Characteristic and Geological Implications of Major Elements and Rare Earth Elements of Triassic Chang 7 Oil Shale in Tongchuan City , Southern Ordos Basin ( China )

The study of the oil shale from Triassic Chang 7 oil layer in Ordos Basin is of great importance to oil and gas resources investigation. Based on systematic analyses of major elements and rare earth elements of twelve oil shale samples and grain size analyses of four sandstone samples from the Yishicun Profile in southern Ordos Basin, the elements characteristics and corresponding geological implications are discussed. The Al/Si and Si/(Si + Al + Fe) of oil shale samples are in small range, with the averages of 0.29 and 0.67, indicating that quartz is the main mineral and the oil shale deposits near terrigenous provenance. Rare earth elements of the oil shale illustrate the enrichment of light rare earth elements (LREEs) and deficit of heavy rare earth elements (HREEs). The Chondriteand North American shale composite (NASC)-normalized distributing patterns manifested that the oil shale have been derived from the same terrigenous source and controlled by a similar sedimentary environment. The vertical variation of major elements and REEs show that the heterogeneity of oil shale samples is relatively low. By grain size analysis, the interlayer sandstone is the typical turbidite. The Chemical index of alteration (CIA) and (Fe + Mn)/Ti of the oil shale samples ranges in 65.02–78.09 and 6.60–25.82, respectively, indicating that during oil shale sedimentation, the paloeclimate is warm and humid with moderate chemical weathering and that there are obviously hydrothermal fliud activities. The correlation between δCeN and δEuN, total rare earth elements (ΣREE) and (Dy/Sm)N, implying that the diagenesis of oil shale is relatively low with middle diagenetic stage A period. The Ceanom of oil shale samples ranges from −0.094 to −0.049, suggesting that the redox condition of oil shale sedimentation is dominated by strong reducing condition, and the (La/Yb)n is from 1.3 to 2.1, manifesting the whole sedimentary rate of oil shale is relatively low. By the comparison with REEs distribution characteristics from surrounding potential provenance, the main provenances of Chang 7 sedimentation in southern Ordos Basin are from Yinshan Mountain and Qinling-Dabie Mountain.


Introduction
Recent years, with the decreasing conventional petroleum, oil shale is gradually becoming one of the most significant unconventional resources and has received unprecedented attention [1][2][3][4].Oil shale from Ordos Basin is regarded as the most typical lacustrine oil shale, from which oil shale of Chang 7 oil layer has the features of wide distribution, abundance, shallow burial, and so on [4][5][6][7][8][9][10][11].According to latest energy investigation, the shale oil resources of the oil shale in southern Ordos Basin have already exceeded 3500 × 10 8 barrels with great exploration prospective, taking up 34% of the whole shale oil resources in China [10].
The element geochemistry studies of oil shale have been a hot topic in recent years.The elements record the geological information in the shale, and can be well preserved, so it can be used to restore the palaeosedimentary environment.By the trace elements of marine oil shales from Bilong Co in northern Tibet, the elements occurrence and environment during oil shale sedimentation were discussed [12].Wang et al. studied the trace and rare earth elements of the black shale from Wuyu Basin, and though the paleosalinity was a brackish water environment and the paleoclimate was semiarid to semimoist [13].Qiu et al. testifies the existence of hydrothermal fluid activities of the oil shale in the southern Ordos Basin by platinum group elements [14].In addition, the rare earth elements of oil shale can also indicate the provenance.By the comparison of rare earth elements distribution, the oil shale from the Central Eastern Desert was proved from Neoproterozoic island arc parent rocks [15].Qiu et al. studied the rare earth elements of fine grand sediments of every sub-unit of Yanchang Formation and found that the provenance of the whole Ordos Basin changed from the Chang 7 period [16].
Oil yield and total sulfur of the oil shale from Chang 7 Formation in southern Ordos Basin showed industrial quality of middle-grade (5% < oil yield ≤ 10%) with extra-low sulfur criterion (total sulfur ≤ 1.0%) [4, [17][18][19].The organic geochemistry studies indicated that most of the oil shale in the southern Ordos Basin has entered maturity [4,20].During oil shale sedimentation, the water is dominated by fresh water, along with an average water depth of 67.01 m and the lake was proved to be an extremely eutrophic lake [5].When considering that the studies of organic geochemistry, such as oil yield, total organic carbon (TOC), and vitrinite reflectance, are of great importance to resources calculation, the researches of inorganic geochemistry, especially major elements and rare earth elements, are still desperately inadequate, leading to many remaining unresolved issues.
In order to have a better understanding of oil shale from the Chang 7 oil layer, systematic major elements and rare earth elements characteristics, grain size analyses, and their geological implications of oil shale outcrops of the Yishicun Profile in Tongchuan City are discussed to reconstruct sedimentary background and tectonic setting for the further development of unconventional resources in China.

