Organic Geochemistry and Elements Distribution in Dahuangshan Oil Shale, Southern Junggar Basin: Origin of Organic Matter and Depositional Environment

International Journal of Coal Geology 115 (2013) 41–51

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International Journal of Coal Geology

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Organic geochemistry and elements distribution in Dahuangshan oil shale, southern Junggar Basin: Origin of organic matter and depositional environment

Shu Tao a,b,⁎, Dazhen Tang a, Hao Xu a, Jianlong Liang a, Xuefeng Shi c a Coal Reservoir Laboratory of National CBM Engineering Center, School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China b Coalbed Methane Resources and Reservoir Formation Process Key Laboratory of Ministry of Education, China University of Mining and Technology, Xuzhou 221116, China c CNOOC Energy Technology & Services-Oilﬁeld Engieering Research Institute, Tianjin 300452, China article info abstract

Article history: The Dahuangshan oil shale, located in the northern Bogda Mountain, on the southern margin of the Junggar Received 3 March 2013 Basin, was deposited in a Late Permian lacustrine environment. A combined investigation of element and Received in revised form 14 May 2013 organic geochemistry was performed to define the source rock potential, the paleoenvironment, and source Accepted 16 May 2013 of the organic matter. Thick sequences of oil shales with an average thickness of 638 m were deposited in Available online 25 May 2013 Lucaogou Formation which mainly consists of oil shale, argillaceous dolomite, silty claystone, tuff, limestone, and dolomitic marl. A spot of plant stem fossils and abundance of pyrite crystals, fishtail and fish skeleton can Keywords: Geochemistry also be found there. Rare earth elements Analyzed oil shale samples from Dahuangshan area are characterized by high total organic carbon (TOC) con- Paleoenvironment tents (5.6–34.75%), S2 (22.65–199.25 mg HC/g rock), hydrogen index (HI, 359–1068 mg HC/g TOC), and oil

Oil shale yield (4.9–26.6%), indicating the oil shales have excellent source rock potential. Tmax values (433–453 °C) Dahuangshan show an early to medium maturation stage of organic matter, which is supported by organic geochemical maturation parameters. All of the obtained kerogen types are types II and I, with oil prone source rock potential.

Dahuangshan oil shale samples are rich in SiO2 (68.59%), followed by Al2O3 (10.18%) and Fe2O3 (5.43%). Com- pared with average shale and North American Shale Composite (NASC), analyzed oil shale samples are obviously

enriched in P (0.71%). There is a signiﬁcant correlation between Al2O3 and Fe2O3,MgO,K2O, MnO, Cu, Ba, Co, and Ni for their association with clay minerals. Besides, the signiﬁcant correlations between Fe2O3 and MnO, Co, and Ni are considered to result from their similarity on geochemical behavior. All selected oil shales are characterized

by distinctly sloping light rare earth elements (LREE) trends (LaN/SmN = 2.70–5.95) accompanied by ﬂat heavy rare earth elements (HREE) trends, with distinct Eu negative anomalies (0.60–0.73). Two slightly different patterns of REEs in the oil shale samples are distinguished by the difference in Ce depletion and Nd anomaly. In addition, Dahuangshan oil shale samples are characterized by short- to middle-chain n-alkanes, low carbon

preference index (CPI) values (0.93–1.24), single peak composed of nC20 or nC22,lowPr/Ph(0.41–0.91), relatively

high Homohop index (0.061–0.99), and high concentrations of C27 sterane, indicating reducing, deep-water, and moderate saline environment with prevalent contribution of algae and microorganisms to organic matter accumulation. © 2013 Elsevier B.V. All rights reserved.

1. Introduction fossil resources that can be produced and converted to liquid fuels, has received increasing attention. The Third Chinese Oil and Gas Resource As- With rapid increases in consumption of energy and chemicals, oil sessment and the 2007 World Energy Survey showed that a total oil shale supply and demand imbalances are intensifying so as to become resource of some 720 Gt is located across 22 provinces, 47 basins, and 80 a restraining factor on economic growth in China. China had imported deposits. The shale oil resource has been estimated at some 48 Gt, which about 179 million (Fu et al., 2010) tons of crude oil in 2008 and over is highly signiﬁcant for alleviating the pressure of petroleum supplies (Liu 250 million tons in 2011. Oil shale, one of the substantial unconventional et al., 2007; WEC, 2007). At present, retorting and combustion for power generation are the main patterns of oil shale application. In 2011, shale oil production by retorting technology was about 1.46 million tons all ⁎ Corresponding author at: Coal Reservoir Laboratory of National CBM Engineering over the world, of which about 650,000 t were produced in China, includ- Center; School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China. Tel.: +86 10 82322011. ing 7 oil shale retorting plants located in 5 provinces (Li, 2012). Estonia, E-mail address: [email protected] (S. Tao). the biggest electricity producer from oil shale in the world (Hamburg,

