Controlling factors of the gypsum-salt strata differences within two adjacent Eocene sags in the eastern Jiyang Depression, NE China and its hydrocarbon significance

Abstract

Gypsum-salt rocks play a significant role in the process of hydrocarbon migration and accumulation. The fourth member of the Shahejie Formation (Es4) in Dongying Sag and the adjacent Bonan Subsag, bordered by the Chenjiazhuang Uplift, contains extensive gypsum-salt layers. However, there are notable differences in terms of the endowed horizons. Therefore, it is crucial to decrypt the factors that control the development of gypsum-salt rocks, such as paleoclimate, tectonic activity, and geomorphological differences. Comprehensive insights demonstrate that an arid-dominated climate is the primary factor for the development of gypsum-salt rocks, while the tectonic transformation and migration of depocenter also control the differential development of gypsum-salt rocks in these two areas mentioned above. Under the geological background of arid evaporation as a whole, in the lower section of the Es4 (Es4L) sedimentary period, the depocenter was situated south of Dongying Sag but far away from Bonan Subsag, resulting in the deposition of lakeshore to the shallow water of Bonan Subsag and the deep lake deposition of Dongying Sag. Therefore, the gypsum-salt deposits are lacking in the Es4L of Bonan Subsag while being well developed in the Es4L of Dongying Sag. However, in the upper section of the Es4 (Es4U) sedimentary period, due to the migration of the regional depocenter and tectonic transformation, a large-scale and relatively stable deep lake was also developed in Bonan Subsag. Consequently, gypsum-salt rocks developed in both Bonan Subsag and Dongying Sag. This study provides valuable insights for further hydrocarbon exploration in the deep strata of Dongying Sag and Bonan Subsag.