An integrated reservoir characterization workflow, for structural uncertainty assessment and continuous updating of the structural reservoir model, by assimilation of production data, is presented. An ensemble of reservoir models, expressing explicitly the uncertainty resulting from seismic interpretation and time-to-depth conversion, is created. The top and bottom reservoir-horizon uncertainties are considered as a parameter for assisted history matching and are updated by sequential assimilation of production data using the Ensemble Kalman Filter (EnKF). To avoid modifications in the grid architecture and thus ensure a fixed dimension of the state vector, an elastic grid approach is proposed. The geometry of a base-case simulation grid is deformed to match the reservoir top and bottom horizon realizations. The method is applied to a synthetic example. The result is an ensemble of history-matched structural models with reduced and quantified uncertainty. The updated ensemble of structures provides a more reliable characterization of the reservoir architecture and a better estimate of the field oil in place.