Organic matter accumulation driven by land-sea interactions during the Late Cretaceous: A geochemical study of the Nenjiang Formation, Songliao Basin

The Late Cretaceous Nenjiang Formation in the Songliao Basin presents a unique setting to examine how climate change and sea-level rise influenced organic matter accumulation. This study combines TOC analysis, Rock-Eval pyrolysis, GC–MS, GC–MS-MS, and elemental geochemistry on core samples from two wells to assess organic matter deposition before and after transgressive events. TOC values range from 0.18 to 14.63 wt%, with significant variations in hydrocarbon potential and thermal maturity. Periodic warm and cool climates triggered intermittent seawater intrusions that created anoxic conditions conducive to marine diatom and lacustrine dinoflagellate proliferation. Extended warm periods, however, suppressed dinoflagellate development and reduced paleo-productivity. The activity of methanogenic bacteria further contributed to the degradation of sedimentary organic matter, hindering its accumulation. While warm climates facilitated flood events that transported terrigenous nutrients, enhancing dinoflagellate blooms and expanding the oxygen minimum zone. These findings highlight the bio-environmental interactions that governed organic matter accumulation during transgressions, offering insights for exploration in similar sedimentary environments.