四川盆地中二叠统茅口组米兰科维奇旋回及高频层序

Milankovitch cycles and high-frequency sequences of the Middle Permian Maokou Formation in Sichuan Basin

四川盆地中二叠统茅口组形成于中二叠世中晚期。通过分析四川盆地中二叠统茅口组露头剖面岩性特征、对比广元上寺长江沟露头剖面碳氧同位素变化趋势与沉积层序发育特征,将茅口组划分为2个可全盆地追踪的Ⅱ型三级层序,并分析了茅口组三级层序的主控因素。在此基础上,对钻穿中二叠统茅口组典型井的自然伽马能谱测井曲线开展频谱分析,识别出多种具有米兰科维奇旋回特征的高频旋回,并计算出茅口组沉积的平均速率及沉积时限等相关参数。最后,采用数字滤波消除掉其他次要旋回因素的影响而仅保留与主控因素相关的旋回信息,建立茅口组的高频层序划分方案。结果表明,茅口组三级层序在形成过程中主要受控于构造升降及全球海平面变化;茅口组米兰科维奇旋回特征明显,其中长偏心率旋回(413.0 ka)和短偏心率旋回(123.0 ka)分别是形成四级层序(准层序组)和五级层序(准层序)的主控因素,与之对应的平均旋回厚度在龙17井区分别为13.44m和4.31m,在安平1井区分别为16.03m和4.68m;茅口组大约发育15个四级层序,其发育时限大约为6.11Ma;根据构造背景曲线和长偏心率旋回曲线的叠加曲线划分高频层序,其高频层序界面更加接近实际地层的发育情况。

The Maokou Formation was formed during in the Sichuan Basin. ges in the carbon and the middle to late period of the Middle Permian By analysing the outcrop lithology of the Maokou Formation and comparing the chan- oxygen isotope levels to the sequence characteristics of the Changjianggou outcrop inShangsi, Guangyuan area, Sichuan, we divided the Maokou Formation into two 3rd-order type II se- quences 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-fre- quency 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 repre- sentative of Milankovitch cycles, including long eccentricity cycles （413.0 ka） and short eccentricity cy- cles （ 123.0 ka） , which control the formation of the 4＇h- and 5th-order sequences （parasequences）. For the Well Long 17, the corresponding average cycle thicknesses for these two types of sequences are 13.44 m and 4.31 m, respectively. The thicknesses are 16.03 m and 4.68 m for the Well Anping 1. The Maokou Formation developed approximately 15 4th-order sequences within approximately 6. 11 Ma. By dividing the high-frequency sequences from the superimposed curve containing tectonic setting and long ec- centricity data, we believe that the high-frequency interfaces are much closer to the actual development of the Maokou Formation.