Identification and division of high-frequency sequence based on Milakovitch cycle: A case study of Xiayoushashan Formation in Nanbaxian oil and gas field

Key words: 
• Milankovitch cycle /
• Spectrum analysis /
• Wavelet analysis /
• High-frequency sequence /
• Southern Eight Immortals /
• Qaidam Basin

Abstract: [Objective] By analyzing the influence of the periodic change of earth orbit on the periodic change of climate, this paper discusses the climate change characteristics of the Lower Oil Sand Mountain Formation in Qaidam Basin, and establishes a high-resolution astronomical scale for the Lower Oil Sand Mountain in Qaidam Basin based on Milankovitch theory to identify and divide high-frequency sequences. [Methods] Firstly, Laskar algorithm is used to calculate the variation period of the earth's orbital parameters during the summer solstice at 35 north latitude from 14.5~23.8 Ma, and the Miocene cycle theory and Miocene cycle ratio in this sedimentary period are determined. Then, taking wells Xianzhong 39, Xianzhong 8-9 and Xianzhong 8-12 in Nanbaxian oil and gas field as examples, the natural gamma data are analyzed by frequency spectrum and continuous wavelet transform. Finally, according to the orbital period, the average sedimentation rate of the Lower Youshashan Formation is calculated, and the "floating" astronomical scale of well Xianzhong 39 is established. [Results] Through the analysis of frequency spectrum and continuous wavelet transform, the Neogene Lower Youshashan Formation is mainly controlled by eccentricity periods of 400 ka and 95 ka. The average sedimentation rate of the Lower Youshashan Formation is 0.094 41 m/ka, and the sedimentation duration is 7.2 Ma. Based on the 400 ka long eccentric period curve and 95 ka short eccentric period curve as benchmark curves, 18 fourth-order quasi-sequence groups and 72 fifth-order quasi-sequence groups were identified. [Conclusion] The results show that the climate change recorded in the Lower Youshashan Formation is obviously controlled and driven by cycles. Identification and division based on Milankovitch theory can reduce the influence of subjective factors, improve the accuracy of division results, and more accurately describe the climate change characteristics in sediments. These research results are helpful to deeply understand the evolution law of the earth's climate and provide important reference for oil and gas exploration and resource evaluation.