Formation water flow and diagenesis in the Colombian Llanos foreland basin

The Llanos Basin of Colombia, a foreland basin of the Eastern Andean Cordillera, is a petroleum-rich province in which the water/oil ratio found in the reservoirs is anomalously large, with up to 80% water and only 20% hydrocarbons. This formation water has low salinity. Within foreland basins, flow of fresh water has generally been explained by basin-scale circulation of meteoric water percolating from adjacent topographic highs. An alternative origin is tested here by 3D modelling: the low salinity water is released by smectite-illite transformation. Since 50% of the sediments within the Llanos Basin are mudrocks, the modelling results show that a significant volume of water has been added by diagenetic reactions in the northern and western parts of the basin. The peak of low salinity water generation by diagenesis occurs at a depth of about 4 km, where compaction is almost complete. Results show that neglecting the mineral dehydration reactions can lead to misinterpretation of overall mass balance and fluid transfer operating at the basin scale.