Organic Geochemistry Characteristics And Metallogenic Conditions Of Oil Shale In Middle Jurassic Shimengou Formation In Tuanyushan Area,Northern Qaidam Basin,China

This paper takes the area of the Tuanyushan in Northern Qaidam Basin as the study area.In this area,abundant oil shale resources develop and mainly deposited at Shimengou Formation.Based on the core observation and a large number of test data,the paper studied the organic geochemical characteristics and metallogenic conditions of the oil shale-bearing succession in the target layer of the study area,and discussed the main controlling factors of the organic matter enrichment.According to the core observation and the organic geochemistry test,the shale member of Shimengou Formation mainly developed mudstone and oil shale,and the Coal-bearing Member mainly composed of mudstone,oil shale and coal.The TOC content of oil shale is 5.65-10.65 wt.%,with an average value of8.16 wt.%and the TOC content of mudstone is 0.57-7.30 wt.%,with an average value of 3.69 wt.%in Shale Member.The abundance of organic matter of coal in coal-bearing member is the highest?with average TOC of 50.13 wt.%?,followed by oil shale?the average value of TOC is 19.65 wt.%?,and the mudstonet is the lowest?with mean TOC of 8.35 wt.%?.Comparedly,the abundance of organic matter of oil shale in Coal-bearing Member is much larger than shale member,but its hydrocarbon generation potential is obviously lower than t shale member.By analyzing the TOC and rock pyrolysis data of QD-1 and QD-2,the main types of organic matter of oil shale in the Shale Member of the Shimengou Formation is I-II₁,the mudstone is mainly II₁,followed by II₂ and type III.Coal and organic-rich mudstones in the Coal-bearing Member are mainly III,followed by II₂,and the oil shale is mainly II₂,followed by III.Through comprehensive analysis of pyrolysis parameters Tmax and biomarker,we can know that the maturity of organic matter in Shale Member and Coal-bearing Member of are at an immature stage.Based on variations of TOC and lithology with depth,five sedimentary units are differentiated and labelled units?to?from base to top in Shale Member of Shimengou Formation.The organic matter enrichment characteristics of different sedimentary units are quite different.The samples with the lowest organic carbon content are mainly distributed in unit II and V,corresponding to the lithology of grayish-green siltstone and mudstone.The highest TOC samples are mainly distributed in units I and III,corresponding to lithologies of oil shale and mudstone with dark-gray.Compared with mudstone,oil shale has higher TOC content and hydrocarbon generating potential and better organic matter type.Compared with oil shale in units I,the TOC content of oil shale in unit III is low,and the organic matter type is poor.The ratio of organic carbon to total sulfur?TOC/S?and element geochemistry?Sr/Ba?indicates that the water column of Shale Member is a stable freshwater environment,only in the part of the sedimentary unit I is sub-brackish water.The biomarker of indicative salinity was not detected in the Coal-bearing Member,indicating that the water body column is stable freshwater environment.The biomarker parameters?Pr/Ph?and the geochemical parameters V/?V+Ni?indicate that,the preservation condition of Shale Member is stable anoxic environments.The Coal-bearing Member,however,is weak oxidation to oxidation environment,where coal is formed in oxidation environment and oil shale and mudstone are formed in weak oxidation environment.According to the comprehensive analysis of the pyrolysis parameters and biomarker,there are certain differences in the organic composition of the Shale Member.In the whole,the aquatic organisms,such as algae and phytoplankton,are the main organic composition,but the proportion of the aquatic organisms in different sedimentary units is different.The proportion of the aquatic organisms of the sedimentary units I,II and V is greater than the unit III and IV.Compared with the Shale Member,the Coal-bearing Member contains abundant terrestrial higher plants,and the organic matter is dominated by higher plants and mixed sources,and the aquatic organisms,such as algae and phytoplankton,are less.The comprehensive analysis of paleo-climate,sedimentary environment,paleo-productivity and paleo-water conditions show that,the oil shale of Shale Member is mainly formed in the shallow lake and semi-deep lake environment controlled by the warm and humid climate.The higher lake productivity provides oil shale with a large amount of organic matter.The salinity stratification caused by local brackish and temperature stratification plays a control role in the preservation of organic matter,which are favorable for the development of oil shale.The oil shale of Coal-bearing Member is formed in lake marshland environment with weak oxidizing and freshwater conditions.Because of the flourishing vegetation and ample organic matter in the lake marshland,the abundance of organic matter in oil shale is high.The input of higher terrestrial organic matter and the weak oxidation environment cause the poor organic matter type and low hydrocarbon potential of the oil shale in Coal-bearing Member.It is found that sedimentary environment and source of organic matter are the main factors controlling the abundance of organic matter,type and hydrocarbon generation potential of oil shale in Shale Member and Coal-bearing Member.