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High Resolution Analysis of Organic Matter Deposition in the Qingshankou Formation, Upper Cretaceous, Songliao Basin (NE China): Implication From Geochemistry and Astronomical Cycles


The Upper Cretaceous Qingshankou Formation (K2qn) are the most prolific source rocks in the Songliao Basin containing abundant oil, gas and oil shale resources. The oil shale layers with high total organic carbon (TOC) contents, developed at the bottom of the Qingshankou Formation first member (K2qn1), are divided into lower oil shale (LOS; TOC up to 18.3%) and upper oil shale layers (UOS; TOC up to 14.4%). Much information on organic matter (OM) deposition is included in oil shales providing good targets for research on OM accumulation of Cretaceous continental basins. High resolution investigations of OM accumulation within the LOS and UOS layers are required to understand the variations on oil shale quality and thickness in the K2qn and its relation to basin evolution with precision analyses of drill cores by geology, astronomical cycles, organic petrology and geochemistry. The establishment of K2qn1 high resolution astronomical time scale shows oil shales experienced 6 obliquity and 2 short eccentricity cycles. Sedimentary rates of oil shale layers ranges between 0.32 and 0.38 m/ka under high frequent deposition cycles. The OM accumulation of oil shales is related to Cretaceous anoxic events evidenced by the calculated time from 91.28 to 91.08 Ma. The geological results demonstrate the predominant Type-I kerogen derived from algal-microbial mats (i.e. lamalginite). However, increased proportions of plant-derived OM are indicated by Rock-Eval parameters and the presence of vitrinite in the UOS layer. As evidenced from lower concentrations of hopanes, higher sinking rates of OM are suggested to have been responsible for a lower extent microbial degradation during the LOS layer deposition. Under an extended zone of freshwater on the surface, depositional conditions of OM are characterized by anoxic and saline water column at the bottom, which favored for OM production and preservation separately. It is found that oil shale thickness and quality are governed by primary productivity, related to accommodation space and nutrient supply. Redox conditions within the water column are the key factor controlling OM preservation. More detailed information of OM accumulation is elucidated to enhance the investigation precision and improve the OM theory. Further insights into exploration and production strategies for shale oil are expected based on the present data about the distribution and heterogeneity in oil shale quality within the Cretaceous strata.