Geochemical characteristics of shales from the first member of Qingshankou Formation in Gulong Sag, Songliao Basin, China: Implication for mechanism of organic matter enrichment

Abstract

Commercial oil production from the first member of the Qinshankou Formation in the Songliao Basin, China has proven it can be a promising target for shale oil recovery. However, geochemical characteristics and the mechanism of organic matter (OM) enrichment in this layer remain unknown. Herein, a large number of samples collected from the first member were analyzed through a combination of organic and inorganic geochemical methods. Results show, bedding oil shale and laminated shale are prominent facies in this member. Moreover, this member mainly consists of type-I and –II kerogen at highly thermal mature stage, belonging to good to excellent source rock. Overall, this member has been deposited in the semi-humid to humid and warm paleoclimate, fresh to brackish water mass, and dysoxic to anoxic conditions. This member is enriched in P2O5, indicating the high primary productivity. Moreover, lack of relation between P2O5, with Al2O3 and Fe2O3 concentrations, suggests that the contribution of terrigenous detritus influx and the effect of Fe cycle for P enrichment in sediments can be disregarded. The terrigenous detritus influx, rapid sedimentation rate and marine transgression were found to be detrimental to the OM enrichment. Moreover, more serious oxygen deficiency in water mass along with slightly lower sedimentation rate, made the lower part of this member to yield better preservation conditions and weaker dilution effect, which resulted in the formation of OM with better quality and higher abundance. Collectively preservation conditions were found to be significant for the OM enrichment.