2011), whose total generating capacities reached up to 3200 MW with 2. Geological setting about 15 million tons of oil shale was used for power generation in 2011. Thick sequences of organic-rich lacustrine oil shales have been Bogda Mountain is situated in the eastern part of Tianshan Mountain reported to underlie much of the Junggar Basin in Xinjiang Uygur range and is located on the southern margin of the Junggar Basin which Autonomous Region, northwest China. Several authors have ranked is a large, organic-rich foreland basin in northwest China (Jiao et al., these organic-rich lacustrine mudstones (oil shales) among the thickest 2007; Tao et al., 2012a). Thick sequences of organic-rich lacustrine oil and richest petroleum source rock intervals in the world (e.g. Carroll shales are exposed in the foothills of Bogda Mountain (Carroll, 1998), et al., 1992; Graham et al., 1990; Watson et al., 1987). Previous researches including 13 different oil shale mining areas, and eight of them have have focused on the oil yield, deposition, development, resources, and been studied by us in recent years (Fig. 1A) (Tao et al., 2010, 2011, metallogenic characteristics of oil shale in this area (e.g. Tao et al., 2010, 2012a,b,c). Carroll et al. (1992) documented three Upper formations 2011, 2012a,b,c). Until now, however, no available publications have that contain organic-rich mudstones. From oldest to youngest, they are addressed the geochemical characteristics of oil shale in this area. the Jingjingzigou, Lucaogou, and Hongyanchi formations, among which In the current study, the petroleum potential and the thermal ma- extremely rich and oil-prone oil shales are discovered in Lucaogou turity of the organic matter contained in Dahuangshan oil shales from Formation. the southern Junggar Basin were studied by Rock-Eval pyrolysis and The Dahuangshan region is located in the eastern part of Bogda some biomarker parameters; the occurrence and distribution of the Mountain oil shale belt (Fig. 1A). The Permian Jingjingzigou, Lucaogou, major and trace elements in the oil shales were studied in order to de- and Wutonggou formations are the major outcropping seams in this termine the geochemical background of this basin; the sedimentary and area (Fig. 1B). The Lucaogou Formation consists of a sequence of dolo- organic geochemical characteristics of selected oil shale samples were mitic mudstone, dolomitic marl, argillaceous dolomite, limestone, silty examined to discuss the source of organic matter and paleoenvironment claystone, tuff, and oil shale (Fig. 2). In Dahuangshan area, the thickness of the Dahuangshan oil shales. of the Lucaogou Formation (average 845 m) is larger than that of other

Fig. 1. (A) Simplified map showing geological setting of northern Bogda Mountain, and the location of the study area. (B) Simplified geological map of the Dahuangshan oil shales, showing three oil shale profiles, and two boreholes. S. Tao et al. / International Journal of Coal Geology 115 (2013) 41–51 43

3. Samples and analytical procedures

Weathering is known to affect amount and quality of organic matter in petroleum source rocks (Clayton and King, 1987; Leythaeuser, 1973). Littke et al. (1991) noted that pyrite, sulfur, and organic carbon content were altered by weathering. Therefore, the profile samples were collected after digging about 40 cm into the rock to minimize the effects of surface weathering. All samples were carefully packed and then im- mediately sent to the laboratory for experiments. A total of 42 outcrop oil shale samples and some interbedded rocks were collected from section No.2 (Fig. 2). All of the oil shale samples were selected for total organic carbon (TOC), Rock-Eval pyrolysis, ash yield, total sulfur, organic sulfur, and Gray-King low-temperature dry distillation analyses. Then ten of them were analyzed by X-ray fluores- cence (XRF), inductively-coupled plasma mass spectrometer (ICP-MS), gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). The samples for geochemical analysis were all crushed and ground to less than 200 mesh. TOC and organic sulfur values were determined in the Geological Laboratory of Exploration and Development Research Institute of PetroChina Huabei Oilfield Company, following the Chinese National Standard methods GB/T 19145-2003 and GB/T 215-2003, respectively. Rock-Eval pyrolysis data were performed on a Rock-Eval II instrument following the guidelines established by Espitalié et al. (1985). The samples were analyzed in the Petroleum Geology Research Center, China Petroleum Exploration and Develop- ment Research Institute. Ash yield, total sulfur, and Gray-King low- temperature dry distillation were conducted at the Xinjiang Institute of Coal Science and Coal Testing Laboratory, following the Chinese National Standard methods GB/T212-2001, GB/T214-2007, and GB/T 1341-2001, respectively. The XRF was used to determine the oxides

of major elements, including SiO2,Al2O3, CaO, K2O, Na2O, Fe2O3, MnO, MgO, TiO2,andP2O5. Trace elements were determined by an Ele- ment 6000 inductively-coupled plasma mass spectrometer (ICP-MS). Both of the tests were determined in the Beijing Research Institute of Uranium Geology, following the method described by Ryu et al. (2011). The extraction and extracts separation were performed by the method of Bolou-Bi et al. (2010). GC of the saturated hydrocarbon frac- tion was examined on an Agilent 7890 with a quartz capillary column (30 m × 0.25 mm × 0.25 μm film thickness). GC–MS of saturated hydrocarbon fractions was acquired on an Agilent7890-5975c instrument (fitted with a HP-5MS quartz capillary column of 60-m length, 0.25-mm inner diameter, and 0.25-μm film thickness). Helium was used as the carrier gas. The oven was held for 1 min at 50 °C, programmed from 50 °C to 120 °C at 20 °C/min, and then 120 °C to 250 °C at 4 °C/min, finally 250 °C to 310 °C at 3 °C/min, with a final holding time of 30 min at 310 °C. The selected ion monitoring capabilities of the data acquisition system permitted specific ions to be monitored, such as n-alkanes (m/z 85), tricyclic terpanes and hopanes (m/z 191), and steranes (m/z 217) (Amijaya et al., 2006; Korkmaz and Gülbay, 2007). Fig. 2. Stratigraphic column for the Lucaogou Formation in Dahuangshan region. 4. Results and discussion

