Sand discrimination optimization with rock physics models of sediments poorly consolidated The area of study associated to Llanos Basin in Colombia presents great potential related to heavy oil from a fluvial tertiary reservoir. Weak mechanical properties contrast between oil and formation water makes the direct hydrocarbon detection impossible. The experience in the basin shows that clean sands facies with high effective porosity are usually associated with productive zones, due to their flux capabilities for viscous crudes. Seismic prestack elastic inversion was selected as the methodology for conducting the identification and mapping of good quality sands based on the elastic properties contrast between different lithologies. However, it was hard to find out rock properties capable to discriminate sand from shales, endangering the success of the project. A detailed analysis of available well information allowed to outline an alternate approach based on a) the redefinition of petrophysical evaluation emphasizing on shale volume and effective porosity and b) the regeneration of shear wave velocity logs by means of poorly consolidated siliciclastic rock physics model. Core analysis revealed the presence of quartzose siltstones that, in terms of mechanical properties, behave as shale but the original petrophysical evaluation based on gamma ray classified them as possible reservoir. The new approach based on density and velocity logs is appropriate for seismic detection due to its sensitivity to mechanical properties. New relations between modeled elastic parameters and redefined reservoir properties produced enhanced lithology discrimination. It allowed seismic information to successfully predict a good reservoir quality geobody as a guide for field development