Geological Setting
Located in central China, the Ordos Basin is a Mesozoic superimposed basin that developed in the Palaeozoic North China Craton with a Proterozoic crystalline basement.As shown in Figure 1a, the basin is bounded by Hetao Basin in the north, Fenwei Graben system in the southeast, and Yinchuan Basin in the west with 25 × 10 4 km 2 [21][22][23].In the late Mesozoic Era, the basin started to evolved to a continental basin from North China Block with a relatively stable tectonic setting.Influenced by Indosinian orogeny, the southern Ordos basin proved to be an intracontinental foreland basin during the period of Yanchang Formation, which is dominated by fluvial-lacustrine depositional system [20,[24][25][26].By Early Cretaceous, the basin, especially its southern part, gradually uplifted and finally underwent reformation.According to sedimentary cycles and lithological assemblages, the Yanchang Formation is divided into ten oil layers, named as Chang 10 to Chang 1 from the bottom to the top, showing an intergrated record of progradation-aggradation-retrogradation cycle (Figure 1b) [4,5,20].During the Chang 7 period, the basin developed the most extensive lake transgression, forming stable but unequal oil shale section.
The oil shale profile is located in Yishicun, Tongchuan City, with distinct outcrops at the bottom of the Chang 7 oil layer (Figure 1c).The whole thick of oil shale section is approximately 17 m and its occurrence is almost horizontal.The lithologies of the oil shale section are mainly oil shale and mudstone with multi-storey thin tuffs and sandstones.

Samples and Methods
Twelve oil shale samples and four sandstone samples were gathered from the oil shale section in Yishicun profile, Tongchuan city.Details of the rock assemblages and numbers of samples are shown in Figure 2. The four sandstone samples were all inside the oil shale seam at an equal spacing from the bottom to the top (Figure 2).Samples were collected in kraft bags prevent from contamination and oxidation.
For major elements analysis, the samples were all crushed and grounded to less than 200 mesh size and the ashed samples were sent to an open muffle furnace with the raising temperature to 950 • C.Then, the samples were dissolved in HClO4 and HF, and the tests were conducted with X-ray fluorescence, AA-6800 atomic absorption spectroscopy, and UV-2600 ultraviolet-visible spectrophotometer.The analytical methods followed Chinese National Standard GB/T14506.1~14-2010[27].The analytical uncertainty was usually less than 5%.
For rare earth elements analysis, the samples were powdered to less than 200 mesh size and were digested in a microwave furnace using an HF + HNO 3 mixture in Teflon bombs at 190 • C for 48 h and were then determined with a Perkin Elmer SciexElan 6000 ICP-MS with the analysis method of Chinese National Standard GB/T14506.30-2010[28].The analytical error was controlled within 5%.
Sandstone samples were reduced to the known quantity by the coning and quartering method.Grain size analysis was handled with the dry sieving technique [29].With finer than 4Φ, slice samples were analyzed using the pipette method that was described from Griffiths [30] and Carvert [31].Various grain size parameters were computed by GRADISTAT software [32].These analyses were conducted at National Key Laboratory of the Northwestern University, Xi'an, China.

