Elemental characteristics and paleoenvironment reconstruction:a case study of the Triassic lacustrine Zhangjiatan oil shale,southern Ordos Basin,China

Delu Li•Rongxi Li•Zengwu Zhu•Xiaoli Wu•Futian Liu•Bangsheng Zhao•Jinghua Cheng•Baoping Wang

1 Introduction

Oil shale,as one of the most important unconventional petroleum resources,has attracted much attention in recent years(Liu and Liu 2005;Lu¨et al.2015;Hakimi et al.2016;Song et al.2016).In general,shale with over 3.5%oil yield is defined as oil shale,and can release thermal energy and shale oil by low temperature carbonization(Fu et al.2015;Liu et al.2015).Prior research on oil shale has mainly focused on the marine environment,especially in northern China—including the Cretaceous oil shale in Qiangtang Basin(Fu et al.2007,2010a,b,2015,2016;Wang et al.2010),middle-upper Neoproterozoic oil shale in Yanshan Region(Bian et al.2005;Liu et al.2011),etc.Based on elemental geochemistry,the oil shale in Qiangtang Basin was deposited in a tropical-subtropical environment(Fu et al.2009;Wang et al.2010).The Qiangtang oil shale formed at the edge of a gulf-lagoon,with fresh water input and high paleoproductivity(Fu et al.2007;Wang et al.2010);petrography and geochemistry indicate that hydrothermal fluid has impacted its formation(Sun et al.2003;Chen and Sun 2004;Chen et al.2004).Major and trace element analyses suggest hydrothermal activity was frequent during sedimentation of the Xiamaling Formation oil shale in the middle-upper Neoproterozoic(Sun et al.2003).Lacustrine oil shale is widely distributed across China(Bai and Wu 2006;Bai et al.2015;Liu et al.2006).However,most previous studies have concentrated on calculating reserves(Bai and Wu 2006;Bai et al.2009;Lu et al.2006).Elemental and mineral characteristics of lacustrine oil shales,and implications for paleoenvironment,are badly in need of additional investigation.

Fig.1 a Geologic map of the Ordos Basin and location of studied section(Li et al.2016)and b stratigraphic column of Upper Triassic Yanchang Formation in study area(modifi ed after Qiu et al.2014)

A typical lacustrine oil shale in China,the Zhangjiatan oil shale of the Triassic Yanchang Formation in the southern Ordos Basin has many merits,such as wide distribution,abundance,and shallow burial(Lu et al.2006;Bai et al.2009;Li et al.2009a,b,2014;Chang et al.2012;Wang and Yan,2012;Deng et al.2013;Luo et al.2014).Organic geochemistry analyses show that the Zhangjiatan oil shale in Chang 7 Member of the Yanchang Formation is dominated by organic matter type II1and approaches mature designation(Liu et al.2009;Ma et al.2016;Wei et al.2016).However,few studies have focused on elemental geochemistry of the oil shale,particularly occurrence of trace elements.Although faunal data,framboidal pyrite,and the ratio of organic carbon to total phosphorus in the oil shale indicate deposition was dominated by oxidizing conditions(Yang et al.2010;Yuan et al.2016),biomarkers have shown reducing conditions(Deng et al.2013).Additionally,discussions of oil shale paleosalinity are still unsettled(Luo et al.2014),with a possible marine transgression event in the southern Ordos Basin.Elemental geochemistry of some oil shale outcrop samples,focusing on Tongchuan City in southern Ordos Basin,has improved understanding of the paleoenvironment(Sun et al.2015).However,two concentrated oil shale samples from one location do not necessarily re fl ect the entire southern Ordos Basin as weathering and alteration can significantly impact elemental characteristics of samples.

Fig.2 Simplified geologic map of study area,showing the location of drill holes(modified after Ma et al.2016)

In order to improve understanding,we evaluated the quality of oil shale and the occurrence mode of trace elements.Then,by using the speci fi c or calculated value,paleoenvironment was reconstructed.Finally,relationships between single elements and paleoenvironment were analyzed to determine the indicators of paleoenvironment of the lacustrine oil shale.This study fills gaps of lacustrine oil shale elemental geochemistry in the southern Ordos Basin and should help guide future exploration.