4.1. Rock-Eval pyrolysis and TOC areas (568 m; Tao et al., 2012a), and the thickness of total oil shale sequence is up to 638 m. Rock-Eval pyrolysis is used to determine the petroleum potential, Samples for the present study were obtained from three measured thermal maturity of the organic matter and its ability to generate oil outcrop sections and two boreholes (Fig. 1B). The oil shale layers of and/or gas (Alaug, 2011). The pyrolysis gives rise parameters as S1, each profile are well exposed. Two types of facies sequences are devel- S2,S3, hydrogen index (HI), oxygen index (OI), production index oped in the Dahuangshan sections. Dark-black oil shale interbedded by (PI) and Tmax. argillaceous dolomite, silty claystone, tuff, limestone, dolomitic marl is The Rock-Eval and TOC data are summarized in Table 1.TheTOC developed in the middle–upper part of the section (b650 m), containing content of 42 oil shale samples ranges from 5.6% to 34.75 wt.% a spot of plant stem fossils, and an abundance of fishtail and fish skeleton. (mean 13.79 wt.%). Rock-Eval S1 and S2 are 0.29–3.29 and 22.65– The thickness of oil shale is obviously thinning out in the lower part 199.25 mg HC/g rock, respectively. The HI differs widely from 359 to

(>650 m), and it occurs as thin layers sandwiched among surrounding 1068 mg HC/g TOC. High TOC contents, with high S2 and HI values indi- rocks (Fig. 2). cate that Dahuangshan oil shales have excellent source rock potential 44 S. Tao et al. / International Journal of Coal Geology 115 (2013) 41–51

(Littke et al., 1998; Peters and Cassa, 1994; Tissot and Welte, 1984). The with moderate to high HI, which ﬁts well with the predominance of extremely low PI-values (0.004–0.037) indicate immature organic oil prone source rock potential. matter whereas elevated Tmax values (433–453 °C) (Table 1) imply an early to medium maturation stage. 4.3. Major element geochemistry

Major element data in conjunction with mineralogical data may 4.2. Kerogen types be used to establish the element–mineral associations for oil shales. Although the element associations may vary from one oil shale to an-

The graph of S2 vs. TOC is used to indicate the kerogen type other, a correlation analysis would demonstrate the general trends present and its hydrocarbon potential (e.g. Clayton and Ryder, 1984; (Fu et al., 2010).

Cooper and Barnard, 1984; Cornford et al., 1998; Dahl et al., 2004; As shown in Table 2, the organic sulfur (So,d) content of Dahuangshan Demaison and Moore, 1980; Langford and Blanc-Valleron, 1990). As oil shale samples varies between 0.01 and 0.26%. The samples have a shown in Fig. 3, all of the obtained kerogen types are type II and I, high ash yield (51–76%), with low total (St,d)contents(0.31–0.89%) with a good potential of oil generation. and relatively high oil yield (4.9–26.6%). SiO2 is the dominant constitu- The kerogen quality and maturity are determined by plotting HI ent with an average of 68.59%, next come Al2O3 (6.37–12.88%) and versus Tmax rather than HI versus OI (Fig. 4; Alaug, 2011). The kerogen Fe2O3 (4.32–6.84%). type designations are entirely based on the HI (Hunt, 1996). As shown The content of SiO2,CaO,Fe2O3,andNa2O is close to the average value in Fig. 4, the studied samples are at early to medium mature stage of average shale (Clarke, 1924) and North American Shale Composite

Table 1 Results of Rock-Eval/TOC analysis and calculated parameters.

