Study on cyclostratigraphy of Shahejie Formation in Chezhen Sag,Jiyang Depression

Chegu29 wells which distributed from southwest to northeast were selected for the study of Milankovitch cycle.With the help of Matlab software and Wavelet transform toolbox and on the basis of the previously constructed recognition method of Milankovitch cycle,cycles identified from logging data of seven wells were selected and then the dominant Milankovitch cycles recorded in strata were determined.The development regularity of Milankovitch cycle and the development characteristic of delta and basin were revealed,the age of strata was calibrated by the analysis of sedimentary rate and depositional thickness.It is found that Milankovitch cycles are widely developed in Chezhen Sag.The development of the low-frequency informations is inferior to the high-frequency ones.The delta and basin exhibit difference of expansion-shrinking-expansion from southwest to northeast,the relatively low-frequency cycles(Eccentricity) are prone to be preserved in the stratum whose sedimentary rate is lower,the high-frequency ones(Precession,Obliquity)are prone to be preserved in the stratum whose sedimentary rate is higher.According to calculation,the age of top boundary of the Member 1 of Shahejie Formation ranges from 31.5 to 32.5 Ma,and the age of top boundary of the Member 2 of Shahejie Formation is from 32.7 to 33.2 Ma.The ages of the uppermost strata of the Member 1 of Shahejie Formation of Chegu29,Che35,Chegu203,Dagu63 wells are 32.4,32.4,32.1,32.5 Ma respectively.