Lithofacies And It's Sedimentary Model Of Peperites,the Early Cretaceous In The Liuhe-Tonghua Basin,NE China

Liuhe-Tonghua basin,a faulted basin,locates in the south of Dunmi fault zone and the northwest of Yalu river fault zone,and develops on the basis of the pre-Mesozoic in the northern margin of the north China plate.During the Mesozoic era,the study area was strongly inpacted by the volcanic avtivity,which resulted in the complex basin filling of the volcanics and sedimentary rock,and formed a characteristic sequence dominated by volcanism-induced sedimentation.This type of rock formed by the mixed accumulation of pyroclastic and terrigenous debris in the Liuhe-Tonghua basin is the focus of the paper,namely the peperites.In light of the previous study and drilling data from the Liuhe-Tonghua basin,it can come to some following conclusions about lithofacies and sedimentary model of peperites as well as the evaluation of source rock in this basin by the methods of comprehensive analysis on drilling core,outcrop,logging information and experimental data.1.Types and characteristics of peperites in the Liuhe-Tonghua basinThree types of peperites can be identified including coarse peperites,fine peperites and lava-pyroclastic lava,and they are detailedly diveded into 15 rock subcategories.The coarse and fine peperites can exactly classfy into 8 subclasses in sandy grade as well as the lava-pyroclastic lava into 5 subclasses.Well log of peperites demonstrates that rhyolite has high GR level?148.9-182.08API?,low density?2.376-2.46g/cm3?while the coarse and fine ones characterize with low GR level?80.802-125.904API?and high density?2.436-2.621 g/cm3?.The transition from lava to clastic rock reflects the decrease of GR level which means that radioactive magmatic fragments are reducing gradually durning the long-distance accumulation,especially reffering to the rhyolitic components.2.Lithofacies'type of peperitesTriangular diagram of fragments shows a clear mixed-interactive relationship among the base surge?tractive current?,hot pyroclastic flow and gravity flow?including turbidity current?.Given the graphic analysis and core's structure,the lithofacies can be divided into two categories,including mixed lake type and mixed fan delta type.And they can be further summarized by five kinds of microfacies consisted of subaqueous pyroclastic flow,subaqueous surge,subaqueous lahar,lava flow and pyroclastic turbidity current.3.Volcano-sedimentary sequence and filling types in the Liuhe-Tonghua basinThere are two types of volcano-sedimentary sequences in study area including mixed lake and mixed fan delta type which respectively belong to LST-TST system.The former one is a type of mixed filling of LST-TST system which consists of the volcanic mechanism and the mixed lake,while the latter develops the subaqueous mixed fan delta filling type in LST-TST system.4.Stage of the volcanism-induced sedimentation and establishment of mixed-accumulation modelThe stage of the volcanism-induced sedimentation in the region contains four mixed accumulated parts which are respectively Guosong-Yingzuilazi formation,Lingzitou-Xiahuapidianzi formation,Hengtongshan formation and Sankeyushu formation.According to above discussions,we can establish a mixed interactive model of peperites from volcano to sedimentary basin.5.Characteristics and analysis of source rock in the Liuhe-Tonghua basinThe index of TOC,Ro and organic types of hydrocarbon source rocks in this basin shows that the TOC and Ro values of mudstones in Hengtongshan formation are1.41-2.76%and 1.18%respectively,with a number of 1.38%-2.9%and 0.98%in Xiahuapidianzi formation.Their organic types are type I and type II₁ and both can be rated as superior level.However,the hydrocarbon source rocks in the Yingzuilazi formation are poor rank.As for the Liuhe basin,Xiahuapidianzi formation's manifestation of mudstones has a value of 1.05%-1.89%in TOC index as well as 0.65%-1.59%in Yingzuilazi formation.Their organic index are also type I and type II₁,but the latter has an average Ro value of3.9%which represents the postmature source rock level.Guosong formation has poor potential in hydraocarbon generation.The Hengtongshan formation and the Xiahuapidanzi formation show the optimal hydrocarbon source rocks,and both of them are in shale deposition condition of lake durning the wax-wane cycle of volcanic activity.However,durning the evolution of source rock,the volcanism-induced sedimentation provides better maturation and burial conditions in the Tonghua basin while makes destructive effects in Liuhe basin.