We propose a workflow to assess the uncertainty about a global reservoir parameter such as net-to-gross during early exploration. As opposed to traditional statistical approaches that assume data independence and cannot easily account for either seismic data or geological interpretation, this model, based on multiple point statistics, integrates the main components of uncertainty, namely: the choice of a geological scenario, probably the most important factor at the appraisal stage.The location of wells, which could have been drilled elsewhere, giving a different picture of the reservoir.The calibration of the seismic to the well data. This global uncertainty model is demonstrated on a large 3D fluvial reservoir.