Micropaleontology and palaeoclimate during the early Cretaceous in the Lishu depression,Songliao basin,Northeast China

Diverse and abundant microfossils,such as palynomorphs,algae and Ostracoda,were collected from lower Cretaceous strata of Lishu depression,located in southeastern Songliao basin,and were identified and classified in order to provide relevant,detailed records for paleoclimate research.The early Cretaceous vegetation and climate of southeastern Songliao basin have been inferred from the analysis of palynomorph genera,algae and Ostracoda of the LS1 and SW110 wells.The lower Cretaceous strata include,in ascending stratigraphic order,the Shahezi,Yingcheng and Denglouku formations.Palynological assemblages for each formation,based on biostratigraphic and statistical analyses,provide an assessment of their longitudinal variations.During deposition of the Shahezi Formation,the climate was mid-subtropical.Vegetation consisted of coniferous forest and herbage.During deposition of the Yingcheng Formation,the climate was south Asian tropical.Vegetation consisted mainly of coniferous forest and herbal shrub.In addition,fresh and saline non-marine water dominated the lacustrine setting during deposition of these formations.Deposition of the Denglouku Formation,however,occurred under a hot and dry tropical climate.The vegetation was mostly coniferous forest and lake waters became saline.Palaeoclimate variation is correlated by the lake level change and the development of sedimentary facies.Palaeoclimate contribute to the formation of hydrocarbon source rocks and reservoir.