| Shale oil and gas resources of the 7th member of Middle Jurassic Dameigou Formation in the Yuka Depression in the northern margin of the Qaidam Basin have great potential and good exploration prospect.However,previous insufficient research on the source rocks depositional environments restricts the comprehensive evaluation and exploration of shale oil and gas.In this paper,we focused on reconstruction the sedimentary paleoenvironment of the 7th member of Middle Jurassic Dameigou Formation in the Yuka Depression using element geochemistry,oil and gas geochemisty,and organic geochemistry,and discussed the main controlling factors of organic matter enrichment from the perspective of the sedimentary microdynamics of lake basin,with a view to providing theoretical basis for exploration and evaluation of shale oil and gas resource in the Yuka Depression.The abundance of organic matter in the 7th member of Middle Jurassic Dameigou Formation in the Yuka Depression is relatively high,and the abundance of organic matter varies greatly from different lithology.Oil shale has the highest average value of 5.94%;followed by shale and mudstone with an average value of 3.79%and 3.23%,respectively;the sandstone has the lowest organic matter content,with an average of 1.13%.The results of rock pyrolysis indicate that hydrocarbon generation potential is also closely related to lithology.The maturity of organic matter is in the low maturity and mature oil window stage.The main mineral compositions are quartz and clay minerals,and the siderite content in all samples is generally high.The organic matter are type ? and ?,and only oil shale is type ? according to the analysis of biomarkers,organic maceral,kerogen carbon isotope and rock pyrolysis.The distribution characteristics of short-mid-long-chain alkanes and steranes,as well as carbon preference index(CPI),odd-to-even predominance(OEP),and wax index indicate that the organic matter in sandstone and mudstone mainly derived from terrestrial higher plants,with a small amount of aquatic organisms;Oil shale and shale are dominated by algae and microorganism,accompanied by a certain amount of higher plants,and bacteria have also been found to have a certain contribution to organic matter.The organic maceral are dominated by the liptinite,among which the suberinite content is relatively high,which is the main hydrocarbon-generating component.The changes of kerogen carbon isotope indicate that the type of organic matter is closely related to the sedimentary paleoenvironment.The shale has developed nano-pores with mainly inorganic pores.Organic pores are rarely observed,and they are mostly distributed with independent single pores.The pore size distribution is mostly between 30-50 nm,and a small number of samples are distributed between 70-100 nm,indicating that a large number of macropores are present.Micropores and mesopores contribute more than 90%to the pore specific surface area,and macropores contribute more than 20%to the pore volume.The content of clay minerals has a greater influence on mesopores,and the correlation with micropores is poor.The methane adsorption capacity(Langmuir volume VL)ranges from 2.91 to 7.90 cm3/g,with an average of 5.11 cm3/g.TOC content is positively correlated with VL,but no obvious correlation between clay mineral content and VL.Comparing with the upper continental crust(UCC),it is found that Al2O3,TFe2O3,MnO and P2O5 are relatively enriched in the samples;CaO,K2O,MgO,Na2O and SiO2 are relatively deficient.P element can reflect the organic matter content in organisms,and the high enrichment coefficient also indicates that the shales in the 7th member of Dameigou Formation is suitable for biological activities during deposition.TFe2O3 and MnO are relatively enriched,which may be related to the depositional environment.A good positive correlation between TiO2 and Al2O3 in the sample,indicating that the terrigenous detrital input is stable during the deposition.According to the lithological composition,element geochemistry and organic geochemistry characteristics of the ZK23-1 well,the 7th member of Dameigou Formation can be subdivided into three depositional stages.The distribution characteristics of Fe/Mn,Mg/Ca,Rb/Sr and Sr/Cu ratios and carbon index indicate that the stage ? and ? of the 7th member of Dameigou Formation are warm and humid sedimentary paleoclimates,and the stage ? is composed of alternately hot-dry and warm-humid paleoclimate.The values of CuEF,ReEF,UEF,MOEF,VEF and Corg/P reflect the changes of oxidation and reduction during the deposition of the 7th member of Dameigou Formation.The depositional water column in the stages ? and ? were in a reducing environment;in the stage ?,the oxidation and reduction conditions changed frequently,especially the siltstone deposition was an oxidation environment.Paleosalinity of the 7th member of Dameigou Formation gradually increased from the stage ? to stage ?,with freshwater to brackish water.However,the salinity of water column fluctuates strongly in the stage ?,which is consistent with paleoclimatic changes and may be affected by paleoclimatic changes.In addition,the S/TOC ratio indicates that the stage ? was a freshwater environment,the salinity of stage ? changed frequently,and the stage ? was an environment of increased salinity.The Nixs and Cuxs parameter values of the samples in this study combined with the TOC content indicate that the 7th member of Dameigou Formation was at the high paleoproductivity level during the deposition.The paleoproductivity in the upper part of the stage ? increased significantly,the stage ? fluctuates greatly;the stage ? is at a high productivity level.Most samples(except stage ? siltstone samples)showed a high degree of chemical weathering(CIA>80),and the sediments did not undergo a sediment cycle process while deposited.Provenance is composed of felsic volcanic rock and intermediate volcanic rock.The studied samples have slightly differentiated rare earth distribution patterns,with a wider Eu anomaly,indicating that the Middle Jurassic has a stable extensional structure setting.The Yuka Depression experienced the Late Indosinian Movement and the episodes ? and ? of the Yanshan Movement in the Early and Middle Jurassic.Based on the rare earth element discriminant diagrams combined with the regional tectonic setting and provenance composition,indicating that the tectonic setting of Yuka Depression was the active continental margin during depostion of the 7th member of the Middle Jurassic Dameigou Formation.Based on paleoenvironmental reconstruction and paleoproductivity level analysis of the 7th member of Middle Jurassic Dameigou Formation in the Yuka Depression,indicating that paleoclimatic conditions play a key role in controlling the accumulation of organic matter during transition period from dry-hot to warm-humid.Under warmhumid paleoclimate conditions,paleoproductivity level and preservation conditions of organic matter are controlling factors that affect the enrichment of organic matter.Tectonic activity and basin subsidence rate have an important influence on the preservation conditions of organic matter.Therefore,the organic matter enrichment of the 7th member of Dameigou Formation in Yuka Depression was affected by many factors,among which paleoclimate,paleoproductivity level and organic matter preservation conditions are the main controlling factors.Ultimately,a comprehensive depositional model is proposed to interpret the evolution of deposition of the 7th member of Dameigou Formation,providing new insights into the exploration of shale oil and gas resources in the Yuka Depression. |
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