SUMMARYThe geomechanical analysis of a highly compartmentalized reservoir is performed to simulate the seafloor subsidence due to gas production. The available observations over the hydrocarbon reservoir consist of bathymetric surveys carried out before and at the end of a ten-year production life. The main goal is the calibration of the reservoir compressibility c M , i.e., the main geomechanical parameter controlling the surface response. Two conceptual models are considered: in one (a) c M varies only with the depth and the vertical effective stress (heterogeneity due to lithostratigrafic variability); in another (b) c M varies also in the horizontal plane, that is, it is spatially distributed within the reservoir stratigraphic units. The latter hypothesis accounts for a possible partitioning of the reservoir due to the presence of sealing faults and thrusts that suggests the idea of a block heterogeneous system with the number of reservoir blocks equal to the number of uncertain parameters. The method applied here relies on an ensemble-based data assimilation (DA) algorithm (i.e., the Ensemble Smoother, ES), which incorporates the information from the bathymetric measurements into the geomechanical model response to infer and reduce the 2 C. Zoccarato, D. Baù, F. Bottazzi, M. Ferronato, G. Gambolati, S. Mantica, & P. Teatini uncertainty of the parameter c M . The outcome from conceptual model (a) indicates that DA is effective in reducing the c M uncertainty. However, the maximum settlement still remains underestimated, while the areal extent of the subsidence bowl is overestimated.We demonstrate that the selection of the heterogeneous conceptual model (b) allows to reproduce much better the observations thus removing a clear bias of the model structure.DA allows significantly reducing the c M uncertainty in the five blocks (out of the seven) characterized by large volume and large pressure decline. Conversely, land subsidence can constrain only partially the partitions that marginally contributes to the cumulative displacements of the seafloor.