2 Geologic setting

The Ordos Basin is a superimposed basin with stable deposition and multiple sedimentary cycles(Fig.1a)located in mid-western China(Liu et al.2008).The Ordos formed as a marine basin of the North China Block by the Carboniferous to Permian and has a Proterozoic crystalline basement(Wan et al.2013;Qiu et al.2015).After the Triassic,the basin gradually departed from the North China Block and evolved into a large inland sedimentary basin with a relatively quiettectonic setting (Lietal.2006,2008).Affected by the Indosinian Orogeny,the whole basin gradually uplifted and subducted in the Early Cretaceous and underwent reformation.According to present tectonic characteristics,basement features and evolutionary history,the basin is divided into six first-order tectonic units:the Weibei Uplift,Yishan Slope,Yimeng Uplift,Jinxi Flexure Zone,Tianhuan Depression,and Western Thrusted Zone.The Triassic Yanchang Formation went through an integrated occurrence-extinction period and deposited a set of progradation aggradation retrogradation strata with a thickness of 1000–1300 m(Wu et al.2004;Li et al.2009a,b;Zou et al.2012).According to sedimentary cycles and rock assemblages,the Yanchang Formation can be further divided into 10 Members:Chang 10 to Chang 1(Qiu et al.2010).During the initial stage of Chang 7,due to extension and sinking of the basin along with orogenies and paroxysmal eruption in the south,the lake reached its largest size(Qiu et al.2015).There are two sets of oil shale in the Yanchang Formation,namely the Zhangjiatan at the bottom of Chang 7 with a thickness of 20–30 m(Wang and Yan 2012)and the Lijiapan(Wang and Yan 2012;Dong et al.2014)at the top of Chang 9 with a thickness of 5–15 m.The Zhangjiatan is the main Mesozoic oil reservoir in the Ordos Basin(He 2003;Deng et al.2013;Li et al.2016).

Fig.3 Zhangjiatan oil shale sections,showing sampling locations

The study area is located in the Weibei Uplift,southern Ordos Basin.Due to the Indosinian Orogeny in the Late Triassic and the later Yanshan Orogeny,the study area experienced unbalanced uplift and lacks upper Yanchang strata.The bottom of Chang 7 contains oil shale,shale,mudstone,and silty mudstone(Fig.1b).Shallow burial conditions and the significant thickness of the Zhangjiatan make it economically viable.Understanding the Zhangjiatan may improve oil shale exploration.

3 Samples and analytical methods

A total of 16 samples were collected from three oil shale sections in the lower Chang 7 Member in the southern Ordos Basin(Figs.2,3).All samples were analyzed for oil yield(Tad),ash yield(Ad),calorific value(Qb,ad),total sulfur(St,d),major element oxides,and trace elements.Four samples were tested by X-ray diffraction(XRD)for mineral content.

For the oil yield analysis,samples were ground to a particle size of less than 3 mm,then 50 g of each sample was enclosed in aluminum retort by low temperature carbonization for analysis.The procedure followed the Chinese standard methods SH/T 0508-1992(1992).

The ash content analysis used the slow-ashing method.Each sample of about 1 g was ground to less than 0.2 mm and tiled to a cupel.Then the cupel was heated to 815 ± 10°C in a muf fl e furnace until the residue content stabilized.We followed the Chinese standard methodsGB/T 212-2008(2008a,b).Precisionwas within 5%.

Fig.4 Relationships between key parameters of oil shale and silty mudstone samples

Table 1 Mineral content tested by XRD in oil shale samples(%)

For the determination of calorific value,samples of 1 g with grain size below 0.2 mm were put into a combustion boat and ignited electrically by oxygen with a purity of more than 99.5%.After 6–7 min,temperatures were recorded for 3 min at 1-min intervals and the highest temperature was recorded as the fi nal temperature.The analytical method followed the Chinese National Standard GB/T 213-2008(GB/T 2008a,b).Analytical error was within 5%.

The samples for total sulfur analysis were powdered to less than 100 μm and heated in a pipe furnace to 1250 ± 20°C with fluxing agent of cupric oxide powder.The method followed Chinese National Standard GB/T 6730.17-2014(2014).

For mineral content analysis,XRD was performed with a D8 ADVANCE powder diffractometer.The analytical procedures followed Chinese National Standard SY/T 6210-1996(1996)and the precision was within 1%.

The above analyses were conducted at Shaanxi Coal Geological Laboratory Co.,Ltd.

