A comparative study of oil shale deposition in the Upper Cretaceous Nenjiang Formation, NE China: Evidence from petrographic and geochemical analyses

The Songliao Basin located in NE China is one of the most petroliferous basins in the world. The Upper Cretaceous Nenjiang Formation is an important source rock in the basin owing to its rich development of oil shale successions. From outcrop observation, we found that the oil shale successions can be divided into two types. The type I oil shale (TY1) is composed of multiple thin oil shale layers (similar to 1 m) interbedded with organic-poor shale layers and is typically developed in the first member of Nenjiang Formation (K(2)n(1)), whereas the type II oil shale (TY2) developed one thick succession of oil shale (similar to 10-15 m) and was mainly found in the second member of the Nenjiang Formation (K(2)n(2)). To compare the difference in forming mechanisms with two types of oil shale successions, petrographic characteristics, stable isotopic compositions, and geochemical elements were analyzed from two outcrops (i.e., Yaojia Station and Houjingou Outcrops). Based on the results, a notable higher delta C-13(org) values are recorded in the TY1, suggesting accelerated primary productivity. The observation of tuff layers and the relatively high P and Fe contents reveal that the frequent volcanic activities would have brought nutrient supply to the Songliao paleolake, thereby stimulating primary productivity and contributing to the deposition of multiple thin-layered oil shales. In contrast, the TY2 show decreased bulk sediment delta N-15 suggests intensified denitrification under anoxic conditions. Combined with elevated trace elemental ratios and increasing trend of CIA, we attributed these changes to marine transgression that occurred at the beginning of K(2)n(2) deposition. Such conditions increased the accommodation space and favored the formation of stratified, anoxic bottom waters, which might have promoted organic matter preservation and further responsible for the deposition of thick oil shale succession in K(2)n(2).