Causes of formation and distribution of abnormally high formation pressure in petroleum basins of Ukraine

The maintenance of abnormally high formation pressure (AHFP) over long periods of geologic time cannot be explained by compaction or structural deformation without the addition of fluids from underlying rocks. Abnormally high pressures develop in the deepest parts of basins that contain 8-10 km of sedimentary rocks. The deeper part of the sedimentary fill typically occurs in the zones of late catagenesis and incipient metamorphism with temperatures ranging from 200 degrees to 300 degrees C. The observed increase of formation pressure above normal hydrostatic pressure with increasing depth and temperature, in conjunction with other factors, indicates that extended zones of AHFP first appear at temperatures of 175 degrees C and higher. At lower temperatures, abnormally high pressures occur only locally at the crests of anticlinal structures. The main cause of abnormal pressure in the zone of late catagenesis is the generation of large volumes of methane, carbon dioxide, hydrogen, water, and other volatile components. The devolatilization process is commonly associated with elevated temperatures, loss of porosity and permeability, and fracturing of rocks. Ln rocks which have experienced a high level of catagenesis, such as those in the structurally inverted parts of the Donbas foldbelt and the western Lviv depression adjacent to the Carpathian foldbelt, abnormally high formation pressures are absent.