Palaeoenvironmental conditions during deposition of the Upper Cretaceous oil shale sequences in the Songliao Basin (NE China): Implications from geochemical analysis

Excellent hydrocarbon source rocks (oil shales), containing Type I organic matter (OM), were deposited in the continental Songliao rift basin during the Late Cretaceous. A major contribution of aquatic organisms (dinoflagellates, green algae, botryococcus) and minor input from macrophytes and land plants to OM accumulation is indicated by n-alkane distribution, steroid composition and delta C-13 values of individual biomarkers. Microbial communities included heterotrophic bacteria, cyanobacteria and chemoautotrophic bacteria, as well as purple and green sulfur bacteria. The presence of methanotrophic bacteria is indicated by C-13-depleted methyl hopane. The sediments were deposited in a eutrophic, alkaline palaeolake. Highly reducing (saline) bottom water conditions and a stratified water column existed during OM accumulation of the Qingshankou Formation and Member 1 of the Nenjiang Formation. This is indicated by low pristane/phytane, gammacerane index and MTTC ratios, and the presence of beta-carotane and aryl isoprenoids. However, an abrupt change in environmental conditions during deposition of Member 2 of the Nenjiang Formation is indicated by significant changes in salinity and redox-sensitive biomarker ratios. A freshwater environment and suboxic conditions in the deep water prevailed during this period. Higher input of terrigenous OM occurred during deposition of the upper Nenjiang Formation. Good oil-to-source rock correlation was obtained using biomarker fingerprints of oil-stained sandstone from the Quantou Formation and oil shales from the Qingshankou Formation. Based on the extent of isomerisation of C-31 hopanes, the oil was most probably derived from oil shales of the Qingshankou Formation in deeper parts of the basin. (C) 2012 Elsevier Ltd. All rights reserved.