鄂尔多斯盆地延长组米氏旋回分析及层序划分

ANALYSES OF MILANKOVITCH CYCLE AND SEQUENCE CLASSIFICATION FOR YANCHANG FORMATION IN ORDOS BASIN

通过典型探井自然伽马曲线的频谱分析,结合前人沉积速率研究,在延长组中识别出A、B、C、D、E等5组高频旋回周期,分别与米氏旋回中偏心率周期、地轴倾角长周期、地轴倾角短周期、岁差长周期、岁差短周期等存在近似的时间长度和对应关系。应用高分辨率层序地层理论,推测延长组的高频旋回发育受米氏旋回天文轨道力控制,其四级层序受123 ka的偏心率周期影响。在米氏旋回偏心率周期基础上,重新厘定了延长组层序地层划分方案,认为延长组存在4个二级层序、12个三级层序及数百个四级层序。

By means of the spectrum analysis of the natural gamma ray curves of the typical exploring wells, and combining with the previous studies of the deposition rate~ five groups of high-frequency cyclical periods are recog- nized in Yanchang Formation： Group A, B, C, D and E, which have approximate time lengths and correlative re- lationships with the eccentricity cycle, long cycle of the earth-axis obliquity, short cycle of the earth-axis obliquity, long cycle of precession and short cycle of precession of Milankovitch cycles. With the help of the theory of high- resolution sequence stratigraphy, the high-frequency cycles in the formation are predicted to be controlled by the as- tronomic orbit force of Milankovitch cycles, the fourth sequence is influenced by 123 ka eccentricity cycle. On the basis of Milankovitch-cycle eccentricity cycle, the program of Yanchang-Formation sequence stratigraphic classifica- tion is reset and recalculated; four second-class sequences and twelve three-class sequences and hundreds of fourth- class sequences are thought in Yanchang Formation.