Abstract:

Based on the division in 3th-sequence about loggings and drillings and the study on high-frequency sequence about the outcrops in Bachu area, the authors identifies the high-frequency sequences with the characteristics of Milankovitch cycle, via the filtering, fast Fourier transform, wavelet analysis and tuning processing of the natural gamma spectrometry log ln(Th/K) curves of sequence OSQ2 of Lower Ordovician Yingshan Formation in Tazhong162 and 43 drillings, Tazhong area, Tarim basin. The results show that the most obvious cycle is the obliquity, the period is 37.0 ka, which is the controlling factor of the development of 6th-meter-scale cycle that shallowing upward oriented. The average thickness of the primary cycle is 4.55 m in Tazhong162 drilling and 3.97 m in Tazhong43 drilling in the platform margin facies belt, while 2-4 m in the half-restricted platform-open platform facies belt. The short eccentricity cycle, which period is 95 ka, is the mainly controlling factor in forming the 5thparasequence. The long eccentricity cycle period, 413 ka, which approximately represents the forming time of 4th-parasequence sets, is not obvious in the stratigraphic record. According to the analysis of the high-frequency sequence superimposition and the preliminary estimates about the average number of main cycles, the forming time is approximately 4.92 Ma in the sequence OSQ2 of Lower Ordovician Yingshan Formation in Tazhong-Bachu area. The thickness of the sequence OSQ2 in Yingshan Formation is eroded for about 192 m due to later tectonic uplift. Combined with the forming time(413 ka) of 4th-sequence(parasequence sets) and the superimposition analysis of Milankovitch cycles, and the tuning curves of the wavelet transform, the sequence OSQ2 in Yingshan Formation is preliminarily divided into 12 parasequence sets in Tazhong-Bachu region. The natural gamma ray spectrometry ln(Th/K) curve contains the relative change information in ancient water depth during the deposition period and thus could better reflect the characteristics of Milankovitch cycles caused by the climate change.

Key words: thorium-potassium ratio logging curve, Milankovitch cycles, spectral analysis, Fourier transform, Tarim basin