Identification of Astronomical Cycles in Fine-Grained Rocks and Their Application in High-Resolution Stratigraphic Correlation: A Case Study of the Fourth Member of the Shahejie Formation in the Leijia Area, Western Sag of the Liaohe Depression, China

In the Western Sag of the Liaohe Depression within the Bohai Bay Basin, the fourth member of the Shahejie Formation (E₂s₄) in the Leijia area is characterised by lacustrine fine-grained sedimentary rocks, primarily composed of clay, felsic minerals, carbonates and analcime. These rocks serve as the key hydrocarbon reservoirs in the region. However, their complex lithology and rapid lateral variations lead to significant reservoir heterogeneity, posing challenges for the identification of favourable reservoirs. This study addresses these challenges through cyclostratigraphic analysis to enhance stratigraphic resolution and improve reservoir prediction. We employed time-series analysis, high-precision U–Pb carbonate dating and gamma ray (GR) logging data to identify astronomical cycles within the E₂s₄ fine-grained mixed rocks. Spectral analysis of GR data from wells lei93, lei37, lei53 and lei14 revealed signals consistent with standard eccentricity, obliquity and precession cycles. Using the 405 kyr long eccentricity cycle for astronomical tuning, we established a detailed chronostratigraphic framework for the E₂s₄, constrained by U–Pb dating results from several wells. In total, we identified six long eccentricity cycles (405 kyr) and approximately 19 short eccentricity cycles (~129 kyr). The correlation between Earth's orbital cycles and high-frequency sequences allowed us to construct a refined stratigraphic division and correlation framework at an eccentricity timescale. This refined framework offers a clearer understanding of reservoir distribution, greatly enhancing hydrocarbon exploration accuracy in the Leijia area. Moreover, our findings highlight the significant role of cyclostratigraphy in guiding future oil and gas exploration efforts in the Bohai Bay Basin, showcasing its potential application in broader geological contexts.