Major Elements Characteristic
Table 1 listed the results for major elements of oil shale samples.The SiO 2 , Al 2 O 3 , and TFe 2 O 3 (total iron) abundances show the most abundant three elements (average value > 5 wt %) and the rest seven major elements are detected with the concentration being less than 5 wt % (Table 1).The whole major elements concentrations of oil shale samples vary slightly and show no obvious vertical variation rules, indicating that the heterogeneity of oil shale samples is relatively low (Figure 2).Notably, slight variations of Al 2 O 3 and TiO 2 concentration can also explain that terrigenous detrital input is relatively stable (Figure 2).Element Si is mainly occurs in quartz and clay minerals in fine-grained sediments [33,34].To some extent, the Al/Si ratio can reflect minerals in oil shale samples and the ratio gradually decreases with the increasing quartz content [4,33].The Al/Si ratios of oil shale samples range from 0.21 to 0.36, with an average of 0.29 (Table 3, Figure 3), indicating that quartz dominates the minerals (Figure 4a), which is in accordance with the previous research using XRD analysis [5,23].The lithology of the oil shale is classified as shale type (Figure 4b).Si/(Si + Al + Fe) ratio can offer judgment on the terrigenous provenance distance, and decreases with the increasing distance [35].The Si/(Si + Al + Fe) of oil shale samples varies from 0.63 to 0.72, with an average of 0.67 (Table 3), indicating that the oil shale samples were formed near terrigenous provenance.

Rare Earth Elements Characteristic
The rare earth elements (REEs) of twelve oil shale samples are presented in Table 2.The total rare earth elements (ΣREE) of the oil shale samples are relative low (104..96ppm, averaging 151.16 ppm), as compared to ΣREE of shale from Chang 9 oil layer in the southern Ordos Basin (average 212.35 ppm) [38].The ratio of total light REEs (ΣLREE) to total heavy REEs (ΣHREE) varies in a small range (8.72-13.41,averaging 11.52) with relatively high value (Table 3), indicating the enrichment of LREEs and a deficit of HREEs.After being normalized by Chondrite, the distribution patterns show sloping LREEs trend and flat HREEs trends with unconspicuous Ce depletion (Figure 5a).Also, after being normalized by North American shale composite (NASC), all of the samples reveal shale like pattern [33].The extremely unified distributing patterns of oil shale samples indicate that the REEs in the oil shale samples have been derived from the same terrigenous source and are controlled by similar sedimentary environment with almost no difference.Vertically, ΣLREE shows consistent variation with ΣREE, while the correlation between ΣHREE and ΣREE is not obvious (Figure 2).In both distribution patterns of REEs, the HREEs all display a slightly rich trend (Figure 5), which is probably influenced by the absorption of organic matter [39].

Grain Size Parameters
The results of grain size analysis are shown in Table 4.The parameters of samples vary a little with mean grain size (Mz) from 3.71Φ to 4.09Φ, which means that the sandstone samples are all siltstone.The C (2 (Phi1) × 1000) and V (2 (Phi50) × 1000) range in 115.82-137.74µm and 59.54-76.95µm.The distribution of particle size displayed one-part form with little rolling and suspension (Figure 6a), and the four samples all plot in Turbidite part (Figure 6b), suggesting that the main transport is saltation and siltstone is the turbidity event deposition results.

