A new method to calculate sedimentary rates based on Milankovitch Cycles: a case study on middle section of 3rd member of Shahejie Formation in well Niu38, Dongying Sag, Bohai Bay Basin

A case study was carried out in the middle section of the 3rd member of the Shahejie Formation in well Niu38,Dongying Sag,Bohai Bay Basin.Spectrum analysis and continuous wavelet spectrum of GR log which was closely related to climate change indicated that the target interval was significantly controlled by the eccentricity period of 125 ka corresponding to the thickness of 12.127 m.The filter analysis of this period revealed that about 40 cycles of 125 ka were preserved in the target interval.High-resolution astronomical time scale was established,by means of which the age of the top of the target interval was 33.975 Ma since that of the bottom of the target interval was 38.975 Ma.Based on regional stratigraphic framework,combined with seismic,core and logging data,the Dongying delta was divided into 9 periods named Z1-Z9.According to the sliding window spectrum analysis,the sedimentary rates of the target interval first increased and then decreased as time went by.The sedimentary rate of Z4 reached the maximum (0.127 m/ka) during the most prosperous period of delta prograding.Finally,the duration of each period and more accurate geological age of the interfaces were calculated on the basis of sedimentary rates.