Impact of climate-driven base-level on the diagenesis of lacustrine turbidites: Insights from Eocene Dongying Sag, Bohai Bay Basin, East China

Abstract

Turbidites have been recognized as significant reservoir units in both marine and lacustrine basins. This study was conducted on the Eocene Dongying Sag (Bohai Bay Basin, East China) using sediment cores, well logs, seismic data, and petrography to investigate the diagenesis and reservoir quality of lacustrine turbidites controlled by climate-driven base-level changes. The early forced regression within a third-order sequence stratigraphy can be divided into rising and falling stages of fourth-order base-level changes. Throughout these stages, extrabasinal turbidites were triggered by river floods and comprised the main channel, distributary channel, lobe, and lobe fringe. Intrabasinal turbidites only occurred during the falling stage, which were caused by sediment failures and corresponded to tongue deposits. Compared to extrabasinal turbidites, intrabasinal turbidites possess enhanced anti-compaction capabilities, reduced clay matrix content, and higher carbonate cement content. Climate-driven base-level changes dominated the diagenesis of lacustrine turbidites. From the rising to falling stage, the climate transitioned from humid to arid conditions. This caused a decline in the formation of clay-sized sediments that were subsequently transported into deep-water environments, thereby enhancing mechanical compaction of extrabasinal turbidites. Stacking patterns of channel complexes changed from vertical aggradation to lateral migration, reducing the occurrence of carbonate cementation near channel amalgamation and sandstone–mudstone interfaces. Therefore, these processes controlled by base-level changes led to a decrease in carbonate cement content and mechanical compaction, simultaneously promoting dissolution in extrabasinal turbidites during burial. Additionally, falling base-level facilitated rapid progradation of clinothems and delta collapse, resulting in the development of limited-scale tongues characterized by lower sorting and clay matrix content inherited from the delta front. Tongues, despite their enhanced resistance to compaction, were surrounded by thick-layer deep-water mudstones, contributing to strong carbonate cementation during burial. This study confirms that extrabasinal turbidites during the falling base-level possessed better reservoir quality, thereby providing guidance for the exploration of gravity flow sandstone reservoirs in lacustrine basins.