The Feixianguan carbonates of Triassic in NE Sichuan Basin are the most important gas reservoir of China. The forming mechanism of high quality reservoir with higher porosity and permeability is related to diagenesis, especially dolomitization. However, the diagenetic models of Feixianguan carbonates are not consistent with the fundamental principles of carbonate diagenesis in the concentrations of strontium and manganese, and the cathodoluminescence of them. The relationship of strontium isotope compositions and concentrations of strontium and manganese of the carbonates also shows that the diagenetic process of crystalline dolomite, which is the most important gas reservoir, has never been influenced by meteoric fluids. It is difficult to explain the origin of crystalline dolomite by traditional dolomitization mechanisms. The diagenesis of the Feixianguan carbonates should be governed by some kinds of marine fluids rich in strontium and poor in manganese. It is valuable to pay further attention to the source and origin of the trapped marine fluids during the diagenesis. The spatio-temporal factors controlling the migration, trapping and water-rock interaction of the fluid are still waiting to be answered. The geochemical study for carbonates and evaporites, especially the strontium isotope composition and concentrations of strontium and manganese, including cathodoluminescence of the carbonates with various fabrics for marine Triassic, in basin scale, will provide an important clue to the origin of the crystalline dolomite as the important gas reservoir.