Abstarct: The production of oil and gas in oil and gas fields is accompanied by production of water (Produced Water). Most of the reservoirs at the beginning of production have fresh water; but with passing time due to the increase in productions and decrease in pressure of reservoir, the produced water gradually becomes saline. The saline Production water causes severe corrosion in pipelines and well head facilitis leading to reduction in gas production. Determining the origin of salinity for reducing the salinity is most important. Khangiran gas field is located in the northeast of Iran which composed of two separate gas formations, Mozduran at lower and Shurijeh at the upper part. The produced water samples were collected from fresh and salty wells in the Mozduran reservoir as well as two deep samples from brine below the gas reservoir (at depth of 3 km) for comparison and different analyzes. The Mozduran reservoir has two major problems, high salinity of the produced water, as well as the volume of water produced, rendering some wells unexploitable. The Mozduran gas reservoir in the Khangiran gas-field is formed in the Kopet-Dagh sedimentary basin in the form of sequential sequences of clastic and carbonate sediments of the old sea water. The reservoir is located in the formations of Shurijeh and Mozduran, consisting of limestone, dolomite, sandstone and shale. The Mozduran Formation was deposited in a carbonate platform and Shurijeh Formation in the river systems to coastal deltas during the Jurassic. The TDS Value of produced waters in the Mozduran gas reservoir varies from 1300 to 73000 mg/L with a Cl-Ca to Cal-Na water types. The TDS content of brine in confined high pressure aquifer was 100,000 mg/L. Probable salinity sources of brine in gas-capped confined aquifer can be evaporated sea water, filtration or salt dissolution. According to hydrogeochemical and geological studies of the study area, the primary source of brine below gas reservoir is the old evaporated sea water, which is formed in the lagoon to margin of the continental shelf. By comparing the hydrogeochemical characteristics of Iran's major gas reservoirs, their primary source of brine was the evaporated sea water, indicating forming in the similar sedimentary environment. Considering the high salinity and volume of produced water as well as the hydrochemical properties of Mozduran gas reservoir wells, it can be concluded that brine of gas-capped deep confined aquifer is the main probable salinity source of produced waters in the study area. The old evaporated seawater has been in contact with various formations since the burial time, and their initial composition has altered. The concentration of all ions has increased to saturation in brine due to the evaporation of sea water. However, over the time, the concentrations of Ca, Li, Sr, B and I ions have been increased compared to the original source and the concentration of Na, Mg and SO4 ions have been decreased. The geochemical evolution of this reservoir has been affected by evaporation, water-gas and water-rock reactions such as dolomitization, albitization of plagioclase, ilitization of Smectite, sedimentation or dissolution of sulfate minerals, magnesium carbonate precipitation. Concentration of potassium and chloride ions was mainly influenced by the process of evaporating the old sea water. The results of this research are used to identify the history of sedimentation; secondary geochemical processes in the reservoir, determination the origin and the salinization mechanism of produced water from the gas reservoirs area to achieve sustainable management of the reservoir.