Abstract:

Spectral analysis was conducted on the natural gamma ray logging data from the Changxing Formation of the wells Pan 1, Weining 1, Nayong 1 and Zhijin 1 in western Guizhou. Major Milankovitch sedimentary cycles are identified, and the stable periods are extracted using the filtering method for the contrasts of well connection. The results show good correlations between spectral peaks and Milankovitch cycles, and it is suggested that the Changxing Formation in the study area has preserved the eccentricity cycle of 123 ka, the obliquity cycles of 45 ka and 35.6 ka, and the precession cycles of 21.2 ka and 17.7 ka. The average stratigraphic thickness of all cycles is 4.59 to 5.73 m, 1.37 to 2.15 m and 0.66 to 0.98 m, respectively. The short eccentricity cycle curve of each well shows that about 18 eccentricity cycles are recorded in the Changxing Formation, which indicate that the diagenesis of the Changxing Formation can be estimated as 2.21 Ma. Based on the well connection contrasts of the short eccentricity cycle layers and the calculated sedimentation rate, it is found that the isochronic contrast effect of the marine facies marker sequence is distinct, which reflects rapid regional transgression, while the change of sedimentation rate may be affected by the sea-level change and the sedimentary supply.

Key words: western Guizhou, Changxing Formation, Milankovitch cycle, natural gamma ray logging data