Overpressures in the South Caspian Basin – Occurrence and Nature

Two basic factors are identified in the South Caspian Basin (SCB) from well logs, downhole measurements up to depths of approximately 7 km and 2D basin modeling: tectonic stress (rapid subsiding and sedimentation) and temperature (hydrocarbon generation). Accordingly to this, two main overpressure zones are established in this basin: upper zone (depth interval 600-1200 m), conditioned by a disequilibrium rock compaction (undercompaction) of rocks and lower zone (zone of decompaction of rocks) resulted from hydrocarbon generation (depth below 5 km). The lower overpressure zone is the most intense and depends on the thickness of the shale sequence, content and type of organic matter and the temperature conditions of its transformation into hydrocarbons. In this zone the greatest risk may be associated with the gas generation at depths greater than 9 km, due to both more intense thermal breakdown of kerogen and the cracking of liquid hydrocarbons generated earlier. Overpressures are a major cause of diapirism and mud volcanism in SCB.