Continental lacustrine fine-grained lithofacies, assemblage and geological significance of unconventional petroleum: A case study of the Paleogene of the 4th member of the Shahejie formation in the Jiyang depression

This study adopted a method involving the characterization of macroscopic observations and microscopic features. Then, based on the analysis results of the Milankovitch cycles, the lacustrine fine-grained sediment grain layers and lithofacies were explored. This study theorized that any conclusions in classification based only on the differences in lithology of the fine-grained intermediated-acidic facies were not necessarily accurate. Therefore, this study proposed an up-dated index and the principles of a lacustrine fine-grained lithofacies division. These included two levels of core indicators as the divisions of the lacustrine fine-grained rock facies, which provided a basis for the classification process. The core indicators were divided into primary and secondary indicators. The basic index proposed in this study was divided into nine index types, and the secondary indicators were utilized for the determination of the fine-grain layer types. The goal was to determine if the lamination of fine-grained sediment was an important indicator of fine-grained intermediated-acidic facies classification, rather than merely a modifier. Therefore, the rock facies were divided four types of lithofacies combination: Calcite/clay grain layer combinations; calcite/ organic grain layer combinations; clay/organic laminae; and calcite/clay/organic laminae. There were 21 types of facies and 33 lithofacies and classes, with each lithofacies having between I and 3 of the classes. Then, based on analysis results of the Milankovitch cycles of the lacustrine fine sedimentary facies, it was determined that the lacustrine fine grained lithofacies long eccentricity control was closely related to the climate and had four long eccentricity cycles of wet climate. In addition, the content levels of carbonate minerals, clastic rock minerals, and organic matter were investigated during the cyclical changes. Then, based on the analysis results, it was believed that the meters of the upper climate cycle may have been the main factors which controlled the sedimentary features and the local land areas for the fine-particle deposition provided favorable conditions. Therefore, since the climate change cycle curves were correlated with the land, it was possible to accurately rebuild in the ancient climate conditions by analyzing of the Milankovitch cycles.