Paleoclimate
Palaeoclimate is the climate of a certain stage in geological history and is of great significance for the resources exploration [43][44][45][46].Chemical index of alteration (CIA) is usually adopted to reflect paleoclimate during the sedimentary period [47][48][49].CIA values of 50-65, 65-85, and 85-100 indicate cold and dry with low chemical weathering, warm and humid with moderate chemical weathering, and hot and humid climate with strong chemical weathering, respectively [49], and the CIA can be calculated as follow: where all of the oxide concentration is calculated in molecular proportions.However, when considering the change of original composition on fine grained detrital rock from recirculation deposit, index of compostitional variability (ICV) should be firstly applied to judge whether experiencing recirculation deposition.The formular is: where CaO* represents CaO in silicate and is calculated by the expression: where CaO* is the minimum between the calculated result and the Na 2 O concentration.
Normally, ICV that is higher than 1 indicates more clay minerals in samples and the samples undergo less recirculation deposit.When the ICV is lower than 1, it means that the samples have less clay minerals and are affected by more sediment recycling or strong weathering [49,50].Although the average ICV of oil shale samples is below 1, some of the samples are still over 1 and most of the other samples are very close to 1 (Table 3).Thus, it can be seen that the oil shale samples were affected by relatively weak recirculation deposit and CIA can be used to judge paleoclimate.
The CIA of oil shale samples ranges from 65.02 to 78.09 with an average of 71.82 (Table 3, Figure 3), showing that the paleoclimate is dominated by warm and humid with moderate chemical weathering during oil shale sedimentation.The ternary diagram of chemical alteration show the same observation with the CIA value (Figure 7).

Hydrothermal Fluid Sedimentation
Major elements can react to hydrothermal fluid activities accurately and have been widely applied [6,35,[51][52][53].Usually, if the sediments were affected by hydrothermal fluid activities, (Fe + Mn)/Ti of sediments would be more than 15 [5,54].As it can be seen from Table 3, the (Fe + Mn)/Ti of the oil shale samples varies in the range of 6.60-25.82(average 17.15) (Figure 3), clearly manifesting that the oil shale experienced hydrothermal fluid activities.Apart from the evidence of major elements, a slightly enriched trend of HREEs has been considered to be a strong association with hydrothermal fluid activities [55].In both normalized distribution patterns, HREEs all display an enriched trend, which can also indicate hydrothermal fluid activities (Figure 5).Besides, when compared with chondrite, an obvious high positive Eu anomaly is considered to be the presence of hydrothermal fluid [6,[51][52][53].As can been seen in chondrite-normalized distribution pattern of REEs, the high positive anomaly of Eu further support the hydrothermal events (Figure 5a).
Previous studies indicated that hydrothermal fluid activities mainly have two influences on organic matter [6,51].On one hand, hydrothermal fluid can bring nutritional ingredients, such as P, Mo, and Cu, which can directly improve organic matter concentration.On the other hand, some saline material that is produced from hydrothermal fluid can facilitate organic matter preservation [6,49].Therefore, hydrothermal fluid activities are of great importance to the formation and preservation of the high quality oil shale.

Diagenesis Stage
Rare earth elements can respond well to diagenesis and δCe N is a sensitive parameter.When diagenesis is strong, δCe N shows intense positive correlation with δEu N and ΣREE, and negative correlation with (Dy/Sm) N .On the contrary, when the diagenesis is weak, the correlation is inconspicuous [56].As it can be seen in Figure 8, the extreme weak correlations between δCe N and δEu N , ΣREE, and (Dy/Sm) N illustrate the relatively weak diagenesis.Previous organic geochemistry studies on the oil shale from Yishicun Profile indicate that the type of organic matter is dominated by II 2 and the orgin of organic matter are mixed with algae, phytoplankton, and terrestrial plants [4].The R o and T max of oil shale ranges in 0.98-1.15%and 422-469 • C, with averages of 1.06% and 437.35 • C, respectively.According to the division of diagenetic stages in clastic rocks [57], the oil shale is mainly in middle diagenetic stage A period.