a b c d e f g h Sample no. TOC S1 S2 S3 HI OI PI Tmax i (wt.%) (mg HC/g) (mg HC/g) (mg CO2/g) (mg HC /g TOC) (mg CO2/g TOC) (S1 /S1 +S2) (°C) O-01 9.71 0.50 56.16 4.72 578 49 0.009 439 O-02 7.45 0.40 30.45 3.51 409 47 0.013 438 O-03 13.20 1.70 95.67 6.30 725 48 0.017 441 O-04 21.12 0.90 133.16 3.70 630 18 0.007 440 O-05 13.90 1.09 80.45 3.40 579 24 0.013 441 O-06 5.86 0.35 39.88 2.56 681 44 0.009 440 O-07 11.00 0.43 53.54 2.63 487 24 0.008 441 O-08 9.12 1.16 53.37 1.57 585 17 0.021 441 O-09 7.86 1.06 29.03 1.44 369 18 0.035 437 O-10 13.92 1.14 75.19 3.16 540 23 0.015 437 O-11 31.17 1.89 199.25 5.90 639 19 0.009 447 O-12 12.56 1.21 58.73 2.27 468 18 0.020 438 O-13 17.77 1.47 88.49 3.29 498 19 0.016 441 O-14 12.35 1.75 69.03 1.86 559 15 0.025 439 O-15 9.35 0.46 43.77 4.23 468 45 0.010 440 O-16 8.51 0.33 86.16 4.23 1012 50 0.004 440 O-17 7.67 0.33 39.92 2.69 520 35 0.008 438 O-18 15.11 1.03 81.65 2.32 540 15 0.012 441 O-19 10.16 0.54 46.25 2.86 455 28 0.012 442 O-20 17.81 2.81 108.54 4.47 609 25 0.025 440 O-21 29.02 2.84 180.49 5.33 622 18 0.015 445 O-22 6.01 0.35 35.38 3.09 589 51 0.010 440 O-23 14.39 0.65 114.88 3.43 798 24 0.006 441 O-24 9.38 0.72 53.55 2.46 571 26 0.013 443 O-25 6.69 0.79 29.49 2.15 441 32 0.026 439 O-26 11.41 1.02 99.92 2.67 876 23 0.010 443 O-27 5.60 0.29 24.65 2.94 440 53 0.012 441 O-28 27.06 2.96 144.86 4.65 535 17 0.020 440 O-29 31.99 3.29 198.88 1.67 622 5 0.016 454 O-30 9.30 0.59 39.3 3.63 423 39 0.015 437 O-31 11.85 1.26 71.64 1.95 605 16 0.017 438 O-32 32.23 1.71 184.03 6.56 571 20 0.009 454 O-33 7.13 0.40 41.07 2.28 576 32 0.010 436 O-34 10.61 0.97 53.65 5.79 506 55 0.018 434 O-35 15.69 2.16 109.14 4.28 696 27 0.019 435 O-36 7.44 0.35 32.78 5.73 441 77 0.011 436 O-37 10.22 0.66 109.13 4.03 1068 39 0.006 441 O-38 13.33 2.18 57.50 2.79 431 21 0.037 433 O-39 34.75 0.96 194.39 3.67 559 11 0.005 453 O-40 6.31 0.64 22.65 1.72 359 27 0.027 436 O-41 9.58 0.66 60.21 3.73 628 39 0.011 442 O-42 13.59 1.96 74.69 6.70 550 49 0.026 443

a TOC = total organic carbon. b S1 = free hydrocarbons. c S2 = pyrolysable hydrocarbons. d S3 = carbon dioxide. e HI = hydrogen index. f OI = oxygen index. g PI = productivity index. h Tmax = temperature of maximum S2. i HC = hydrocarbon. S. Tao et al. / International Journal of Coal Geology 115 (2013) 41–51 45

Meanwhile, SiO2 mainly occurs in compositions of coarse sediments, whereas Al2O3 is the characteristic component of clay minerals, therefore, there is a signiﬁcant negative correlation between them (r = −0.951). The elements K and Ti are also mainly associated with clay minerals, therefore, there is a highly signiﬁcant correlation

between Al2O3 and K2O(r = 0.959), Al2O3 and TiO2 (r = 0.928), K2OandTiO2 (r = 0.812). On the contrary, the concentration of SiO2 correlates negatively with TiO2 (r = −0.87) and K2O(r = −0.954). Table 3 also shows that there is a signiﬁcant correlation be-

tween Al2O3 and Fe2O3 (r = 0.847), and MgO (r = 0.836), and MnO (r = 0.819), and Cu (r = 0.946), and Ba (r = 0.787), and Co (r = 0.952), and Ni (r = 0.905). The correlation coefﬁcients exceed 0.8 mostly, which reﬂects the fact that these elements are associated

with clay minerals. Besides, the correlation between Fe2O3 and MnO, Fig. 3. Plot of TOC wt.% versus S2 mg HC/g rock indicating the kerogen types. Co, Ni is also strong (r = 0.858, 0.903, 0.786, respectively), which reflects the similarity on geochemical behavior of these iron group elements. (NASC; Gromet et al., 1984); while, Al2O3,MgO,TiO2 and K2Ocontents are lower than both of them; MnO contents of oil shale samples vary from 0.09% to 0.29%, and the average being 2.57%, close to average 4.4. REE distribution patterns shale (0.36%), but much lower than that of NASC (2.98%). Particularly, Dahuangshan oil shale is obviously enriched in P, The concentration of REEs and some other trace elements of 10 with a mean value of 0.71%, which is much higher than that of aver- samples from the Dahuangshan oil shale are presented in Table 4. age shale (0.19%) and NASC (0.13%). This phenomenon is probably The content of total rare earth elements (ΣREE) varies considerably, caused by the input of the volcanic ash which is indicated by a tuffa- ranging from 31.49 to 111.18 μg/g. The weighted mean value is ceous composition of the interbedded mudstone and carbonate layers 79.48 μg/g, which is higher than that of the average REE contents of US (Fig. 5). Volcanic ash provided ample nutrients for growth of algae coals (53.59 μg/g; Finkelman, 1993) and the marine oil shale from the leading to formation of a reducing depositional environment favorable Changshe Mountain area, northern Tibet, China (68.19 μg/g; Fu et al., for preservation of organic matter. As a result, the oil yield values of 2010), but lower than those of world-wide black shales (134.19 μg/g; the Dahuangshan oil shale samples are relatively high, with an average Ketris and Yudovich, 2009), common Chinese coals (162.51 μg/g; Dai of 15.0%, which is even higher than the mean value of the Green River et al., 2008), and the NASC (167.41 μg/g; Haskin et al., 1968). The concentration of the light rare earth elements (LREEs) is higher oil shales (11.44%, Ruhl, 1982). The analysis results indicate that SiO2 is the dominant constituent of the outcrop samples, the variation of than that of the heavy rare earth elements (HREEs), which is in accor- which influences directly on the contents of other elements (Table 3); dance with the general distribution of REEs in shales (e.g. Condie, 1991; Fu et al., 2010; Gromet et al., 1984; Ketris and Yudovich, 2009). therefore, due to a dilution effect SiO2 exhibits a negative correlation with most major elements and trace elements, which is so-called The LREE/HREE ratios range from 5.16 to 9.22. The value of (La/Yb)N has the same significance with the LREE/HREE ratios, ranging from “Diluent Effect” of SiO2. 6.67 to 10.69 (Table 4), indicating the enrichment of LREEs. (La/Sm) Al2O3 is also an important component in surface sediments, the N values vary from 2.70 to 5.95, showing a strong fractional degree contents of Al2O3 and SiO2 always change in the reverse direction. among LREEs, suggesting the deep water sedimentary environment of oil shale. All oil shale samples exhibit a negative Eu anomaly (Table 4), with a mean δEu value of 0.64. The δCe values of all samples vary from 0.81 to 1.11, showing a slightly or negligibly negative anomaly in some plies. The oil shale samples exhibit two slightly different types A and B of the chondrite-normalized REE patterns although they show coher- ent, subparallel REE patterns (Fig. 6). Both types A and B are charac-