The samples for element analysis were all powdered to less than 200 mesh,and analyzed by X-ray fluorescence spectrometry(XRF)for major elements and inductively coupled plasma–mass spectrometry(ICP-MS)for traceelements with AA-6800 atomic absorption spectroscopy,UV-2600 ultraviolet–visible spectrophotometer and Perkin Elmer SciexElan 6000.The analytical procedures followed Chinese National Standard GB/T 14506.1～14-2010(2010)and GB/T 14506.30-2010(2010).Analytical precision was within 5%.The analyses were conducted at the Analytical Center,No.203 Research Institute of Nuclear Industry.

Fig.5 Zhangjiatan oil shale outcrops of Bawang zhuang Proifle in Tongchuan City,southern Ordos Bain

Table 2 Clay mineral content in oil shale samples(units in%)

Table 3 Oil yield,calori fi c value,ash yield,total sulfur,and major elements in oil shale samples(units of Qb,ad are MJ/kg;others are%)

4 Results

4.1 The industrial quality of shale

In resource assessment of oil shale,four key parameters(oil yield,calori fi c value,ash yield,and total sulfur)are always considered(Liu et al.2009).Oil yields of oil shale samples are in the range of 4.3%–9.1%(average 6.63%)and calorific values are from 3.85 to 9.49 MJ/kg(average 6.73 MJ/kg).The better the quality,the higher the values of the two parameters(Liu et al.2009;Zhang et al.2013;Sun et al.2015).Ash yields of oil shale samples range from 68.15%to 86.08%(average 76.93%)and total sulfur from 2.47%to 7.48%(average 4.98%).The better the quality,the lower the value of these two parameters(Liu et al.2009;Zhang et al.2013;Sun et al.2015).We found that oil yield correlated positively with calorific value(Fig.4a)and total sulfur(Fig.4b)and negatively with ash yield(Fig.4c).Meanwhile,superb negative correlation was observed between calorific value and ash yield(Fig.4d).According to National Resource Assessment of Oil shale(Dong et al.2006),oil shale is classified as low,middle,and high grade by oil yield of 3.5%–5%,5%–10%,and＞10%,respectively.Thus,the oil shale in the southern Ordos Basin is middle grade.

Fig.6 Relationships among SiO2,Al2O3,K2O,and TiO2 concentrations in oil shale and silty mudstone samples

4.2 Minerals in oil shale

The minerals identi fi ed by XRD in oil shale samples were clay minerals,quartz,feldspar,and pyrite(＞5%)with minor amounts of calcite,siderite,ankerite,gypsum,and jarosite(Table 1).Pyrite was mostly in the form of nodules(Fig.5a).For clay minerals,mixed-layer illite/smectite was predominant,followed by illite(Table 2).Kaolinite and chlorite were measured in trace amounts(Table 2).Compared with marine oil shale,these samples had higher contents of quartz,feldspar,and clay minerals,and much lower average content of calcite(data from 13 marine oil shale samples in Quse Formation of the Early Jurassic,Qiangtang Depression show average quartz:12.72%;feldspar:3.38%;clay minerals:14.28%;calcite:68.53%,)(Fu et al.2016).The relatively low calcite and high clay mineral contents are likely associated with terrigenous clastic input(Yu and Zhu,2013;Zeng et al.2013,2014;Xie et al.2014).

4.3 Major element geochemistry

Major elements can be used to construct associations between elements and minerals in oil shale,as demonstrated by prior study(Fu et al.2010a,b).Table 3 lists major element concentrations of the oil shale samples.Average contents of SiO2(47.28%),Al2O3(13.13%),and TFe2O3(6.00%)were relatively high and the rest were relatively low(＜5%).Some major elements show positive correlation with ash yield at95%confidence level,suchas Si(r=0.978),Al(r=0.571),Mg (r=0.574),Ca (r=0.397),Na(r=0.683),K (r=0.719),Mn(r=0.123),and Ti(r=0.355),indicating that these elements have relationships with minerals.P2O5(r=0.654)and TFe2O3(r=0.836)show positive correlation with oil yield,suggesting P and Fe are mainly associated with organic matter.

Fig.7 Relationships between TFe2O3concentration and total sulfur,oil yield,SiO2,and Al2O3concentrations in oil shale and silty mud stone samples

The elements Si,Al,Ti,and K have some connection with quartz,feldspar,and clay minerals(Fu et al.2010a,b).Relatively higher correlations among them(Fig.6)illustrate that they mainly stem from a mixed clay assemblage,in accordance with clay mineral analysis.The Al/Si ratio can reflect the source of SiO2(Fu et al.2010a,b).The Al/Si of oil shale samples varied from 0.26 to 0.37(average 0.32),implying most SiO2is in the form of quartz,with some as clay minerals,which is consistent with the high abundance of quartz in XRD analysis.