Paleo Aqueous Medium
The Ce anomalies in sediments have been considered as a suitable indicator for understanding contemporaneous paleo-redox conditions [58][59][60][61][62][63].Because of the changeable valence state of Ce, Ce could exist in the stable valence of Ce 3+ in aqueous solution.However, Ce could be oxidized in the form of Ce 4+ and easy to subside [64].So, the redox condition can affect Ce anomaly, which can track ancient redox conditions [13,65].Generally, Ce anom is used to reflect redox condition.When Ce anom is greater than −0.1 and less than −0.1, it means reducing and oxidation environment, respectively [7].The Ce anom = lg(3Ce n /(2 × La n + Nd n )).Subscript n represents the NASC-normalized value.Ce anom of oil shale samples ranges from −0.094 to −0.049 (average −0.069) (Table 3), implying that the redox condition of oil shale sedimentation is dominated by strong reducing condition.

Sedimentary Rate
With the changing sedimentary environment, REEs are differentiated in electrovalence and adsorbed ability and can be perceived by detention time during sedimentation [7,66,67].Specifically, when the detention time is short, REEs would deposit sharply and a weak exchange interaction takes place with water, showing that NASC-normalized REEs distribution pattern is flat and (La/Yb)n approaches 1. Vice versa, the (La/Yb)n deviates from 1 [7,66].
The (La/Yb)n of oil shale samples ranges from 1.3 to 2.1, with an average of 1.8 (Table 3), indicating that the whole sedimentary rate of oil shale is low.When compared with the central part of the basin, (La/Yb)n is relatively high, illustrating that although the study area is located in deep lacustrine facies, some event deposits would lead to transitory variation of the sedimentary rate.In oil shale seam, syndepositional deformation structure can also explain the relatively high sedimentary rate to some extent [4].

The Provenance of Oil Shale and Orogeny Coupling Response
By the diagrams of ΣREE-La/Yb, Al 2 O 3 /SiO 2 -(MgO + TFe 2 O 3 ), and SiO 2 -log(K 2 O + Na 2 O), the oil shale from southern Ordos Basin has been proved to originate from felsic rocks and deposits in the continental island arc tectonic setting.Hence, in this contribution, there is no further discussion on the parent rock and tectonic setting of the oil shale [7].However, due to complex orogeny during the Chang 7 period, the source direction of sediments is still discussed controversially.Massive detrital minerals indicate that there are two similar sedimentary periods in the Yanchang Formation [68].One is Chang 10-Chang 8 and the other one is Chang 7-Chang 1.The source of conversion period between Chang 8 and Chang 7 is changed, referring the possible provenance of southern Ordos Basin in Chang 7 period.However, average element geochemistry data of individual samples in the southern Ordos Basin reveals that Yinshan Mountain, which locates the north of the Ordos Basin, provided provenance to the whole basin in Chang 9-Chang 7 period without northern provenance [19].While in Chang 6-Chang 3 period, Qinlin-Dabie, which is located to the south of the Ordos Basin, started to offer provenance to southern Ordos Basin.Therefore, it is certain that the northern Yinshan Mountain provided provenance to the whole basin.Whether there was a source in southern Ordos Basin during Chang 7 period is not clear and this is of great importance to the sedimentary environment and the tectonic response.However, due to the sensitive and stable characteristics of REEs, REEs can be indicators for sedimentary and orogeny response and can explain this issue well.
As discussed in Section 4.2, the Chondrite-normalized distribution pattern of REEs of oil shale samples shows a sloping LREEs trend and flat HREEs trends.The pattern plots present two different kinds of provenances (Figure 5a), indicating that the source of Chang 7 is mixed.One source comes from magmatic rocks in Yinshan Mountain and the other one stems from the basement of the northern Ordos Basin.
The southern Ordos Basin is adjacent to the Qinling-Dabie Orogenic Belt and East Qilian.During the Chang 7 period, the Yangtze plate collided the North China Block with Qinling orogeny.Thus, the collision orogeny and uplift denudation affected tectonic, depositional background, and the source of sediments [21].Also, the zircon SHRIMP U-Pb age of laminated tuff in southern basin founded that the tuff was from the Qinling-Dabie Orogenic Belt in 230 Ma ± [69], which demonstrates that the Qinling-Dabie Mountain has the possibility of providing source for the southern basin both in time and space.In addition, Li et al. [4] found that the organic matter type of the oil shale in the studied profile is dominated by Type II 2 and the source of organic matter is mixed with algae, phytoplankton, and terrestrial plants.While, previous recognitions demonstrated that the organic matter type of oil shale in the basin center is mainly Type I with little terrestrial plants and oil shale in whole basin all deposites in semi-deep to deep lake [4, 70,71].So, the only reasonable explanation for the above abnormal phenomena in oil shale must be attributed to Qinling orogeny, which brought a considerable amount of terrestrial plants to the southern paleolake, accompanied with turbidite events.All of these researches can also be one of the responsive evidence on the other side, which is in accordance with the results of Si/(Si + Al + Fe), as well.
Therefore, the REEs of the oil shale samples exactly reflect that the provenance of Chang 7 oil layer in southern Ordos Basin comes from both Yinshan Mountain and Qinling-Dabie Mountain.

