were uplifted to the surface and large-scale atmospheric freshwater karstification.In the deeper strata, the thermal convection of fluids caused by volcanic activity accelerated the thermochemical sulfate reduction (TSR), and the generated H2S gas accompanied the upward transport of hydrothermal fluids, which further dissolved and modified the original karst system, increasing the storage space. However, away from the heat source, calcite (positive Eu anomaly, higher 87Sr/86Sr ratio, lower δ18O value,) reprecipitated due to the temperature reduction, so the role of hydrothermal activity in reservoir reconstruction is limited. The two karst action patterns indicated that epigenetic karstification is an important process for forming carbonate reservoirs in the Tahe Oilfield and the basis of hydrothermal karst reservoir formation. This study demonstrates the research and exploration value of karstification for the geo-energy field, which could benefit sustainable development in the Tarim Basin.
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