Volcanic activities exert a significant influence on pore fluid property and related diagenetic processes that substantially controlled reservoirs quality. Analysis of Paleogene medium-deep sandstones on the Huanghekou Sag provides insight into relating the diagenetic processes to pore fluid property evolution influenced by volcanic activities. Three distinct types of pore fluids were identified on the basis of an integrated and systematic analysis including core and thin section observation, XRD, SEM, CL, and trace element. Alkaline aqueous medium environment occurred in E₂s₁₊₂ where volcanic activities have insignificant influence on pore fluids, evidenced by typical alkaline diagenetic events such as K-feldspar albitization, quartz dissolution, feldspar dissolution, and carbonate cementation. During the deposition of E₃d₃, influx of terrestrial freshwater and alteration of ferromagnesian-rich pore water result in the formation of mixing aqueous medium environment through volcanic eruption dormancy causing zeolite dissolution, clay mineral transformation, and K-feldspar albitization. Ferromagnesian-rich aqueous medium environment developed resulting from the intensive hydrolysis of the unstable ferromagnesian minerals formed due to intense volcanic activities during E₃d₁₊₂ and corresponding predominant diagenetic processes were characterized by the precipitation and dissolution of low-silica zeolites. Therefore, the differential properties of pore fluids caused various diagenetic processes controlling reservoir quality.

The late Eocene succession of the Dongying Depression forms a highly productive hydrocarbon source. However, due to lack of an unambiguous fine chronostratigraphic framework for the late Eocene stratigraphy, it is challenging to understand the paleolake’s evolution and the driven mechanism of lake-level variation, a limitation which hinders hydrocarbon exploration. In this work, high-resolution gamma-ray logging data were analyzed to carry out the cyclostratigraphic analysis of the third member (Es₃) of the Shahejie Formation in the Dongying Depression. Significant 405-kyr eccentricity cycles were recognized based on time series analysis and statistical modeling of estimated sedimentation rates. We abstracted ~57 m cycles of the GR data in the Es₃ member, which were comparable with the long eccentricity cycles (~405-kyr) of the La2004 astronomical solution, yielding a 6.43 Myr long astronomical time scale (ATS) for the whole Es₃ member. The calibrated astronomical age of the third/fourth member of the Shahejie Formation boundary (41.21 Ma) was adopted as an anchor point for tuning our astrochronology, which provided an absolute ATS ranging from 34.78 ± 0.42 Ma to 41.21 ± 0.42 Ma in Es₃. According to the ATS, sedimentary noise modeling for the reconstruction of lake-level changes was performed through the late Eocene Es₃. The lake-level changes obtained based on sedimentary noise modeling and spectrum analysis reveal significant ~1.2 Myr cycles consistent with global sea level variations which were related to astronomical forcing. Potential driven mechanisms of marine incursion and/or groundwater table modulation were linked to explain the co-variation of global sea level changes and regional lake level changes. Our results suggest global sea level fluctuations may have played an important role in driving the hydroclimate and paleolake evolution of the late Eocene Dongying Depression.

This study conducts a cyclostratigraphic analysis of the third member of the Eocene Shahejie Formation (Es3) in the Laizhou Bay Sag, Bohai Bay Basin, to investigate the influences of astronomically driven climate variations on sea-level changes, sedimentation rates, and depositional environments. We integrated high-resolution geophysical well logs, ostracod fossils, and palynological data from Well B-2 for cyclostratigraphic and paleoclimate analyses. Time series analysis identified orbital cyclicity in the natural gamma-ray (GR) log, with its significance confirmed by correlation coefficients and statistical significance tests. By tuning the GR log to the 405 kyr eccentricity cycle, we constructed a ~7.695 Myr floating astronomical timescale. Integrating the preliminary biostratigraphic framework (based on ostracods and palynology) with the La2010d astronomical solution yielded a high-resolution absolute astronomical timescale for the 1317–2594 m interval of Well B-2, spanning from 33.9 to 41.6 Ma. Sedimentary noise modeling reconstructed the Eocene sea-level curve in the study area, revealing that the 1.2 Myr obliquity modulation cycle was a key driver of sea-level fluctuations. The δ¹³C and δ¹⁸O records confirm the presence of the Middle Eocene Climatic Optimum (MECO), indicating that its stratigraphic signature constitutes a robust marker for regional stratigraphic subdivision in the southern Bohai Bay Basin. Our findings provide new insights into the climatic evolution of the Es3 member in the southern Bohai Bay Basin.