terized by distinctly sloping LREE trends (LaN/SmN = 2.70–5.95) accompanied by ﬂat HREE trends, and there is no notable difference in δEu between types A and B oil shales. However, No Ce depletion is seen in the REE distribution patterns of type A (samples O-11, O-13, O-20, O-28, O-29, and O-39), showing negligible Ce anomalies. A small to medium Eu depletion is observed and shows a small or middle “V” shape in the REE distribution patterns. In contrast, Type B (samples O-04, O-21, O-32, and O-35) shows a little negative Ce anomaly with slight Ce depletion (Fig. 6B). At the same time, Type B exhibits obviously Nd anomaly with a medium increase and shows a re-

versed “V” shape. The average values of ΣREE, LREE/HREE, (La/Yb)N,and (La/Sm) N of type A samples are relatively higher than those of type B samples.

4.5. Molecular geochemistry of organic matter

4.5.1. n-Alkanes Aliphatic hydrocarbons are dominated by n-alkanes. Gas chro-

Fig. 4. Plot of Tmax °C versus HI mg HC/g TOC identifying the kerogen types. matograms of saturated hydrocarbons from Dahuangshan oil shales 46 S. Tao et al. / International Journal of Coal Geology 115 (2013) 41–51

Table 2 Concentrations of ash, oil yield, total sulfur, organic sulfur and major elements in samples from the Dahuangshan oil shale (unit in %).

Sample no. Oil yield Ad St,d So,d SiO2 Fe2O3 Al2O3 CaO MgO K2ONa2O TiO2 P2O5 MnO O-04 7.8 52.35 0.88 0.31 66.14 6.68 12.39 1.33 2.34 2.52 2.2 0.68 0.86 0.22 O-11 4.9 64.33 0.48 0.21 69.01 4.96 10.26 3.14 1.97 2.34 0.7 0.58 1.15 0.14 O-13 9.9 55.08 0.89 0.3 63.84 6.84 12.88 3.8 2.08 2.82 1.93 0.64 0.98 0.29 O-20 12.2 67.14 0.59 0.15 64.7 5.34 11.3 5.02 2.09 2.76 2.28 0.55 0.3 0.13 O-21 20.5 65.45 0.32 0.13 71.09 5.18 8.6 3.41 1.84 2.05 0.61 0.51 0.42 0.18 O-28 26.6 62.37 0.35 0.09 71.5 4.32 7.82 4.1 1.81 1.82 0.56 0.48 0.48 0.11 O-29 15.1 71.03 0.31 0.09 76.78 4.76 6.37 3.3 1.73 1.5 0.44 0.41 0.36 0.09 O-32 17.5 64.18 0.34 0.18 70.14 4.76 9.4 3.7 1.86 2.14 0.73 0.5 0.78 0.18 O-35 9.4 56.91 0.52 0.23 65.42 5.98 12.69 3.6 2.63 2.75 1.46 0.62 0.86 0.25 O-39 26.2 50.2 0.58 0.23 67.29 5.46 10.12 3.96 1.98 2.12 1.23 0.6 0.92 0.20 Ave. 15.0 60.90 0.53 0.19 68.59 5.43 10.18 3.54 2.03 2.28 1.21 0.56 0.71 0.18 Ave. shalea ––––64.21 6.71 17.02 3.44 2.7 3.58 1.44 0.72 0.19 0.5 NASCb ––––64.8 5.66 16.9 3.63 2.86 3.97 1.14 0.70 0.13 0.06

Ad, ash yield, dry basis; St,d, total sulfur, dry basis; So,d, organic sulfur, dry basis. a From Clarke (1924). b From Gromet et al. (1984). are presented in Fig. 7. The n-alkanes patterns of oil shales from deep water environment with bottom waters of elevated salinity

