Division of high-frequency sequences of different orders in fine-grained deposits and its geologic significance: A case study of mud shale from the lower section of the third member to the upper section of the fourth member of Shahejie Formation in Dongying Sag, Bohai Bay Basin

The sequence division of mud shale is a focus in basic research. Based on lots of sensitivity parameters, using the natural gamma curves (GR) and the improved TR cycle theory, it was classified in this paper the fourth and fifth orders of sequences in mud shale. Moreover, using continuous testing data, based on the astronomical cycle theory, some parameters such as eccentricity, slope and precession were used to establish an astronomical scale for mud shale in the study area, by which the sixth and seventh orders of sequence units can be identified. Four deposition processes were concluded in the upper section of the fourth member of Shahejie Formation in well A in the Dongying Sag, Bohai Bay Basin, and quantitative calculations were made for each process to estimate the deposition rates in different units. We divided the strata from the lower section of the third member to the upper section of the fourth member of Shahejie Formation into eight fourth order and 31 fifth order units. The quantitative analyses of deposition process showed that the deposition period of the second quasisequence in the upper section of the fourth member of Shahejie Formation was the most conducive to biological development.