Divergent mechanisms of organic matter enrichment in contrasting members: A case study of coal‐bearing and oil shale successions

The formation mechanisms of coal and oil shale remain controversial due to their distinct depositional environments. The integrated geochemical and multivariate statistical analyses provide a new perspective to explore organic matter (OM) enrichment, making it possible to better understand the genesis of organic‐rich coal and oil shale. This study investigates the Yilan Basin, a representative coal‐oil shale co‐depositional basin in Northeast China, using integrated elemental geochemistry, biomarker analysis and multivariate statistics to reveal two distinct depositional intervals within the Eocene Dalianhe Formation: (1) the coal‐predominant ‘high salinity‐degradation interval’ featuring pyrrophyta and salinophilic archaea microbial communities, and (2) the coal‐oil shale interbedded ‘high lake productivity interval’ dominated by green algae and cyanobacteria. Principal Component Analysis demonstrates that differential terrestrial OM inputs created contrasting enrichment patterns between the two key intervals: the ‘high salinity‐degradation interval’ achieved optimal coal preservation through balanced terrestrial contribution under stable brackish conditions, while the ‘high lake productivity interval’ experienced OM dilution from excessive terrestrial input despite higher productivity. This reveals a fundamental divergence in accumulation mechanisms: while coal formation relied primarily on terrestrial supply and preservation conditions, oil shale development showed distinct phase‐dependent characteristics. In the Coal‐Bearing Member, optimal thick, high‐quality oil shale accumulation occurred during lake flooding periods with moderate terrestrial input and reducing conditions. Conversely, higher lacustrine productivity supported the OM enrichment of the oil shale from the Oil Shale Member, yet the oil shale ultimately exhibited lower quality due to OM dilution caused by gravity flows and excessive terrestrial input.