Dahuangshan area are dominated by short (nC15–nC19, Bechtel (Zhu et al., 2005). The values Paq of Ficken et al. (2000),where et al., 2002)- to middle (nC21–nC25)-chain n-alkanes with highest Paq =(C23 +C25)/(C23 +C25 +C29 +C31), obtained from 10 relative intensities in the nC20 to nC22 range without a marked carbon samples (averaging 0.75) are close to the data measured in sub- preference index (CPI b 1.3) in the nC23 to nC31 range. The results are merged/ﬂoating macrophytes (Ficken et al., 2000). The pristane/ different from the long-chain n-alkanes with a marked odd over phytane (Pr/Ph) ratio is a commonly-used parameter for the study even preference in Tertiary oil shales in the central Tibetan plateau of oxic/anoxic conditions and sources of organic matter (Didyk (Wang et al., 2011). Brassell et al. (1978) reported that the single et al., 1978; Escobar et al., 2011), although some studies have dem- peak with the maximum peak carbon of nC18–nC24, low CPI value, in- onstrated that there are multiple possible relationships between dicated the organisms were derived from phytoplankton, zooplank- depositional environment and Pr/Ph ratio (ten Haven et al., 1987). ton, benthic bacteria with no photosynthesis and terrestrial plants. The pristane/phytane ratios in Dahuangshan oil shales vary from In addition, Allen et al. (1971) indicated that microbial reworked 0.41 to 0.91 and average 0.60. According to Peters and Moldowan organic matter in sediments is characterized by low CPI values at (1993),Pr/Phb 0.6 within the oil-generative window indicates low maturation stage. Therefore, the organic matter in Dahuangshan anoxic conditions; whereas Pr/Ph > 3 indicates suboxic to oxic de- oil shales mainly comes from thallogen microorganisms living in a positional environments. In Dahuangshan area, Pr/Ph ratios in most

Td Do

Td Q

Td Q Ta Sd

200µm Do 200µm A B

Ta Td Td Td Q Td

Qa Td

Ta 200µm 200µm C D

Fig. 5. Minerals in the Dahuangshan proﬁle samples (existing as thin bed or thin interbedded with oil shale), Thin section, Polarized light. A — limestone with tuff debris (Td), tuffaceous matrix (Ta), and quartz (Q); B — dolomite with tuff debris, dolomite (Do), quartz, and siliceous debris (Sd); C — tuffaceous mudstone with tuff debris and tuffaceous matrix; D — tufﬁte with tuff debris, tuffaceous matrix, quartz, and authigenic quartz (Qa). S. Tao et al. / International Journal of Coal Geology 115 (2013) 41–51 47

Table 3 Correlation coefﬁcient values of major elements and trace elements.

SiO2 Al2O3 Fe2O3 CaO MgO Na2OK2O TiO2 P2O5 MnO Ba Sr V Cu Co Ni

SiO2 1.000

Al2O3 −0.951 1.000

Fe2O3 −0.735 0.847 1.000 CaO −0.087 −0.150 −0.402 1.000 MgO −0.715 0.836 0.690 −0.260 1.000

Na2O −0.850 0.836 0.792 −0.061 0.660 1.000

K2O −0.954 0.959 0.739 0.053 0.762 0.830 1.000

TiO2 −0.870 0.928 0.854 −0.372 0.767 0.751 0.812 1.000

P2O5 −0.455 0.564 0.451 −0.378 0.395 0.148 0.415 0.687 1.000 MnO −0.730 0.819 0.858 −0.257 0.656 0.547 0.703 0.809 0.619 1.000 Ba −0.867 0.787 0.429 0.353 0.697 0.696 0.892 0.600 0.179 0.431 1.000 Sr −0.412 0.333 −0.032 0.469 0.478 0.130 0.390 0.146 0.013 0.301 0.601 1.000 V −0.287 0.041 −0.154 0.838 0.001 0.148 0.147 −0.074 −0.297 −0.008 0.422 0.609 1.000 Cu −0.898 0.946 0.881 −0.132 0.876 0.852 0.874 0.896 0.452 0.803 0.737 0.320 0.143 1.000 Co −0.872 0.952 0.903 −0.228 0.904 0.835 0.867 0.900 0.465 0.843 0.696 0.330 0.050 0.988 1.000 Ni −0.843 0.905 0.786 −0.132 0.739 0.633 0.825 0.900 0.773 0.816 0.638 0.228 0.051 0.881 0.860 1.000

of the samples are b0.6, indicating an anoxic deposition. In addition, hopanes above C31 gradually decreases. Homohopanes (C31–C35)are syngenetic pyrite can be found in studied samples, which can also believed to be derived from bacteriohopanetetrol as well as from prove the anoxic sedimentary environment of these samples (e.g. Dai other hopanoids in bacteria (Ourisson et al., 1984). The homohop indi- et al., 2002; Fu et al., 2010; Hackley et al., 2009; Kara-Gülbay et al., ces [C35 homohopane/(C31–C35) homohopanes] of all samples are more 2012; Nowak, 2007; Sabel et al., 2005). than 0.06, indicating a reducing depositional environment (Peters and Moldowan, 1991). 4.5.2. Terpanes Gammacerane occurs in small amounts for all samples (Fig. 8). A

