Distribution and origin of overpressure in the Paleogene Dongying Formation in the southwestern sub-sag, Bozhong Sag, Bohai Bay Basin

Overpressure is extensively distributed in the Paleogene Dongying Formation in the southwestern sub-sag of the Bozhong Sag, Bohai Bay Basin. However, there is a lack of a clear understanding of the pressure distribution, the formation mechanisms of overpressure, and the effects of overpressure on hydrocarbon accumulation in this formation. In this study, we explore the present-day pressure in the formation and its spatial distribution using measured formation pressure and logs. Based on a comprehensive analysis of log curves and the cross plots of sonic velocity vs. density, we analyze the genetic types of overpressure in the formation. Furthermore, through basin modeling, we reconstruct the paleopressure evolution during hydrocarbon accumulation to assess the effects of the formation paleopressure on hydrocarbon accumulation. Key findings are as follows: (1)The Dongying Formation exhibits a complete overpressure system vertically. Horizontally, the overpressure is distributed around hydrocarbon-generating sub-sags, intensifying closer to the sag center; (2)The genetic type of overpressure shifts from predominant undercompaction in shallow strata to a combination of undercompaction and hydrocarbon generation-induced expansion in deep layers; (3)The overpressure within the source rocks of the Dongying Formation, including the 3^rd member and the lower sub-member of the 2^nd member, offers a sufficient dynamic for hydrocarbon migration. Furthermore, it forms seals for the underlying overpressured fluids, preventing the upward escape of hydrocarbons. It is proposed that ancient buried hills represent favorable targets for future deep and ultra-deep hydrocarbon exploration in the southeastern sub-sag of the Bozhong Sag.