The element Fe is generally associated with pyrite in oil shale(Fu et al.2010a,b).The significantly positive correlation between TFe2O3and total sulfur indicates a high proportion of Fe arising from sulfide oxidation and organic sulfur as the dominant component of total sulfur(Fig.7a).This is corroborated by the positive correlation between TFe2O3and oil yield(Fig.7b).TFe2O3showed negative correlations with SiO2and Al2O3(Fig.7c,d),indicating a low frequency of iron in the clay minerals,which contrasts with observations made of marine oil shale(Fu et al.2010a,b).Moreover,according to XRD,there was a small amount of Fe present as siderite,ankerite,and jarosite,demonstrating the occurrence of segmental Fe.

Calcium presents in various forms(Mukhopadhyay et al.1998).In marine oil shale,a high abundance of calcite indicates that Ca is associated with calcite(Fu et al.2016).However,in the Zhangjiatan oil shale samples,the calcite content was relatively low,suggesting different forms of Ca are present.Low correlation(r=0.396)between CaO and Al2O3suggests presence of Ca not only in clays,but also in other minerals.XRD analysis showed Ca in gypsum.Generally,low CaO content indicates sparse fossil remains in oil shale(Mukhopadhyay et al.1998).Fish fossil remains observed in Zhangjiatan samples(Fig.5b)support Ca being of biological origin.

Table 4 Trace element contents(ppm)and corresponding UCC values

4.4 Trace element geochemistry

The trace element contents of samples are presented in Table 4.Ba,Sr,V,Zr,Cu,Rb,and Zn were in relatively high abundance(＞100 ppm on average).Compared with the average value of the upper continental crust(UCC)(McLennan 2001),As,U,Cd,and Mo were highly enriched,while Sr,Nd,and Ta were depleted.Element enrichment was quanti fi ed by the enrichment factor(EF):EF=(X/Al)sample/(X/Al)ucc.All analyzed trace elements from oil shale samples were classified into three groups by EF(Fig.8;Table 5):(1)Cu,As,U,Cd,Bi,and Mo(average EF≥5);(2)Cr,Ba,Sc,V,Zn,Ga,Pb,Rb,Cs,Th,Co,Ni,Hf,Li,In,and B(1≤average EF＜5);and(3)Sr,Zr,Nb,Ta,and Be(average EF＜1).In general,Cu is associated with biological growth and Mo with chalcophile elements(Sun et al.2015).The highly enriched Cu and Mo imply that there was high productivity during oil shale formation.The extremely high U concentration in oil shale is probably due to redox conditions(Cao et al.2012;Bai et al.2015).

Fig.8 Spider diagram of trace elements of oil shale samples

Ba content gradually decreases and can reflect organic matter lakeshore(Sun et al.1997).In addition,Ba is often associated with paleoproductivity and can reflect organic matter content(Sun et al.1997).The average EF of Ba(1.34)in this study indicates that the oil shale was deposited in a depocenter with high organic matter.

The Sr EF of oil shale samples was higher than that of mudstone from Chang 6 to Chang 3 in the southern Ordos Basin(EF＜1.0;Qiu et al.2015).Increased Sr is usually associated with increased salinity and calcite(Zhang et al.2004).This suggests the paleosalinity of southern Ordos Basin water gradually reduced after Chang 7 deposition.In addition,the EF of U in oil shale samples(Table 5)was clearly higher than that in mudstone from Chang 6 to Chang 3(EF just above 1.0),suggesting that reducibility of water gradually declined.Compared with marine oil shale from Bilong Co in China(Fu et al.2016),lacustrine oil shale from Chang 7 returned a significantly lower EF of Sr and higher EF of U.The variation of EF may be due to mineral content,water depth,redox conditions,or paleoclimate(Zhang et al.2004,2011;Ma et al.2016).

