Quantification models of overpressuring in Paleogene source rocks of the Raoyang Depression, Bohai Bay Basin, China
Abstract Much progress in studies on the quantifications of overpressuring has been made during the past decades, and various quantitative models for the overpressure have been built based on overpressure transfer, tectonic extrusion, disequilibrium compaction, hydrocarbon generation, and other processes. In the Raoyang Depression of the Bohai Bay Basin in China, overpressure is mainly generated due to disequilibrium compaction and hydrocarbon generation. However, the overpressure-generation mechanism varies significantly in the overpressured formations and sags. The overpressure genesis in the Dongying Formation (Ed) occurs by disequilibrium compaction, but the overpressure mechanism in the first member (Es1) and third member (Es3) of the Shahejie Formation is a combination of disequilibrium compaction and hydrocarbon generation. The overpressure is caused by the difference in hydrocarbon generation before and after hydrocarbon expulsion. On the basis of the law of volume conservation, three quantitative models have been established for the under-compacted interval, the hydrocarbon-generation-to-expulsion interval, and the post-hydrocarbon-expulsion interval. The contribution of hydrocarbon generation differs in four sags developed in the Raoyang Depression due to differences in the hydrocarbon-generation capacity. Calculations based on these quantitative models indicated that the contributions of hydrocarbon generation to overpressure at the bottom of the Es1 and Es3 members in the Maxi Sag are 54.3% and 39.1%, respectively. The excessive pressure in the lower strata was stronger than that in the upper strata due to the fact that the contribution ratio of hydrocarbon generation to overpressure in the Es1 member is lower than that in the Es3 member, unlike in the case when the pressure in the upper strata is stronger than that in the lower strata. Therefore, it indicated that hydrocarbon generation plays an important role in the overpressure-generation mechanism.