Milankovitch cycles and high-frequency sequences of the Middle Permian Maokou Formation in Sichuan Basin
Zhang Yunbo1, Wang Genhou1, Yu Zhengwei1, Zhao Zongju2, Wang Mingjian3, Sun Yanpeng4
1 School of Earth Sciences and Resources,China University of Geosciences(Beijing), Beijing 100083 2 Chinese Petroleum Society,Beijing 100724 3 Qingdao Institute of Marine Geology,Qingdao 266071,Shandong 4 School of Energy,China University of Geosciences(Beijing),Beijing 100083
Abstract：The Maokou Formation was formed during the middle to late period of the Middle Permian in the Sichuan Basin.By analysing the outcrop lithology of the Maokou Formation and comparing the changes in the carbon and oxygen isotope levels to the sequence characteristics of the Changjianggou outcrop in Shangsi,Guangyuan area,Sichuan,we divided the Maokou Formation into two 3rd-order type II sequences that can be tracked over the entire basin.The major controlling factors of these two sequences were analysed.Based on these,we carried out spectral analysis of natural gamma spectroscopy data from two typical wells that were drilled through the Maokou Formation. We identified a variety of high-frequency cycles that exhibited the characteristics of Milankovitch cycles and calculated out the average deposition rate and time,in addition to other parameters related to the Maokou Formation.To establish the high-frequency sequence division,we employed a digital filtering process to eliminate the effect of minor factors and only retain the information associated with the major controlling factors.Our research shows that the 3rd-order sequences of the Maokou Formation were mainly controlled by tectonic movements and global sea-level changes during the formation process.The Maokou Formation shows the stratigraphic features representative of Milankovitch cycles,including long eccentricity cycles(413.0 ka)and short eccentricity cycles(123.0 ka),which control the formation of the 4th- and 5th-order sequences(parasequences). For the Well Long 17,the corresponding average cycle thicknesses for these two types of sequences are 13.44m and 4.31m,respectively.The thicknesses are 16.03m and 4.68m for the Well Anping 1. The Maokou Formation developed approximately 154th-order sequences within approximately 6.11 Ma.By dividing the high-frequency sequences from the superimposed curve containing tectonic setting and long eccentricity data,we believe that the high-frequency interfaces are much closer to the actual development of the Maokou Formation.
Zhang Yunbo,Wang Genhou,Yu Zhengwei et al. Milankovitch cycles and high-frequency sequences of the Middle Permian Maokou Formation in Sichuan Basin[J]. JOPC, 2013, 15(6): 777-786.
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