Genetic mechanisms and distribution characteristics of overpressures in the Paleogene reservoirs of the Bohai Bay Basin, East China

Abstract

The Bohai Bay region is a primary accumulation area of oil and gas in offshore China, in which overpressure commonly occurs in the Paleogene strata; the analysis on distribution characteristics and genetic mechanisms of the overpressure would provide geologic evidences for making plans of well drilling and logging as well as oil and gas exploitation; additionally, it could lay the geological foundation for studying how overpressure controlled hydrocarbon accumulation. Based on research, the overpressure of the study area starts from the second member of the Dongying Formation and ends in the third member of the Shahejie Formation. The distribution of overpressure is mainly controlled by the sag-salient tectonic framework within the basin, which means overpressure mainly develops in sags or slopes; however, high areas stay normal pressured. In the study area, pressure develops around Bozhong Sag and in northern Liaodong Bay reaches the peak. The genetic mechanisms of overpressures in the Paleogene reservoirs are mainly disequilibrium compaction, hydrocarbon generation of the organic matter, fluid charging, and transmission or the superimposition of the former two. Different strata have different genetic mechanisms of overpressure. The chief genetic mechanisms for the generation of overpressure of the Dongying Formation are disequilibrium compaction while the genesis of the formation of overpressure in the Shahejie Formation is more complicated in some extent. The first member of the Shahejie Formation dominated by disequilibrium compaction and hydrocarbon generation of the organic matter plays a supplemental role, while the second member of the Shahejie Formation, as the primary reservoir strata, is dominated by fluid charging and transmission, and the third member of the Shahejie Formation is the main source rock interval; its overpressure is closely related to hydrocarbon generation. Each contribution ratio for overpressure forming by

Exploring Sources and Biogeochemical Dynamics of Dissolved Methane in the Central Bohai Sea in Summer

“…However, the Laotieshan Channel, which is located in the northern part of the Bohai Strait and has a maximum depth of 86 m, has neither Holocene sediments nor Late Pleistocene sediments due to strong tidal erosion (up to 250 cm s −1 ) (Liu et al, 1998). In addition, the Bohai Sea is the largest offshore crude oil/gas development base in China, where hydrocarbon-related overpressure commonly occurs in the Paleogene strata (Xu et al, 2018) and oil/gas leakages have been previously observed .…”

“…The erosional character of the seabed means that it cannot host methanogenic processes due to an absence of sedimentary organic carbon. Therefore, the high [CH 4 ] values in the water column were probably contributed to natural release of CH 4 due to fault activities related to hydrocarbon-generation sag (Xu et al, 2018).…”