ABSTRACT The eastern forelands of the South American Andes thrust belt contain prolific petroleum systems. In Colombia, this foreland, called the Llanos Basin of Colombia (LBC), is currently subsiding, and the maturation of the source rocks, as the migration of the hydrocarbon that started at the Miocene times, is ongoing. Although the success ratio of the exploration companies in this area is very high, the exploration and production are complicated by the presence of a high water/oil ratio in the produced fluids. The geochemical data show evidence of mixing of different fluids within the LBC, not only hydrocarbons from different sources but also waters from different origins. The primary goal of this chapter is to verify a model of the fluid flow history by a quantification of the main processes (meteoric water inflow, water expelled by the smectite-to-illite transformation, maturation and migration of the hydrocarbon [HC]) and by a calibration to various data (temperature, maturation of the source rock, salinity of the formation waters). We have built a 3-D model taking into account not only the subsidence, the compaction, and the source rock maturation but also the dehydration of the shale, rather extensive in that basin, as well as the meteoric water infiltrations to complete the water budget. A 3-D modeling is the only way to incorporate all the processes described earlier to compare them semi-quantitatively and check the consistency of the conceptual model. We use the salinity of the formation waters as a geochemical tracer and take into account the water released by the smectite-to-illite transformation, which is fresh water, for the salinity calculation. Results show the very strong impact of the shale diagenetic evolution in the fluid flow. In the LBC, this impact is major between the depths of 3 and 4 km (1.9 to 2.5 mi) and relatively short term (5 Ma) since the sedimentation rate is high. Data as model results show that the meteoric water is flowing down in the upper part of the foreland sediment but does not reach 3 km (1.9 mi). Our results confirm that the classical way which only links fresh water with meteoric water can lead to erroneous conclusions and that the salinity is a powerful additional calibration parameter to better define the fluid flow in a basin. In the LBC case, complementary geochemical and isotopic data of the formation waters are used to specify up to which depth the meteoric water may be found and how the upward fresh water flow due to the smectite-to-illite transformation interacts with the downward meteoric water flow. These complementary data are qualitatively compared with the outputs of the 3-D modeling.