Characteristics of trace elements and hydrogen and oxygen isotopes in the formation water of the Xujiahe Formation, Sichuan Basin

Large quantities of natural gases have been found in the Upper Triassic Xujiahe Formation of the Sichuan Basin in recent years. At the same time, formation water with high salinity was also found widespread in the Xujiahe Formation and it contains not only the particularity of constant ion development, but also particularity of stable isotopes and trace element distributions. This paper established an evaporation-fractionation trend line of stable isotopes for the Jurassic to Permian systems in the middle-western Sichuan Basin, and preliminarily determined standards to discriminate origins of formation water in the study area. Influenced by gypsum-salt layers in the Leikoupo Formation, the isotopes evaporation-fractionation evolution of formation water was characterized by the dominance of a rapid increase of δD and δ18O values, respectively, for Jurassic to the 3rd member of the Leikoupo Formation and the 2nd member of Leikoupo Formation to Permian. The formation water in the Xujiahe Formation was identified to be a metamorphic mixture of the high-salinity formation water from the upper Leikoupo Formation and the depositional water excreted by compaction after the burial of the Xujiahe Formation. Meanwhile, some gasfield water of the Xujiahe Formation was found to contain a considerable amount of condensate water, and the presence of this condensate water might also indicate a strong uplift of the Xujiahe Formation and some gas-accumulation events by exsolution of water-soluble gases.