5 Discussion

5.1 Element associations

Vertical distributions of selected elements are shown in Fig.9.Compared with silty mudstone,most oil shale samples returned high Cu,U,and V concentrations and low Al and Si concentrations.The variations of Fe,Cu,U,and V roughly paralleled oil yield and total sulfur,indicating that the four elements are closely associated with organic matter.The vertical distributions of Al and Si are similar to that of ash yield,suggesting quartz and clay minerals are the bases of ash yield and Si is present in clay minerals.The element patterns are relatively consistent,indicating that these elements are controlled by a common sedimentary environment(Fu et al.2010a,b,c).

According to the established sedimentary cycle of the Yanchang Formation,the lake reached its largest scale during oil shale deposition(He 2003),leading to different sedimentary environments between oil shale and silty mudstone.The variation of environment may have in fl uenced the presence of Al and Si.

Correlation coefficients and tree diagrams in cluster analysis help quantify correlations and attribute relationships in samples by grouping samples to optimize withingroup similarity(Zhu et al.2000;Zhao et al.2015).Statistical Program for Social Sciences(SPSS)version 20.0 developed by IBM was used to analyze element associations.By cluster analysis,trace elements,major elements,total sulfur,oil yield,and ash yield were classi fi ed into two groups(Fig.10).

Group A includes total sulfur,Fe,Cu,U,V,Sr,Ba,Zn,Ga,As,Mo,Pb,Cd,Cs,Na,P,Ca,Mn,and Tad.The correlation coefficients of Fe-Cu(0.918),U-Cu(0.944),and V-Cu(0.847)were all higher than 0.80,indicating aclose relationship among Fe,Cu,U,and V(Table 6).Elements from this group generally had negative correlation coefficients with ash yield(15 out of 17 elements had negative correlation,with values ranging from-0.890 to 0.277)and generally positive correlation coefficients with oil yield(12 out of 17 elements had positive correlation,with values ranging from-0.424 to 0.566)(Table 6).The positive correlations with oil yield indicate that these elements are mainly associated with organic matter.Previous research has suggested an extremely high positive correlation between total organic carbon and oil shale yield(Liu et al.2009;Zhang et al.2013).

Table 5 Enrichment factors of trace elements

Group B includes Tad,Si,K,Rb,Be,In,Hf,Th,Mg,Ti,Ni,B,Cr,Al,Li,Co,Nb,Ta,Bi,Zr,and Sc.These elements generally had positive correlation coefficients with ash yield(13 out of 17 elements had positive correlation,with values ranging from-0.469 to 0.954)and generally negative correlation with oil yield(14 out of 17 elements had negative correlation,with values ranging from-0.707 to 0.326)(Table 6).In addition,the elements in this group showed a positive relationship with Al(with the exception of Bi),indicating a terrigenous source.

Fig.9 Vertical variation of oil yield,total sulfur,ash yield,certain trace elements,and major elements in oil shale sections(trace elements in ppm,others in%)

5.2 Reconstruction of paleoenvironment

Previous studies have suggested that the ratios and calculated values of certain elements can be used to reconstruct lacustrine paleoenvironment(Fan et al.2012;Bai et al.2015;Sun et al.2015;Fu et al.2016;Jemaı¨et al.2016).

The Sr/Cu ratio is sensitive to paleoclimate(Deng and Qian 1993;Fu et al.2010a,b,c;Liang et al.2015).Sr/Cu ratios from 1.3 to 5.0 indicate a warm and humid paleoclimate;＞5.0,dry and hot.Sr/Curatios of our oil shale samples ranged from1.01to12.16,with an average of3.26(Table 7),implying that the integral paleoclimate was warm and humid.

Fig.10 Cluster analysis of major elements,trace elements,oil yield,ash yield,and total sulfur in oil shale samples(cluster method,Furthest Neighbor;interval,Person correlation;transform values,maximum magnitude of 1)

The Ba/Al ratio can be used to semiquantitatively reconstruct paleoproductivity of lakes(Dean et al.1997;Luo et al.2013).Higher values are associated with higher paleoproductivity;a ratio of 0.010 to 0.012 has been designated as high productivity(Dean et al.1997;Sun et al.2015).Ba/Al in our samples varied from 0.005 to 0.012,with an average of 0.009(Table 7),suggesting relatively high paleoproductivity.

The Sr/Ba ratio can be used to reconstruct paleosalinity(Wang et al.1979,2014;Wang and Wu 1983;Li and Chen 2003;Guo et al.2015;Li et al.2015).Sr/Ba＞1,0.6–1,and＜0.6 re fl ect sea water,brackish water,and fresh water,respectively.Sr/Ba ratios of our samples were generally lower than 1.0(from 0.27 to 0.69,with an average of 0.38)(Table 7),suggesting that the oil shale mainly formed in fresh water.This could support the opinion that there was no large-scale marine transgression in the southern Ordos Basin during the Triassic(Yin and Lin 1979;Zhang 1984;Zhang et al.2011;Qiu et al.2015).

