Benchmarking High-Frequency Sequence Stratigraphy of Lacustrine Fine-Grained Deposits: A Case Study of the Paleogene Shahejie Formation in Dongying Sag, Bohai Bay Basin

Summary The sequence division of fine grains has always been the focus of sequence stratigraphy, as it is more difficult to divide the sequence of the sedimentary strata with very fine grain size. Previous studies often have used different indicators and methods which have led to differences in division results. At the same time, the current research ignores the characteristics of the indexes of sedimentology, paleontology, and localization, especially the high-frequency sequence of fifth level and above. To further strengthen the understanding of the sequence interface and sequence characteristics of the lake fine grain deposit, we studied the upper interval of the upper submember of the fourth member of the Paleogene Shahejie Formation (Es4scs) in Dongying Sag, Jiyang Depression, Bohai Bay Basin as an example, using methods and techniques such as macro- and microscopic identification, X-ray diffraction whole-rock mineral analysis, X-ray fluorescence spectroscopy, total organic carbon (TOC) analysis, and magnetic susceptibility testing. Based on sedimentology, paleontology, and geochemical data and supplemented by drilling and logging data, we systematically and carefully discuss the interface identification marks of lake fine grain deposits and carry out subhigh-frequency sequence division at all levels to establish a “benchmark” for the sequence research of this type. The research results are as follows: First, the Niuye 1 well of Es4scs was divided into one three-order sequence, three system domains, six fourth-order parasequence sets, 22 fifth-order parasequences, 48 sixth-order high-frequency cyclics, and 106 seventh-order rhythm layer/meter-order cycles. The Fanye 1 well of Es4scs was divided into one three-order sequence, three system domains, six fourth-order parasequence sets, 22 fifth-order parasequences, 68 sixth-order high-frequency cyclics, and 110 seventh-order rhythm layer/meter-order cycles. Second, the stratigraphic sequences of different levels were composed of superimposed lower-order sequences, which were made up of smaller layers. The fourth-order parasequence set consisted of three or four fifth-order parasequences, respectively. The fifth-order and sixth-order high-frequency cyclic consisted of two sixth-order and two seventh-order rhythmic layers/meter-order cycles, respectively. Third, the establishment of a “benchmark” can greatly solve the problem of high-frequency sequence division of lake fine-grained sedimentary strata. It also provides a test scale for methods such as time-series analysis. The “benchmarking” study of the sequence division of fine sedimentary areas provides a strong supplement for the study of traditional sequence stratigraphy, and can provide reference for the study of high-frequency sequence and the division of high-resolution sequence strata, effectively solving the scientific problem of the division and comparison of lake shale strata.