Mineralogy and Element Geochemistry of Oil Shales in the Lower Cretaceous Qingshankou Formation of the Southern Songliao Basin, Northeast China: Implications of Provenance, Tectonic Setting, and Paleoenvironment

Abstract

Songliao Basin is a large Mesozoic oil shale-bearing basin in the northeast of China, where multiple layers of oil shales and mudstones developed in the first member of the Qingshankou Formation (K2qn1). Mineralogy and geochemical data were used to reveal the provenance and tectonic setting of the sediments; furthermore, the differences in the paleoenvironment between mudstones and oil shales as well as between different oil shale layers were revealed. Core observation shows that oil shales are light brown with horizontal bedding, while mudstones are dark gray with a massive structure. X-ray diffraction data revealed abundant clay minerals and a certain amount of syngenetic pyrite in the oil shales, indicating that oil shale forms in a reducing water environment. The element ratios revealed that the parent rocks were mostly felsic volcanic rocks, and the discriminant diagrams indicated that the tectonic setting was of a continental island arc. The paleoenvironment and the genetic mechanism of oil shales are revealed by geochemical data, and the oil shales and mudstones were deposited under warm-humid paleoclimate, salty brackish, and strong reducing water environments. The geochemical curves show four high and low change cycles in the column, which means that the oil shale layers have a higher organic matter content than the mudstone layers because of the higher primary productivity, more saline water environments, and stronger reducing conditions. From the lower layer 1 to the upper layer 4 of oil shales, the extent of the warm-humid climate gradually weakened, whereas the salinity of the water environment gradually increased.