Overpressured zones in the Precaspian Basin occur at depths below 4000 m. The most extensive zone is in the subsalt reservoirs throughout the basin. In spite of the extended literature database on overpressure phenomena, the Precaspian Basin and its Palaeozoic rock sequence is of a special interest because of its long geological history. The delineation of overpressure generation in the completely compacted sediments is very important in order to exclude disequilibrium compaction as the most significant overpressure mechanism for the Tertiary basins. Seal effectiveness and pressure compartment longevity have mainly resulted from other processes: origin of secondary minerals, hydrocarbon saturation and bitumen incorporation in mudrocks. Solid bitumen precipitation in clays forms a more perfect seal than the capillary pressure. The Astrakhan gas field is the best example, where the compositional evolution of formation fluids has changed the clay permeability.