The recent discovery of deep and ultra-deep gas reservoirs in the Permian Changxing Formation reefs, northeastern Sichuan Basin is a significant development in marine carbonate oil & gas exploration in China. Reef dolomites and their origins have been major research topics for sedimentologists and oil & gas geologists. The petrography, trace element and isotope geochemistry of the reef dolomites indicated that the dolomites are characterized by low Sr and Mn contents, relatively low Fe contents, very similar δ¹³C and δ¹⁸O values and very different 87Sr/86Sr ratios. Although the calculated results of the fluid mixing suggested that a mixture with 85%-95% meteoric water and 5%-15% seawater seemed to be the dolomitizing fluids of the reef dolomites, the low Mn contents, relatively low Fe contents, high δ¹³C values and high homogenization temperatures of the dolomites did not support that there were large proportions of meteoric water in the dolomitization process, and the ⁸⁷Sr/⁸⁶Sr ratios which were close to coeval seawater also did not support the possibility of the mixture of deep-burial circulated fluids from clastic rocks. High temperature deep-burial circulated seawater with low Mn and Fe contents, high Sr content and high δ¹³C values from the dissolution of widely distributed Middle Triassic evaporates during the burial diagenetic processes (including dehydration of water-bearing evaporites) could have been the dolomitizing fluids of the reef dolomites.