Paleoenvironmental reconstruction of the coal- and oil shale-bearing interval in the lower Cretaceous Muling Formation, Laoheishan Basin, northeast China

Abstract

The Laoheishan Basin in northeast China has been filled with Lower Cretaceous coal- and “oil shale”-bearing sediments. The basin fill includes from base to top alluvial conglomerate (lower member of Muling Formation), fan delta sediments interbedded with coal and oil-prone mudstone (denominated as “oil shale”) layers in the upper member of Muling Formation, and volcaniclastic rocks (Dongshan Formation). In the present study, the maturity of organic matter, oil shale quality, and paleoenvironment of the coal and oil shale accumulation are investigated based on macro- and micropetrographic data, proximate and ultimate analyses, bulk geochemical parameters, biomarkers analysis and stable isotope geochemistry.In the Laoheishan Basin, both coal and “oil shale” layers are derived from land plant organic matter. This contrast the “oil shale” of this basin from oil shale/coal intercalations in fault-related basins, in which oil shale has been found to be of algal origin. The coal is sub-bituminous in rank, hydrogen-rich and oil-prone. The “oil shale” is of low-medium grade and the relatively low oil yield may relate to the abundant resinite and sporinite, because of their lower generation potential compared with alginite. Accumulation of the high-ash coal commenced in low-lying mire, drowned during frequent floodings. Subsequently high-ash, low-sulfur coal was deposited in a stable low-lying mire, under oxic conditions and limited bacterial activity. Afterwards the mire was drowned and formed a freshwater, dysoxic to oxic shallow lake, in which “oil shale” layers accumulated. Finally, the depositional environment returned to low-lying mire but probably with a brackish influence, as indicated by elevated sulfur contents in the uppermost samples. Petrography- and biomarker-based proxies indicate that gymnosperms dominated the paleovegetation of the mire, accompanied by variable amounts of herbaceous plants, such as ferns.