Sedimentary microfacies and environment of Paleogene carbonate-rich shale in Dongying Sag, Bohai Bay Basin

The identification, classification, and integration of microfacies characteristics of carbonate-rich shale in the Dongying Sag of the Bohai Bay Basin are fundamental for understanding the sedimentary origin and spatial structure of related sediments. Leveraging core and rock thin section data, along with analytical techniques such as X-ray diffraction and scanning electron microscopy, high-resolution core sampling and testing were conducted to analyze the rock mineralogy and paleontology of the carbonate-rich shale in the slope zone of the Dongying Sag. The research findings identified the presence of 10 types of microfacies within the carbonate-rich shale, including algal mat microfacies and shell shale microfacies. These microfacies combined in various ways to form a spectrum of microfacies combinations, exhibiting shale phase transition characteristics on a meter scale. Corresponding to the different stages of saline lake evolution in the Dongying Sag, three types of rock microfacies combinations emerged: shallow water evaporite microfacies combination, oscillatory semi-deep water shale microfacies combination, and oscillatory deep water shale microfacies combination. These combinations were typically controlled by the cyclical rise and fall of lake levels against a backdrop of high-frequency oscillations. In the upper submember of the fourth member of the Shahejie Formation, from bottom to top, as the climate became more humid and hot, weathering intensified, lake levels rose and lake salinity decreased, the proportion of oscillatory deep water shale microfacies combinations gradually increased, while that of shallow water evaporite microfacies combinations decreased. This series of upward-deepening depositional cycles recorded the continuous subsidence and co-evolution of biology and environment during the middle Eocene in the Dongying Sag.