The basic mechanisms governing the hydrodynamic patterns in sedimentary basins are ultimately related to the pressure, reservoirs, water salinity and chemical composition, and the regional geological environment, all of which are interdependent. In Iraq, there are four major hydrogeological units in the Tertiary‐Jurassic section, which are generally governed by two hydrodynamic systems — by gravity and by high‐pressure zones.1. Slightly saline water moves by the action of gravity from sites of high potential, namely the Zagros foothills zone and the outcrops of the Arabian shield (where reservoirs are recharged) towards the centre of the basin, which is topographically low. The Tigris, Euphrates and Arabian Gulf form a drainage axis due to their comparatively low topographic positions.The vertical movement of formation water in the basinal area where artesian phenomena exist is effectively possible, favoured by major faults and fractures connecting the deep aquifers to the surface. This mechanism is even more notable in areas of minimum potential.2. High pressure zones have their origin in the diagenesis of organic matter and “cracking” of hydrocarbons beneath thick sediments, which lead to an increase in pressure within a closed system. Abnormally high pressures have been encountered in the Lower Cretaceous in Southern Iraq, mostly related to the presence of gas.Pressure anomalies may also be related to the compaction of sedimentary rocks; if during compaction the fluid expulsion can be normally effected due to the presence of satisfactory drainage channels, the hydraulic gradients will be low. On the other hand, if the fluid expelled is drained into a confined reservoir embedded within argillaceous beds, then an abnormally high pressure will be created.The hydrodynamic systems are related to the geometry of the sedimentary basin; in addition, factors other than compaction (for example, the existence of Miocene saliferous beds, tectonic forces, temperature, etc.) may be involved in the development of high pressures in the Tertiary aquifers of the north and NE of the Mesopotamian basins.The salt series of the Hormuz and Gotnia formations encountered in the Arabian Gulf, which extend to Southern Iraq, could be one of the factors related to the pressure anomalies, when they remain within a closed system.The hydrodynamic systems and the geodynamic evolution of the sedimentary rocks are responsible for the confined formation water, the migration of expelled water towards the surface, and the distribution within the basin of various parameters such as the hydraulic potential, salinity, heat flow, and d