Abstract: The Lower Paleozoic carbonate rocks in the Tarim Basin are widely developed,representing an important target of hydrocarbon exploration.Taking the Lower Ordovician in Tahe and Yubei areas in the Tarim Basin as examples,we discussed the impact of hydrothermal activities on reservoir formation in the fault belts and along stratigraphic framework,through core and thin section observation,electron probe microanalysis (EPMA),seismic and logging data,as well as the analyses of C,O,and Sr isotopes and REEs.The results show that medium to fine-crystalline dolomites are strongly modified by silica-rich hydrothermal fluids.A series of characteristics occurred as a result of this process,such as:thermal fading,saddle dolomite filling,and mineral precipitation including dolomite,calcite,chalcedony,pyrite and so on,as well as higher SiO₂,FeO,MnO and BaO contents,lower SrO contents and ⁸⁷Sr/⁸⁶Sr values,more negative δ¹⁸O values,lower ∑REE contents,obvious positive Euanomalies and negative Ce anomalies.While the dolomite reservoir transformation resulted from hydrothermal fluids mainly includes dissolution,recrystallization and filling,the secondary pores with dissolution serve as the most important reservoir space.The conduit system for deep hydrothermal fluid migration is composed of faults of various scales,third-order sequences boundaries,or sequence boundaries of even shorter periods,playing a deci-sive role in controlling hydrothermal activities and reservoir development.Finally,a conceptual model displaying the impact of hydrothermal activities on reservoir formation was established under the dual control of faults and sequences boundaries in the study area.

Key words: silica-rich hydrothermal fluid, secondary dissolved pore, dolomite reservoir, fault, sequence, Lower Ordovician, Tarim Basin