Astronomically Forced Cyclicity and Cyclostratigraphic Framework of the Middle Jurassic Bath–Bajocian Formation in the West Siberian Basin

We aim to elucidate the sedimentary cyclicity of the Middle Jurassic Bath–Bajocian Formation in the northern S Oilfield of the West Siberian Basin, address the lack of high-resolution Milankovitch cycle research in this region, and support hydrocarbon exploration and development. This study employs the gamma-ray (GR) logging data of Well 79 as the primary dataset. Using Acycle V2.8 software implemented on the MATLAB 2020b platform, we conducted a systematic astrochronological analysis. After improving data quality through preprocessing procedures—including outlier removal, linear interpolation, and detrending—we identified significant cyclic signals via spectral analysis. These cyclicities were subsequently validated using multitaper spectral analysis (MTM), sliding spectral analysis, COCO correlation testing, and wavelet analysis. Band-pass filtering was then applied to facilitate sequence subdivision and sedimentation rate estimation. The results reveal well-preserved Milankovitch cyclicity in the Bath–Bajocian Formation of Well 79. The observed cycle thicknesses corresponding to the 405 kyr long eccentricity, 100 kyr short eccentricity, 41 kyr obliquity, and 20 kyr precession are 34.57 m, 8.26 m, 3.44 m, and 1.73 m, respectively, with thickness ratios deviating by less than 5% from the theoretical 20:5:2:1 proportion. Sliding spectral analysis indicates an alternating pattern of increasing and decreasing sedimentation rates. Based on the identified orbital signals, 12 fourth-order sequences and 52 fifth-order cycles were recognized. Sedimentation rates among the three wells range from 6.49 to 12.08 cm/kyr, averaging 9.29 cm/kyr, and exhibit a decreasing trend from west to east. These findings provide a robust astrostratigraphic framework for refined stratigraphic division and reservoir prediction in the study area.