Depositional evolution of Eocene deep-lake mudrock lithofacies driven by astronomical forcing in the Dongying Depression, China

Abstract

With the deepening of shale oil and gas exploration and development, researchers are more eager to understand the depositional evolutionary pattern of mudrock lithofacies, and the astronomical cycle is an important mechanism driving the depositional evolution of mudrock. In this paper, stratigraphy is divided based on astronomical cycle, and a stratigraphic sequence is established by using long eccentricity and precession cycles to dissect well sections with cores in the connecting well profiles. The relationship between the type of lithofacies development in response to the long eccentricity cycle is found. The lake basin mainly developed mudstone-dominated facies during high-value periods of long eccentricity, while sandstone facies developed in the slope zone and marl facies developed in the deep-lake zone during low-value periods. The relationship between lithofacies subcycles and amplitude variations in precession within the long eccentricity cycles has been determined. Sandstone or marl developed during periods of maximum precession, while mudstone developed during periods of minimum precession. The model of climate and depositional cycles controlled by long eccentricity and precession has been summarized. First, during periods of low long eccentricity, the amplitude of precession is small. This leads to cold and arid climates with reduced rainfall, resulting in less terrestrial input to the basin and a period of contraction. During periods of high long eccentricity, the climate is hot. This leads to hot and humid climates with increased rainfall, resulting in stronger terrestrial input to the lake basin and a period of expansion. Second, the amplitude variations in precession also affect the strength of terrestrial input. During periods of maximum precession, the received solar radiation is low, resulting in a dry and cold climate with reduced rainfall, weakening the terrestrial input to the basin. During periods of minimum precession, the received solar radiation increases, resulting in a hot and humid climate with increased rainfall, strengthening the terrestrial input to the basin. This study systematically investigates the depositional model and factors controlling the mudrock layers in the research area. The research results can contribute to the prediction of “sweet spots” for shale oil and gas and provide references for similar sedimentary basins.