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Summary A wide variety of hydrogeochemical data were obtained through the analysis of the formation water samples collected from 118 producing wells from the San Francisco Oilfield (SFO) in the Upper Magdalena Valley (UMV) basin, Huila, Colombia. The study area is composed of deposited sandstone in fluvial–lacustrine marine environment, which characterized the formation waters as chloride-sodium water type. The brackish-saline facies identified can be attributed to evaporation, halite and dolomite dissolution along with water recharge of meteoric waters somewhere in the basin, probably from Magdalena River, the contribution of the SFO injection water system and significantly by the rock–water interaction. Some ionic ratios were used to confirm clearly that water–rock interactions play a significant role in the evolution of the hydrogeochemistry process in the SFO. The charge balance error ranges between −5.88% and 2.62% indicating very well balanced and mature water for blocks 1, 2 and 3 (north part of the field), and partially equilibrated and immature waters for blocks 4, 5, 6, 7, and 8 as is confirmed by Piper, Giggenbach, and Schöeller diagrams. Calculated scale and stability indices (Langelier, Ryznar, and Puckorius), besides halite and dolomite indexes, indicate that these formation waters are slightly saturated with respect to the calcite and dolomite and have strong tendency to the scale formation as well. Concentration maps were used to identify the geological factors that could have influenced its chemical composition and the hydrogeochemical processes involved in the field, such as halite dissolution, calcite or dolomite precipitation and cationic exchange reactions. The mineralogical distribution leads to the conclusion that the halite dissolution is mostly distributed in the Northeastern area of the SFO between B1, B3, B4, B5, and B6 blocks, the dolomite and calcite have greater concentrations between B5 and B6 blocks. The dissolved minerals followed the trend halite > calcite > dolomite > silvite. HCA gives two main clusters divided in six water classes, while PCA explains near of 48.85% of the variance in the samples. CP1 explains 25.74% of the correlation between variables and factors revealing a high positive loading for electrical properties and cations. This factor may be related with the high dissolution of halite (marine environment) caused by the intensive flushing of the rock formation. CP2 explains the 13.15% of the data showing a good correlation between alkalinity parameters. CP3 explain the 10%, which establishes a good negative correlation for pH, and positive correlation between iron, zinc, and ammonia. [ABSTRACT FROM AUTHOR]