The Geochemical Characteristics And Paleo-environment Reconstruction Of Triassic Chang 7 Oil Shale In The South Of Ordos Basin

The Triassic oil shale in the in ordos basin have huge resources and the exploration prospect is very promising.Under the control of the oil yield and calorific value of oil shale industry grade parameters,in the exploration of oil shale,the corresponding researches are often lack of using geochemistry.Restoring sedimentary environment of the oil shale can provide geological basis data and theoretical guidance for the next oil shale exploration.Geochemistry is an effective means to explore the palaeosedimentary environment.By the study of petrology,mineralogy,element geochemistry and organic geochemistry of the oil shale in Chang 7 Formation in the southern ordos basin,this paper analyses the ancient climate,redox conditions,ancient salinity,water depth and hydrothermal deposition conditions during the oil shale depositional,and evaluates the hydrocarbon source rocks with discussion of organic matter source.And finally,the oil shale depositional model is put forward.In southern Ordos Basin,the Triassic Chang 7 Formaion develop a large number of horizontal bedding with bottom-mould tectonics,pudding structure,drainage,lapped bedding,slump structure and massive ancient fossils,reflect relatively weak water hydrodynamic and the sedimentary facies is deep lake-deep lake facies.In the southern part of the ordos basin,the stratum of Chang 7 oil shale is relatively horizontal,and the lithologic are black or dark brown thin pages,which are brittle and easy to break,and can be lit directly.The minerals in oil shale are mainly clay minerals,quartz and feldspar with a small amount of dolomite,phosphorus iron ore and calcite.Clay minerals mainly include illite,illite/smectite mixing layer,kaolinite and chlorite.The sandstone and tuff are mainly seen in the interlayer of oil shale.The lithology of sandstone is mainly composed of lithic sandstone and feldspathic litharenite,and the sandstone particle size is mainly medium-fine grained.The results of particle size analysis show that the sample are plottd in the characteristics of gravity flow.The tuff is dominated by maroon,gray and grayish yellow,and there is a thin layer of yellow-gray silty ash in the tuff.According to the oil yield?4.3 to 9.1%,averaging 6.5%?,calorific value?3.85 to 9.49MJ/kg,averaging 6.73 MJ/kg?),ash?68.15-86.08%,averaging 76.93%?and total sulfur?2.47to 7.48%,averaging 4.98%?of the oil shale drilling samples in the southern basin,the industrial grade of the oil shale is classified with medium grade-high sulfur oil shale.In the major elements of the oil shale,SiO₂ content is the highest,followed by Al₂O₃ and TFe₂O₃,and the rest are lower than 5wt%.The Al/Si ratios are relatively low?averaging 0.33?,refleting the SiO₂ is in occurrence of quartz.Si/?Si+Al+Fe?shows an average of 0.62,implying that the oil shale deposited not far from the terrigenous.The Log?SiO₂/Al₂O₃?-Log?TFe₂O₃/K₂O?diagram shows the oil shale belongs to shale and Fe-shale.Compared to UCC,the Cu,U,Cd and Mo abundances are extremely rich.The light rare earth to heavy rare earth ratios are 7.05-10.85 with an average of 8.95,showing the richement of light rare earth elements and the depletion of heavy rare earth elements.Trace elements characteristics,La/Th-Hf and Co/Th-La/Sc discrimination diagrams show that the parent sources of the oil shale are acidic rock.The rare earth element distribution curves indicate that the provenance of the oil shale comes from Yinshan magmatic rocks in the north and Qinling-Dabie mountain in the south.The chemical alteration index?CIA?of the oil shale sample is 62.07⁷3.09,with an average of 68.12,and the Sr/Cu was 1.01³.45,with an average of 2.06,reflecting the moderate weathering degree and the warm and humid palaeoclimate conditions.V/Cr is2.18⁵.27?averaging 3.72?;V/?V+Ni?is 0.82⁰.92?averaging 0.88?;U/Th is0.91⁶.39?averaging 3.52?;?U is 1.46¹.90?averaging of 1.77?;AU is 8.03⁶2.93?averaging39.84?,indicating that the redox condition is mainly reducing conditions.The boron element method and Sr/Ba of the oil shale show that the water is mainly composed of fresh water or brackish water,and there is no possibility of transgression.The ancient water depth is quantitatively reconstruction by Co element method,and it is considered that the average range of ancient water depth during oil shale sedimentary is 29.55⁸3.58m,which is the deep water environment.The Eu anomaly,Fe/Ti,?Fe+Mn?/Ti,U/Th,and Fe-Mn-?Cu+Co+Ni?×10with SiO2/?K2O+Na2O?show that there is a strong hydrothermal fluid activity in Chang 7period.Hydrothermal fluid activities have two main effects on hydrocarbon source rocks.On the one hand,the thermal fluid activity can promote the enrichment of organic matter.On the other hand,it can enhance the reducing nature of water body,which is beneficial to the preservation of organic matter.The total organic carbon?TOC?of the oil shale is 0.29²4.68%?averaging 12.27%?;the amount of hydrocarbon generation potential?S₁+S₂?is 0.29⁷5.25 mg/g?averaging 34.63mg/g?,chloroform bitumen"A"content is 0.06⁰.65%?averaging 0.34%?.All these suggest that the the oil shale is good hydrocarbon source rocks.According to the organic maceral and kerogen type diagram shows the organic matter type of the oil shale is Type II.The Tmax is422-469? with an average of 437.21? and Ro is 0.98¹.15%?averaging 1.07%?,implying that the oil shale is in oil-generating window stage?maturity stage?.The???R regular sterane C₂₇,C₂₈,C₂₉ is given priority to with"V"type,in which the C₂₇ content?40.17 43.00%?and C₂₉ content?23.38 29.89%?are higher than that of C₂₈?28.4833.77%?.By the triangular figure of???R regular sterane can be seen that the organic matter source of the oil shale is mixed type with both lower aquatic organisms and terrigenous higher plants.Pr/nC₁₇ is 0.22⁰.76?averaging 0.33?,and Ph/nC₁₈ is 0.35¹.62?averaging 0.61?,reflecting that the oil shale is in the mature stage.Pr/Ph is 0.39⁰.60?averaging 0.52?,implying the reduction environment.In addition,the low content of gammacerane is detected in the oil shale,and the average value of gammacerane/hopane is 0.05,indicating that the water body should be fresh or brackish water environment.The sedimentary environment determined by the organic geochemistry is consistent with the conclusion of inorganic geochemistry.In Indosinian period,the South China plate subduced to the North China plate with the beginning of the Qinling orogenic.Located in the North China plate,the Ordos Basin started regional uplift,and gradually turned to continental sedimentation.According to this study,the organic matter sources in the southern Ordos Basin are affected by the Qinling orogenywith a considerable proportion of terrestrial higher plants.The water of the oil shale deposit is deep,and the climate is warm and humid,and the water is mainly fresh with obvious thermal fluid activities.According to these features,the model of the oil shale in the southern Ordos Basin is established,which is expected to provide some theoretical basis for the oil shale exploration in the southern Ordos Basin.