Links between marine incursions, lacustrine anoxia and organic matter enrichment in the Upper Cretaceous Qingshankou Formation, Songliao Basin, China

Abstract

There is a general relationship between the formation of terrestrial hydrocarbon source rocks and marine incursions during the Mesozoic–Cenozoic in China. To better understand this scientific issue, this study focused on the little-studied Upper Cretaceous Qingshankou Formation (early Late Cretaceous) in the Songliao Basin, China, which was investigated using organic petrological and organic and inorganic geochemical methods. The mudstone and shale in the Qingshankou Formation were deposited in an anoxic, reducing, slightly brackish water environment and contain benthic macroalgae, which is a typical marine fossil in Qingshankou Member I. The δ34S values are generally similar to those of Late Cretaceous marine sulfate, and the total N contents and isotopic compositions (δ15N) are both high, suggesting that the Qingshankou Formation was affected by marine incursions that triggered lake anoxia. The total organic carbon (TOC) contents of the mudstones and shales in the Qingshankou Formation are 0.21–3.86 wt%, with an average value of 1.91 wt%, which meet the criterion of a very good–excellent source rock. The organic matter varies from type I to III kerogen but is mainly types I and II, which generate oil. The hydrocarbon-generating bio-precursors are mainly planktonic and benthic algae. The hydrocarbon generation potential of the Qingshankou Member I is high, mainly due to the high productivity during its deposition. Increased productivity caused by marine intrusions, water stratification caused by high salinity, and reducing bottom-water environment provided favorable conditions for organic matter enrichment. The marine incursions during deposition of the Qingshankou Formation (especially Member I) improved the quality of the source rocks by enhancing the primary productivity, which might be a common feature of terrestrial source rocks affected by such incursions in general.