The distribution and relative abundances of pentacyclic and tricyclic high gammacerane index [Gammacerane/C30–αβ-hopane] is interpreted terpanes obtained from m/z 191 ion chromatograms are shown in Fig. 8 to indicate highly reducing, hypersaline conditions during deposition. and their parameters are given in Table 5. Triterpenoids are relatively rare However, relatively high gammacerane abundances are also seen in in Dahuangshan oil shale samples. Traces of unsaturated triterpenes freshwater lacustrine sediments, and Sinninghe Damsté et al. (1995) are found in most of the samples. The hopanoids are dominated by the thus proposed that gammacerane index is in fact an indicator for presence of C30-hopane, C29-norhpane, 17α(H)-trisnorhopane (Tm), water column stratiﬁcation. The compound is abundant in saline lacus- and a considerable quantity of homohopanes (C31–C35)(Fig. 8). trine deposits just because the water columns in hypersaline deposi- Hopanes are ubiquitous constituents of sedimentary organic matter tional environments are often density stratiﬁed (Zhu et al., 2005). The (Piedad-Sánchez et al., 2004; Zumberge, 1987). They are derived from a gammacerane index varies from 0.08 to 0.36 in the Dahuangshan oil degraded bacteriohopane C35 (Ourisson et al., 1979). As shown in Fig. 8, shale, combining with the low Pr/Ph ratio and other biomarker param- the hopane C30 is the highest whereas the relative abundance of eters, thus probably indicates a moderate saline lacustrine environment.

Table 4 Rare earth element contents (in μg/g) in samples and associated geochemical parameters.

Element O-4 O-11 O-13 O-20 O-21 O-28 O-29 O-32 O-35 O-39

La 18.1 6.27 21.9 16.5 23.0 13.4 9.68 17.6 17.6 12.6 Ce 33.8 11.7 42.8 36.6 35.5 25.4 17.4 25.3 34.5 28.3 Pr 4.80 1.47 5.31 4.03 4.79 3.17 2.24 2.85 4.56 2.98 Nd 19.9 5.67 21.2 16.8 26.2 12.7 8.8 14.7 18.3 11.2 Sm 4.21 1.16 4.40 3.35 2.91 2.60 1.83 1.86 3.72 2.30 Eu 0.84 0.25 0.92 0.74 0.63 0.56 0.44 0.41 0.84 0.46 Gd 4.22 1.30 4.33 3.53 3.44 2.95 1.86 1.89 4.08 2.36 Tb 0.68 0.20 0.69 0.58 0.52 0.46 0.30 0.29 0.65 0.35 Dy 3.68 1.19 3.87 3.11 3.03 2.57 1.70 1.71 3.90 1.99 Ho 0.79 0.26 0.77 0.67 0.63 0.56 0.37 0.36 0.88 0.42 Er 2.12 0.85 2.30 1.79 1.95 1.73 1.08 1.10 2.61 1.30 Tm 0.32 0.13 0.33 0.28 0.28 0.26 0.17 0.17 0.38 0.19 Yb 2 0.88 2.04 1.64 1.82 1.69 1.05 1.11 2.51 1.35 Lu 0.31 0.16 0.32 0.27 0.28 0.26 0.16 0.17 0.39 0.22 Y 20.7 8.25 21.1 17.2 17.9 16.4 9.59 10 24.7 11.9 V 65.7 101 128 184 133 143 86.3 120 147 189 Ni 41.2 36.6 43.5 39.7 37.8 37.2 41.5 38.4 42.9 41.5 Sr 129 110 146 185 163 170 133 198 237 176 Ba 221 97.2 268 340 195 212 235 191 322 207 Cu 68.20 48.67 69.81 63.25 52.87 52.69 55.76 52.10 71.24 62.08 Co 13.41 8.58 13.28 11.57 9.74 9.23 10.05 9.82 13.89 11.28 ΣREE 95.77 31.49 111.18 89.89 104.98 68.31 47.08 69.52 94.92 66.02 L/H 5.78 5.34 6.59 6.57 7.78 5.52 6.04 9.22 5.16 7.07

(La/Yb)N 6.10 4.80 7.24 6.78 8.52 5.35 6.22 10.69 4.73 6.29 δCe 0.87 0.93 0.96 1.08 0.81 0.94 0.90 0.86 0.93 1.11 δEu 0.61 0.62 0.64 0.66 0.61 0.62 0.73 0.67 0.66 0.60

(La/Sm)N 2.70 3.40 3.13 3.10 4.97 3.24 3.33 5.95 2.98 3.45

0.5 0.5 L/H = LREE/HREE; (La/Yb)N,(La/Sm)N; subscripts N stands for chondrite-normalized value (Boynton, 1984); Ce/Ce* = CeN/(LaN ×PrN) ; Eu/Eu* = EuN/(SmN ×GdN) ;subscriptsN stands for chondrite-normalized value (Boynton, 1984). 48 S. Tao et al. / International Journal of Coal Geology 115 (2013) 41–51

Fig. 6. Distribution patterns of rare earth elements in oil shale samples by chondrite-nomalized. REE patterns.

In general, Pr/nC17 and Ph/nC18 ratios would decrease along with the steranes, except sample O-04 having C29 >C27 steranes distribution, increasing in maturity of organic matter (Moldowan et al., 1985). The reﬂecting a high contribution of aquatic algae (Peters and Moldowan,

