The Early Cretaceous (Late Barremian–Early Aptian) organic-rich shales in the Qiangtang Basin constitute a significant hydrocarbon source rock series, whose formation is closely linked to specific paleoenvironmental conditions and organic matter (OM) enrichment mechanisms. Studies show that this shale sequence was deposited in a semienclosed relict bay–lagoon transitional environment. Based on lithological assemblages and geochemical characteristics, it can be divided into three distinct sedimentary units: micritic limestone at the base, black shale in the middle, and marl at the top. The black shale unit exhibits the highest total organic carbon (TOC) content. Paleoenvironmental reconstruction indicates that OM enrichment was controlled by multiple synergistic factors, including redox conditions, paleoproductivity, and paleoclimate. Redox proxies reveal that the micritic limestone was deposited under dysoxic– anoxic conditions, the black shale formed in a more strongly reducing anoxic–euxinic stratified water column, and the marl corresponds to oxic–suboxic conditions. This vertical variation directly influenced the preservation efficiency of OM. Paleoproductivity and paleoclimate indicators collectively indicate that the basal and middle units were deposited during periods of high marine primary productivity alongside a warm and humid temperate climate. In contrast, deposition of the upper unit occurred under a hot, arid climate with significantly reduced productivity. Particularly important is the presence of intermittent intense hydrothermal activity during the deposition of the black shale. This activity not only supplied abundant nutrients to the marine environment, stimulating productivity blooms, but the released reducing gases also helped maintain sulfidic conditions in the water column. Consequently, the development of Early Cretaceous organic-rich shales in the Qiangtang Basin resulted from a warm climate that enhanced the hydrological cycle and terrestrial nutrient input, thereby boosting productivity. Coupled with the topography of the semi-restricted bay, which facilitated water column stratification, these conditions collectively created an optimal environment for OM preservation. This study provides important geological insights for understanding the formation mechanisms of transitional facies source rocks and for hydrocarbon resource assessment in the Qiangtang Basin.
Wang et al. (Thu,) studied this question.