Fluid overpressures observed in sediments cored in the Western Mediterranean Sea during Leg 161 are explained as the result of disequilibrium compaction associated with the presence of free methane, high sedimentation rates, and the alternating layers of coarse and fine grain sediments. Most of the methane is probably formed by in situ microbial degradation of the organic matter present in the sediment. We have shown that capillary phenomena can produced capillary seals at the sites investigated during Leg 161. These seals are fixed or migrating in a framework attached to the sedimentary layers. A general theory of compartmentalization and sealing will be published elsewhere and applied to these data to understand the dynamics of these seals.