The lithologic differences between the third and fourth members of the Eocene Shahejie Formation in the Bohai Bay Basin and the associated climatic evolution

Abstract

The Bohai Bay Basin straddled the Eocene boundary between arid and wet climates in eastern China. The basin was simultaneously affected by both arid and wet climates, and contemporaneous sediments developed under both extremely arid and extremely wet conditions. Data from every well drilled in the oil fields in the Bohai Bay Basin have been used to study the lower fourth member and the upper third member of the Shahejie Formation. The spatial distribution of climate indicators in the lower fourth member of the Shahejie Formation shows that the climate boundary extended from the northern Jizhong Depression to the Bozhong Depression. The portion of the basin to the north of this boundary mostly developed coalbeds, dark mudstone, and oil shale (indicative of a warm and wet climate), whereas the portion of the basin to the south of this boundary mostly developed gypsum, rock salt, and magenta dolomitic mudstone (indicative of a hot and arid climate). During the formation of the upper third member of the Shahejie Formation, the climate boundary shifted southward to the region between the southern part of the Jizhong Depression and the Jiyang Depression. The wet climate zone in the north expanded southward, and the arid climate zone in the south shrank. This shift is evident in the vertical lithologic characteristics of the members that formed before and after this shift in the climate boundary. The lithologic succession in the central part of the basin exhibits significant changes, whereas the lithologic successions in the southern and northern parts of the basin remain homogeneous and stable through time. Therefore, between the formation of the third and fourth members of the Eocene Shahejie Formation, the Bohai Bay Basin experienced a significant climate change. This change directly affected the pattern of the Eocene climate zones in eastern China and is indicative of changes in the paleoclimate zones near the Tropic of Cancer.

Sedimentology of the deltaic Paleogene Shahejie Formation in the D‐1 area of Liaodong Bay, East China

“…However, the wells included both thick mudstones (E 2 s 3 and E 3 s 1 ) and thick sandstone (E 3 s 2 ), suggesting the possibility of a good hydrocarbon play in this area, especially in wells D‐1‐1 and D‐1‐2SA. The Shahejie Formation is the main source for oil and gas production in Bohai Bay Basin (Wu, Wang, Wang, Fu, & Jia, 2017; Zahid et al, 2016; Zhu et al, 2008). The oil and gas tests of drilled wells suggest the highest average capacity of daily production peak up to 698 m 3 oil and 69,484 m 3 gases in the E 3 s 2 member.…”

“…E 3 s 2 mainly developed sandstone constituting a typical source‐reservoir‐cap assemblage. In Liaodong Bay and even in the entire Bohai Bay Basin, the deltaic sandstone reservoir of Shahejie Formation was the main source for the production of oil and gas (Wu et al, 2017; Zahid et al, 2016; Zhu et al, 2008). The new discovery of crude oil encountered four oil layers in E 3 s 2 , with a total thickness of 34.7 m in D‐1‐2SA.…”