V,Ni,U,and Th are sensitive to redox conditions and V/(V+Ni),U/Th,AU(U-Th/3),and δU(2U/(Th/3+U))can reflect paleo-redox conditions(Ernst 1970;Deng and Qian 1993;Jones and Manning 1994;Dypvik and Harris 2001;Teng et al.2005;Tribovillard et al.2006;Zhao et al.2016a,b).V/(V+Ni) ＜0.5 and δU＜1 both indicate oxidation conditions;V/(V+Ni)＞0.5 and δU＞1,reducing conditions(Deng and Qian 1993;Zhao et al.2016a,2016b).Strong oxidation conditions are reflected by U/Th＜0.75 and AU＜5 ppm;strong reducing conditions by U/Th＞1.25 and AU＞12 ppm;intermediate values indicate weak oxidation–weak reducing conditions(Qiu et al.2015;Sun et al.2015).Average values of V(V+Ni),δU,U/Th,and AU in oil shale samples were 0.88,1.71,3.13,and 35.32 ppm,respectively(Table 7),indicating that the Zhangjiatan oil shale was mainly deposited in reducing conditions.

The Fe/Ti ratio can be used to measure hydrothermal influence(Bostro¨m 1983;Zhong et al.2015;Chuet al.2016).Fe/Ti＞20 reflects definite hydrothermal influence,with higher values indicating greater influence(Chu et al.2016).Sixsamples(ZK702H14,ZK702H18,ZK702H24,ZK702H32ZK1501H3,and ZK1501H6)from the eastern study area had Fe/Ti＞20(Table 7),meaning there was hydrothermal influence during oil shale sedimentation. However, three samples(ZK2709H3,ZK2709H6,and ZK2709H8)from the western study area had Fe/Ti＜20(Table 7),indicating less hydrothermal influence.

6 Conclusions

The oil yields of Zhangjiatan oil shale samples ranged from 4.3%to 9.1%(average 6.63%),classifying the oil shale as middle grade.

Minerals in the oil shale were mainly clay minerals,quartz,feldspar,and pyrite(＞5%),with illite/smectite dominating the clay minerals.In contrast with marine oilshale,quartz,feldspar,and clay minerals in lacustrine oil shale were clearly higher while calcite content was much lower.Si mainly occurred in quartz and in clay minerals.Fe was mainly associated with organic matter and barely present in the clay minerals,which is opposite to marine oil shale.Ca occurred in various forms.

Table 6 Correlation coefficients of oil yield,calorific value,ash yield,total sulfur,and major elements in oil shale samples

Table 7 Parameters of paleoenvironment

According to cluster analysis,Fe,Cu,U,V,Sr,Ba,Zn,Ga,As,Mo,Pb,Cd,Cs,Na,P,Ca,and Mn were generally associated with organic matter,while Si,K,Rb,Be,In,Hf,Th,Mg,Ti,Ni,B,Cr,Al,Li,Co,Nb,Ta,Bi,Zr,and Sc were not;all showed a positive relationship with Al(with the exception of Bi),indicating these elements are closely associated with a terrigenous source.

The Sr/Cu ratio of oil shale samples ranged from 1.01 to 12.16,with an average of 3.26,indicating a warm and humid paleoclimate.Ba/Al was between 0.005 and 0.012,suggesting that paleoproductivity was high.Sr/Ba varied from 0.27 to 0.69,suggesting that the oil shale was mainly deposited in fresh water and there was no large-scale marine transgression in the southern Ordos Basin.The average values of V/(V+Ni),U/Th,AU,and δU of oil shale samples were 0.88,3.13,35.32 ppm,and 1.71,respectively,indicating that the shale was mainly deposited under reducing conditions.The Fe/Ti ratio of the six oil shale samples from the southern study area was＞20,indicating clear hydrothermal influence;the ratio of samples from the western study area was＜20,suggesting lesser hydrothermal influence.

AcknowledgementsThis work was supported by funding from the National Natural Science Foundation of China(No.41173055)and the Fundamental Research Funds for the Central Universities(No.310827172101).

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