249 / 2019-06-30 12:27:36

Analysis of Milankovitch Orbital Cycle and High Frequency Sequence Division of Coal-bearing Strata of Middle Jurassic Yan'an Formation in Ordos Basin

Milankovitch cycles; high-frequency sequence; Yan’an Formation; Ordos basin;

Abstract: The Yan'an Formation in the Ordos Basin of Middle Jurassic is a good coal-bearing stratum, and the coal seam has a good record of sedimentation. Through spectrum and wavelet transform analysis of the natural gamma curve of the Yan'an Formation stratum by the program which based on the Matlab software developed, there are five stable cycle thickness preserved in the stratum is obtained: 36.9～56.4m、10.6～17.8m、8.4～16.3m、2.5～6.1m and 2～3m. Ratio of thickness of main cycles is 1：0.3156:0.2890:0.1082:0.0532，Theoretical calculation shows that the main periods of eccentricity of orbital cycles in the Middle Jurassic Yan'an Formation period (174.1+1～168.3+1.3 Ma) are 408 kyr, 128 Kyr and 97 kyr, the obliquity period is 35 kyr, and the precession period is 22 kyr. The ratio between the theoretical cycles of Milankovitch orbital cycles is 1:0.3128:0.2374:0.0862:0.0530. There is a good consistency between them, which reflects that the sedimentary development of Yan'an Formation in Ordos Basin is controlled by Milankovitch orbital cycle. Among them, the cyclic thickness of 36.9～56.4m formation is controlled by 408 kyr long eccentricity period，the cyclic thickness of 10.6～17.8m formation is controlled by 128 kyr short eccentricity period，the cyclic thickness of 8.4～16.3m formation is controlled by 97 kyr short eccentricity period，the cyclic thickness of 2.5～6.1m formation is controlled by 35 kyr obliquity period, the cyclic thickness of 2～3m formation is controlled by 22 kyr precession period. The variation of orbital cycles is an important factor controlling the development of coal-bearing strata in Yan'an Formation of Ordos Basin, and the long eccentricity period of 408 kyr is the most important controlling fact. Using the identified 408kyr long eccentricity cycle, the Yan'an Formation in the Ordos Basin is divided into high frequency sequence, and 11 long eccentricity cycle cycles are identified.