The Upper Triassic Xujiahe Formation functions as one of the key sequences for natural-gas exploration in Sichuan Basin. In recent years, the exploration of Xujiahe 2 Member in Yingshan area has achieved better results. Formation water in sedimentary basins is of great significance for revealing reservoir diagenesis, and hydrocarbon generation and distribution. In existing reservoir-forming achievements on Xujiahe 2 Member, there are few reports on exploring diagenesis and fluid activity based on formation water. Thus, both chemical composition and distribution characteristics of formation water were investigated via counting hydrochemical data about this member. By comparing present-day seawater evaporation marks, some enrichment and depletion degrees of main ions were obtained. Moreover, combined with typical cross-plots, not only source but genesis were analyzed preliminarily for formation water. Results show that (1) with CaCl2 as the main type and (Na++K+), Cl-, and Ca2+ as the main ion components, the formation water in Xujiahe 2 Member features high salinity; (2) for most formation-water samples, both sodium-chloride and metamorphic coefficients are less than 0.85 and 8, respectively, the desulfurization coefficient is 0, and the cation exchange index is more than 0.129, all reflecting a good sealing of this member during its geological history; and (3) the present-day formation water in Xujiahe 2 Member is characterized by high salinity, relatively rich Ca2+, and poorer Na+ and Mg2+, which may be caused by a joint effect including the evaporating concentration of formation water, the adsorption alternating of cations, the albitization of plagioclase, and the massive generation of chlorite.
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