Effect of hydrothermal fluids on the ultra-deep Ordovician carbonate rocks in Tarim Basin, China

Abstract Carbonate rocks of Ordovician formations make up many important reservoirs in Tarim Basin. This study focuses on the formation mechanism and evolution model of Ordovician hydrothermal reformed carbonate reservoirs in the Yubei area of Tarim Basin. The observations of core samples, thin sections and cathode luminescence images shows that the carbonate rocks were subjected to multiple stages of karstification and hydrothermal cementation. The results of fluorescence observations and fluid inclusions indicate that the study area was affected by two periods of hydrothermal migration during the late Hercynian period and late Himalayan period. Controlled by the freshwater and hydrothermal fluids, the δ13C and δ18O of the fracture fillings are distributed from −1.8‰ to −0.8‰ and from −13.6‰ to −10.1‰, respectively, with δEu>1 and high Mn and Fe. According to the distribution characteristics of trace elements and metallic elements, the two phases of hydrothermal fluid were rich in SiO2, Ho, and Tm. H2S produced by thermochemical sulfate reduction (TSR) changes the pH of the formation-hydrothermal fluids to acidic values. The hydrothermal fluid was converted to weakly alkaline by the reversible reaction between CO32− and HCO32−, building a silica-calcite-filled karst-fracture reservoir. Eventually, this paper establish a schematic diagram of hydrothermal migration along a single fracture and reveal the evolution mode of the study area.