<p>The change in sedimentation conditions from the shelf to deep-water, including deltas and underwater cones of outflow in the Northern Caspian Sea near the Mangyshlak Threshold can be traced on high-resolution seismic reflection profiles. The profiles of the Shirshov Institute of Oceanology RAS show that the upper part of sedimentary cover here consists of two types of sedimentary units or seismic sequences with essentially different acoustical image. Two deltaic sequences were accumulated during Enotanian and Mangyshlak regressions. Their zones of hummocky reflections had formed in active lithodynamics of shallow-water and subaerial environment. Associated acoustically transparent zones seem to be gas-charged sediments, probably free biogenic gas CH₄ formed due to the decomposition of large amounts of organic plant remains which the Volga River carries away from the land. The marine sequence separating the two deltaic sequences has an acoustically layered structure formed by continuous, parallel, and slightly undulating internal reflectors. This sequence accumulated during the Upper Khvalynian transgression under quiet hydrodynamic conditions of deep seas far from the coastline. The Caspian Sea is a hydrocarbon basin of great value. Many hydrocarbon fields are situated on the Northern Caspian shelf. The shelf geohazards are shallow gas and unconsolidated filling of the buried river valleys, which mainly affect exploration constructions.</p>