Mechanisms for overpressure generated by the undercompaction of paleogene strata in the Baxian Depression of Bohai Bay Basin, China

Abstract

Abstract Undercompaction is an important mechanism for developing fluid overpressure in the mudstone strata. The main factors that affect undercompaction are the deposition rate (DR) and mudstone content (MC); however, the manner in which the DR and the MC influence the development of undercompaction is still unclear. To clarify the pressurization mechanism of undercompaction and the influence of DR and MC, the acoustic and density logging as well as the sonic-velocity–effective-stress schematic are used to identify the undercompaction overpressure. Furthermore, the PetroMod software is used to simulate the development of overpressure and to analyze the influence of the DR and MC on the undercompaction of Paleogene strata in the Baxian Depression of Jizhong Subbasin. The Baxian Depression causes the development of overpressure in the Dongying Formation, the first member of the Shahejie Formation, the third member of the Shahejie Formation, the fourth member of the Shahejie Formation, and the Kongdian Formation, whereas the undercompaction overpressure is observed to mainly develop in the Dongying Formation and in the first member of the Shahejie Formation. A prerequisite for performing the simulation is that the total thickness and deposition time of the formation should remain constant. Further, the effects of the varying DR and MC on the development of undercompaction are simulated. The results exhibit that the DR and MC are prerequisites for developing undercompaction overpressure. However, the DR only affects the undercompaction development period and exhibits little effect on the final overpressure amplitude. Meanwhile, the amount of MC not only increases the amplitude but also results in an early development of the undercompaction overpressure.