Abstract: Recent oil and gas exploration shows that silica-bearing hydrothermal fluid is an important acidic fluid in carbonate sequences. Knowledge on the interactions between silica-bearing fluid and carbonate rock is critical to understand the origin of the silicified carbonate reservoir and the prediction of reservoir distribution. In this study, experimental investigation was carried out on the interaction between calcite and silica-bearing fluid at temperatures ranging from 200 to 375℃ by using fused silica capillaries and
hydrothermal reactor as the reaction chambers. In situ Raman spectroscopy was used to describe the process of the reaction. Besides, Scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS) was used to observe the morphology and to identify the composition of the quenched solids. Firstly, the temperature condition of the decarbonization reaction between silica-bearing fluid and calcite is revealed. Calcite reacts with silica-bearing fluids at temperatures above 275℃ to form CO₂, and the solid phase is non-wollastonite calcium silicate. The detailed structure of this calcium silicate needs further investigation. This result indicates that the dissolved silica itself cannot react with limestone at the reservoir temperatures. Secondly, the high salinity and the CO₂-bearing nature of the silica-bearing fluid are the important factors causing limestone dissolution. Finally, the presence of CO₂ can promote the precipitation of siliceous component, including quartz. Based on the above experiments, the formation of the silicified carbonate reservoir in the Shuntuoguole area of the Tarim basin is proposed, integrated with the previous studies. The silica-bearing hydrothermal fluid migrates upward along the deep and large faults, passing through the Sinian-Lower Ordovician dolomite layer, where the siliceous components will react with the dolomite to form magnesium-rich silicate and CO₂. CO₂ is an important acidic component, which is conducive to the dissolution of the shallow carbonate and the preservation of pores. Decrease in fluid temperature and pressure, and the presence of CO₂ result in the precipitation of quartz, forming large amounts of intercrystalline pores.

Key words: calcite, silica-bearing fluid, water-rock interaction, decarbonization, carbonate reservoir