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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. Highlights • The starting depth of overpressure differs among different source rock intervals. • High deposition rate (DR) and mudstone content (MC) are necessary for developing undercompaction overpressure. • High DR advances undercompaction formation with negligible effect on the final overpressure amplitude. • The undercompaction overpressure amplitude increases as the MC and the formation period increase. [ABSTRACT FROM AUTHOR]

The Dongpu Depression, located in the southwest of the Bohai Bay Basin, east China, is enriched in natural gas with proven reserves of about 480 billion m 3 . However, their genesis and distribution are not completely clear. On the basis of molecular, stable carbon isotope and noble gas isotope (Ar) composition of a large amount of gas samples and combined with the geological background, this paper analyzed the genesis and migration features of Paleogene gas from major gas fields in the Dongpu depression. The results show that Paleogene gases in the Dongpu Depression are of thermogenic origin, and can be divided into coal-derived gas, oil-associated gas and their mixture. Furthermore, Natural gases from the E 2 s 3 L and E 2 s 3 M are predominantly oil-associated gas generated from E 2 s 3 source rocks, while those from E 2 s 4 , and E 2 s 3 U and E 2 s 2 in the Baimiao area are derived from the Carboniferous-Permian coaly source rocks. In addition, the coal-derived gas can also mix with the oil-associated gas, which can be called mixed coal-derived gas and oil-associated gas. Carbon isotope data suggest that gases of E 2 s 3 and most E 2 s 2 are predominately unaltered, while those of E 2 s 4 and several E 2 s 2 mostly from Baimiao area are altered and undergoing strong dissipation. The geographical variation in the gas composition can be attributed to the geochemical fraction caused by vertical migration through the basement faults for the coal-derived gas in E 2 s 4 and several E 2 s 3 U and E 2 s 2 in Baimiao area. While the oil-associated gas in E 2 s 3 L and E 2 s 3 M are resulted from the near-source or even in situ accumulation. [ABSTRACT FROM AUTHOR]