Objective Deep carbonate reservoirs have an enormous potential for oil and gas exploration. However， due to their complex diagenesis， they are highly heterogeneous and dense. To clarify the modifying effect of hydro-thermal fluid activity on the carbonate reservoirs in a petroliferous basin， this study investigated Upper Carboniferous strata in the southwestern Tarim Basin. Methods The main study area includes some key well locations in the Bachu Uplift and Maigaiti Slope， and typical field outcrop profiles in the Kunlun piedmont thrust belt. Previous research on hydrothermal fluid activity has mainly focused on the northern and Tazhong region of the Tarim Basin， mostly concentrating on the Ordovician System. These studies have accumulated rich experience and obtained many results； however， generally little exploration has been carried out in the southwestern Tarim Basin. To strengthen the basic research， the method of combining geology and geochemistry was adopted in the study area： core observation， thin sections， cathodoluminescence， scanning electron microscopy， isotope data， rare earth element characteristics and previous information were used to identify the indicators of hydrothermal activity in the target formation， defining the alteration mode of carbonate reservoirs in the study area and establishing their logging responses to alteration by the heated fluid. Results A detailed classification of the petrology of the study area found that the presence of typical hydrothermal minerals （e.g.， fluorite， barite， and saddle-shaped dolomite） is an indicator of hydrothermal fluid activity. The geochemical responses of these carbonate rocks to modification by hydrothermal fluids includes high ⁸⁷Sr/⁸⁶Sr values， very low δ¹⁸O， light rare earth element enrichment， loss of heavy rare earth elements， and positive Eu anomalies. Hydrothermal fluids often contain many chemically active substances and release a large amount of heat energy in their interaction with rock and produce physical and chemical reactions with the rock. In particular， such reactions promote strong dissolution of carbonate rocks. Conclusions It has been demonstrated that hydrothermal fluids have both positive and negative effects on the alteration of carbonate rock reservoirs. In the constructive role， the high temperature and rapid flow rate of hydrothermal fluids have strong chemical dissolution and mechanical erosive effects on the rock， expanding migration channels， increasing reservoir porosity， improving pore structure， and generally enhancing reservoir physical properties. Most of these occur at the earliest stage of hydrothermal activity and in the proximity of fault zones. They are identified in the logging response by high acoustic time difference， neutron porosity， low natural gamma and low resistivity values. The dissolution pores and fractures are developed over a large zone， and most are not filled. Conversely， a destructive effect occurs at lower temperature and pressure of the hydrothermal fluid， and is recognized by hydrothermal mineral infill in pores and fractures. As the fluidity decreases， the material carried by the hydrothermal fluid is constantly precipitated， forming new mineral combinations filling pre-existing pores and fractures， which reduces reservoir porosity and damages pore structure. This type of alteration mostly occurs in the late stage of hydrothermal activity far from the fault zone. It is evident in logging response by high resistivity and low natural gamma and acoustic time difference， and low neutron porosity； the pores and fissures are mostly isolated and extensively filled. Overall， however， the target strata were mainly affected by constructive hydrothermal dissolution and its positive impact on reservoir development.