Discussion on origin of dolomite reservoirs in Feixianguan Formation, Puguang Gas Field, Sichuan Basin

As an important natural gas reservoir in the Puguang Gas Field, the Feixianguan Formation in the Lower Triassic is featured by big thickness and good physical property. The formation mechanism of high-quality reservoirs is directly related to carbonate diagenesis, and the dolomitization of carbonate rocks is one of the key controlling factors. Based on rock structure analysis and combined with the order degree of dolomites, the geochemical characters of carbon and oxygen isotopes and sedimentary environments, it has been analyzed in this paper the dolomite genesis of the Feixianguan reservoirs in the Puguang Gas Field. It has been concluded that the dolomites in the Feixianguan Formation generated from seepage reflux during penecontemporaneous stage with the following evidences: 1) The structural characteristics of the original rock minerals remain obviously in the dolomites in the Feixianguan Formation, indicating for rapid dolomitization. 2) The sparry oolitic dolomites composed by micritic oolites have the same low order degree with the contemporaneous micro-micritic dolomites. 3) The carbon and oxygen isotopes composition and the paleosalinity of dolomites show that the dolomitizing fluids have high salinity, which is obviously different from the mixing water dolomitization. 4) The platform edge oolite beach facies of the Feixianguan Formation is adjacent to restricted platform, so it is easy to obtain quantities of saline water with high Mg/Ca ratios. What's more, this environment is beneficial to the development of dolomites and the formation of quantities of early dolomites. Thereby, the dolomites in the Feixianguan Formation mainly came from seepage reflux during penecontemporaneous stage. The reef-bank facies adjacent to the high-salinity environments such as restricted platform or evaporate platform facies are favorable for the generation of high-quality reservoirs.