Positive and negative effects of marine transgression on the quality of lacustrine source rocks in the Upper Cretaceous Songliao Basin, China

The linkage between marine transgression and lacustrine source rock development has been highlighted for decades. Here, improved understandings were obtained on this challenging issue based on valuable comprehensive profiles from the Upper Cretaceous Songliao Basin, NE China (Qingshankou Formation, brief as K2qn1). 48 shale samples from the Central Depression in the northern Songliao Basin were analysed to establish three geochemical profiles of K2qn1. Multiple biomarkers suggested that organic matter in the source rocks of K2qn1 was mainly sourced from aquatic organisms. Systematic organic and inorganic geochemical analyses reveal that at least five transgressive events occurred during the sedimentary period of K2qn1, denoted as TI, TII, TIII, TIV, and TV from bottom to top. In the context of multiple transgression events, the quality of K2qn1 source rocks was primarily controlled by lacustrine primary productivity. Importantly, transgression events have both positive and negative effects on the quality of lacustrine source rocks. During earlier transgressive events (e.g., TI and TII), the quality of source rocks was negatively correlated with transgression intensity, which is likely due to the difficulty of freshwater organisms adapting to salinity, resulting in a decline in primary productivity. In the later transgression events (e.g., TIII and Tv), the quality of source rocks was positively correlated with transgression intensity, which is probably due to the gradual dominance of salt-tolerant or halophilic organisms during multiple stages of salinization. This study provides a comprehensive research approach for evaluating the influence of transgression events on lacustrine source rocks.