total dissolved solids, trace elements, hydrogen and oxygen isotopes, formation water, permian, hangjinqi area, ordos basin, Geophysics. Cosmic physics, QC801-809, Geology, QE1-996.5

The Permian stratum in Hangjinqi area is one of the important fields to increase gas reserves and outputs in the Ordos Basin in recent years. The studies of the origin and source of formation water are still insufficient due to the complexity of relationship between gas and formation water. Based on the analysis of major components, trace elements, as well as hydrogen and oxygen isotopes, the indication of geochemical characteristics for the source of formation water is then revealed. The Permian formation water in Hangjinqi area is mainly of CaCl2 type, and the average total dissolved solids (TDS) of formation water to the north and south of the Borjianghaizi fault are 52.1 and 41.9 g/L, respectively. Different water chemical parameters indicate favorable formation sealing conditions, and the formation water has experienced strong concentrated metamorphism with beneficial preservation conditions for natural gas. The formation water is mainly terrigenous one after evaporation and concentration, and the stratum has experienced albitization of plagioclase and dolomitization of cements. The δD and δ18O values of the formation water are mainly in the ranges of -81‰ to -75‰ and -12.1‰ to -8.8‰, respectively, suggesting weaker water-rock interaction than that in the Sulige gas field. Due to the mixing of condensate water in regional uplift since the Late Cretaceous or the seepage of surface water along faults, a small number of formation water samples display more negative δD values.