Controlling Factors for the Development of the Late Oligocene Interlayer in the Qiongdongnan Basin Driven by Astronomical Orbital Periods

The Oligocene Lingshui Formation in the Yacheng Uplift, Qiongdongnan Basin, constitutes an important hydrocarbon reservoir. The development of internal interbeds strongly controls reservoir quality, yet the coupling relationship between high frequency sequence framework and astronomical cycles remains unclear. Based on natural gamma ray logging data, this study applied time series analysis and the sedimentary noise model to establish a high precision astronomical timescale for the 3rd member of the Lingshui Formation and reconstruct the relative sea level curve of the basin. The results show that deposition of the Lingshui Formation was controlled by astronomical orbital cycles. The 3rd member was deposited over approximately 3.3 Ma, with an optimal sedimentation rate of 7.3 cm/kyr. It can be subdivided into 8 fourth order sequences and 32 fifth order sequences. Distinct depositional responses to different orbital cycles are recognized: peaks of the precession cycle (~23 kyr) correspond to the development of thin mudstone layers, whereas peaks of eccentricity cycles correspond to thick mudstone deposits. These form hierarchically nested sedimentary successions, indicating a composite control mechanism of low amplitude sea level fluctuations superimposed on high amplitude changes. This study reveals that precession scale sea level fluctuations and variations in sedimentation rate governed the development frequency and thickness distribution of reservoir interbeds, providing an astronomical geochronological framework for reservoir quality prediction. For the first time, a high frequency sequence framework constrained by a high precision astronomical timescale has been established for the 3rd member of the Lingshui Formation in the Yacheng Uplift. The dominant controls on the development frequency and thickness of interbeds at orbital scales are clarified, providing new scientific evidence and technical approaches for high quality reservoir prediction and refined hydrocarbon exploration in the Lingshui Formation, Qiongdongnan Basin.