Occurrence and Nature of Overpressure in the Sedimentary Section of the South Caspian Basin, Azerbaijan

Abstract

Two basic factors are identified that contribute to overpressure in different sedimentary basins of the world, including the South Caspian Basin (SCB): tectonic stress and subsurface temperature. Two overpressure zones are identified in the SCB: 1. An upper zone (depth interval 600–1200 m), conditioned by disequilibrium rock compaction (undercompaction) and 2. A lower zone (zone of decompaction) conditioned by hydrocarbon generation (depth below 5 km). The lower overpressure zone is the most intense and depends on the thickness of the shale sequence, the content and type of organic matter, and the temperature conditions of kerogen transformation to hydrocarbons. In this zone the greatest risk is associated with 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. Overpressure is a major cause of diapirism and mud volcanism in SCB.