The increasing of hydrocarbon production in the Llanos Basin of Colombia (LBC) confirms that its petroleum system is rather well understood, however, question marks remain concerning the fluid migration pathways and the large water/oil ratio within the producting fields. To have a better understanding of the water origin and of the fluid circulation, formation water have been sampled and analysed. The low salinity values of the formation waters (sampled in the hydrocarbon producing wells) were historically interpreted as the result of a dilution of the saline connate waters with the fresh meteoric waters. New analytical data of stable isotopes, measured on rivers and formation waters, allow us to highlight areas where indeed meteoric waters infiltrate within the basin. However the stable isotopic data (dD, d18O) suggest a third source of water in the north-westward deepest part of the LCB. We link it with the diagenetic evolution of the clay minerals; indeed the LCB infilling is for a large part shally and the smectite – illite transition release fresh water (Gonzalez et al., 2011). XRD and CEC data acquired on six wells confirm the occurrence of this transformation in the LBC over the last couple of Myr and allow us to propose a kinetic reaction for since transformation taken into account the thermal evolution already calibrated in the nuemericla model. The quantity of water due to this dehydration is far to be negligible. Finally, to check the consistency of the data, 3D basin modeling of formation fluid circulation in the LBC were carried out taking into account the water expelled by clays diagenesis as well as the meteoric water infiltration. The model allows us to predict the ratio of mixing between the waters of various origins.


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