LOG IDENTIFICATION OF ASTRONOMICAL CYCLE IN LOWER 3RD MEMBER OFSHAHEJIE FORMATION IN DONGYING SAG

Astrostratigraphy cycle refers to the sedimentary cycle caused by the periodic perturbation of the earth gravitational field. The astronomical periodic information contained in stratigraphic cycle appears as a regular change of sedimentary texture, sedimentary structure, lithology and lithofacies in the strata. The key to identify astrostratigraphy cycle is to select all kinds of geophysical and geochemical parameters that can reflect climate change from the stratigraphic records, these parameters are used as a substitute indicator for the cycle.

Logging data can not only achieve high resolution on the meter scale, but also sensitively and continuously reflect the physical properties of the measured formation. Because natural gamma logging data can reflect the changes of the rock cycle and rhythmic layers of rock in the depth of field well. Therefore, the researchers have used the natural gamma logging data to study the sedimentary cycles and have made it as an alternative index to identify the astrostratigraphy cycle.

This study is based on the semi-deep lake-deep lake facies shale in the lower 3rd member of Shahejie Formation in Dongying Sag, Bohai Bay Basin, eastern China. The natural gamma logging data of the well is processed by spectrum analysis, wavelet transform and power spectrum estimation. Through the spectrum analysis, the dominant cycle thickness corresponding to the ratio of the three elements in the earth orbital parameters (eccentricity, obliquity of the ecliptic, precession) is identified. The scale value corresponding to different cycles are obtained by the method of the Morlet wavelet basis for natural gamma logging curve of continuous wavelet multi-scale decomposition. The dominant scale value is obtained by power spectrum method based on wavelet analysis, and wavelet decomposition is performed under the dominant scale value. Then the scale value close to the frequency of the eccentricity theoretical curve is determined. Finally, the astronomical stratigraphic cycle can be identified in the research layer. The lower 3rd member of Dongying Sag is divided into 14 short eccentricity cycles and 4.5 long eccentricity cycles in this study. And the isochronous stratigraphic framework can be established. The scientific problems of the isochronic division and comparison of lacustrine shale strata are solved effectively by this way.