Conclusions
This paper presents a systematic research on major elements, rare earth elements characteristics, grain size analysis, and the geological implications of an oil shale section from the Chang 7 oil layer in southern Ordos Basin.Based on the above experimental results, the following conclusions can be obtained: (1) Al/Si and Si/(Si + Al + Fe) of oil shale samples from Chang 7 oil layer in southern Ordos Basin indicate that quartz is dominated in minerals and the oil shale deposits near terrigenous provenance.ΣLREE/ΣHREE of the shale samples varies in the range of 8.72-13.41,suggesting the enrichment of LREEs and the deficit of HREEs.The Chondrite-and NASC-normalized patterns show that the oil shale has been derived from the same terrigenous source and is controlled by a similar sedimentary environment.The sandstone lithology is mainly siltstone caused by turbidity sediment possibly.
(2) CIA of oil shale samples ranges from 65.02 78.09, indicating that the paloeclimate during oil shale sedimentation is mainly warm and humid with moderate chemical weathering.(Fe + Mn)/Ti of oil shal samples is in the range of 6.60-25.82,with an average of 17.15, implying that there are obviously hydrothermal fliud activities during the formation of oil shale.
(3) The diagenetic stage stays in middle diagenetic stage a period.The redox condition of paleo aqueous medium is a strong reducing condition.The whole sedimentary rate of oil shale is relatively low.The sedimentary rate may have been associated with some event deposits.
(4) In the Chang 7 period, there should be two main provenances existing in the southern Ordos Basin.One is Yinshan Mountain, to the north of Ordos Basin; the other one is Qinling-Dabie Mountain, to the southern Ordos Basin.

Figure 2 .
Figure 2. Vertical variation of major elements and rare earth elements of oil shale samples (unit of major elements and rare earth elements in wt % and ppm, respectively).

Figure 3 .
Figure 3. Vertical variation of geochemical parameters of oil shale samples.

Figure 4 .
Figure 4. (a) Ternary diagram showing relative proportions of major elements (the base map is from He et al. [36]); (b) lithology classification of oil shale samples (the base map is from Herron.[37]).

Figure 5 .
Figure 5. (a) Chondrite-normalized distribution pattern of rare earth elements (REEs) (the base map is from Qiu et al. [14]) and (b) North American shale composite (NASC)-normalized distribution pattern of REEs.

Figure 7 .
Figure 7. Ternary diagram of chemical alteration (the base map is from Li et al. [6]).

Figure 8 .
Figure 8. Relationship between δCe N and δEu N , total rare earth elements (ΣREE), and (Dy/Sm) N of oil shale samples.

Table 1 .
Major elements concentration in oil shale samples (unit in wt %).

Table 2 .
Rare earth elements concentration in oil shale samples (unit in ppm).

Table 3 .
Geochemical parameters of oil shale samples.

Table 4 .
Grain size analysis of four sandstone samples (units of C and M in µm, the rest units in Φ).