The formation mechanism of overpressure can be divided into following three types:1.the increase of compressive stress;2.changes in the volume of pore fluid or rock matrix

and 3.hydrocarbon buoyancy or liquid flowage.Loading during burial can produce considerable overpressure due to disequilibrium compaction

particularly during the rapid subsidence of low-permeability sediments.Horizontal stress changes can rapidly produce or release large amounts of overpressure in tectonically active areas.The increases fluid volume caused by aquathermal expansion and clay dehydration are too small to produce significant overpressure unless perfect sealing occurs.Hydrocarbon generation and cracking could possibly produce overpressure

but it needs a long period of time for the pressure to cumulate.This requires to be further proved.