Pr/nC17 ratios range between 0.31 and 1.04, and Ph/nC18 ratios vary be- 1993), which is consistent with the n-alkanes distribution. The predom- tween 0.45 and 1.45 (Table 5), which are lower than those of immature inance of C29 steranes in sample O-04 may reﬂect a high contribution of oil shale in the central Tibetan plateau (Wang et al., 2011), showing the bacteria or/and microorganisms (Riboulleau et al., 2007; Vandenbroucke early maturation stage of oil shale in Dahuangshan area. This result is and Bchar, 1988). consistent with the relatively high Tmax values as discussed above. Moreover, 20(S)/(20S + 20R) and ββ/(ββ + αα) sterane ratios increase with increasing maturity of organic matter (Hunt, 1996; Goodarzi et al., 1989; Seifert and Moldowan, 1980). As shown in 4.5.3. Steranes Table 5, 20(S)/(20S + 20R) and ββ/(ββ + αα)ratiosareintherange The steranes composition could be correlated with the type of 0.2–0.34, and 0.15–0.32, respectively, indicating the early maturation environment (Huang and Meinschein, 1979). They proposed that a stage of organic matter (Grantham, 1986; Goodarzi et al., 1989). dominance of C27 sterols (steranes) mainly derive from algae, while the C sterols are more typically associated with land plants. Volkman 29 5. Conclusions (1986) indicated that the low C28 levels are typical of limnic environments. However, they also considered that microalgae or cyanobacteria can also be important sources of C29 sterols. From the m/z 217 mass chro- (1) Two types of facies sequences are developed in section No.2. matograms of our samples, the relative abundances of the C27,C28,and from Dahuangshan area. The middle–upper part of the section C29 steranes and their 20S and 20R epimers have been determined consists of dark-black oil shale interbedded by thin-bedded argil- (Fig. 8; Table 5). Dahuangshan oil shale samples show a higher propor- laceous dolomite, silty claystone, tuff, limestone, and dolomitic tion of C27 (24–55%) compared to C29 (26–47%) and C28 (13–29%) marl, containing a spot of plant stem fossils, and an abundance

Fig. 7. Mass chromatograms (m/z = 85) for the saturated alkanes of selected samples from the Dahuangshan oil shale. S. Tao et al. / International Journal of Coal Geology 115 (2013) 41–51 49

Fig. 8. m/z 191 and m/z 217 ion fragmentograms of the saturated fractions showing the distribution of the terpanes and steranes for selected oil shale samples.

of ﬁshtail and ﬁsh skeleton, while, the lower part is lack of oil (4) The content of ΣREE in oil shale samples vary considerably, shale and fossils. The sedimentary characteristics of oil shale sug- ranging from 31.49 to 111.18 μg/g, with a weighted mean gest a lacustrine deposition. value of 79.48 μg/g, which are higher than those of US coals,

(2) Dahuangshan oil shales contain high values of TOC, S2, HI, and but lower than those of world-wide black shales, common oil yield, indicating the oil shales have excellent source rock Chinese coals and the NASC.

potential. Kerogen types of organic matter are type II and I, (5) The contents of Al2O3 and SiO2 always change in the reverse which further verify the oil prone source rock potential. Tmax direction. There is a highly significant correlation between values, Pr/nC17 and Ph/nC18 ratios, 20(S)/(20S + 20R) and Al2O3 and K2O(r = 0.959), Al2O3 and TiO2 (r = 0.928), and ββ/(ββ + αα) sterane ratios show an early to medium matu- K2OandTiO2 (r = 0.812). On the contrary, the concentration ration stage of organic matter. of SiO2 correlates negatively with TiO2 (r = −0.87) and K2O (3) The oil shale samples are rich in SiO2 (68.59%), Al2O3 (10.18%), and (r = −0.954). Moreover, the significant correlations between Fe2O3 (5.43%). Element P is especially enriched compared with av- Al2O3 and Fe2O3, MgO, MnO, Cu, Ba, Co, and Ni are because erage shale and North American Shale Composite (NASC), which is of their strong affinity to the clay minerals, and the significant

probably caused by the input of the volcanic ash, representing an correlations between Fe2O3 and MnO, Co, and Ni are due to excellent situation for organic matter production and enrichment. their similar geochemical behavior. 50 S. Tao et al. / International Journal of Coal Geology 115 (2013) 41–51

Table 5 Organic geochemical data for extracts of samples from the Dahuangshan oil shale.

a b c d e f g h Sample no. CPI Pr/Ph Pr/nC17 Ph/nC18 Gammacerane index Homohop index 20(S)/(20S + 20R) ββ/(ββ + αα) %C27 %C28 %C29 O-04 1.12 0.41 0.97 1.24 0.09 0.112 0.21 0.19 24 29 47 O-11 0.98 0.59 0.59 1.02 0.23 0.066 0.23 0.24 55 14 31 O-13 1.24 0.82 0.45 0.98 0.26 0.061 0.28 0.32 37 27 36 O-20 1.08 0.60 0.31 0.47 0.18 0.077 0.24 0.18 50 24 26 O-21 0.93 0.91 0.91 0.56 0.36 0.084 0.34 0.28 48 16 36 O-28 1.01 0.56 0.56 1.02 0.11 0.106 0.31 0.20 44 25 31 O-29 1.05 0.57 0.37 0.45 0.13 0.109 0.21 0.15 53 13 34 O-32 0.99 0.61 0.72 0.67 0.08 0.990 0.29 0.26 47 22 31 O-35 1.16 0.42 1.04 1.45 0.12 n.d. 0.20 0.17 46 22 32 O-39 1.01 0.52 0.82 0.85 0.15 0.145 0.22 0.18 52 20 28 n.d. not detected. a Pr/Ph = pristane/phytane ratio. b Gammacerane index = Gammacerane/C30αβ-hopane. c Homohop index = C35αβ, (22S + 22R)-/C31–C35αβ, (22S + 22R)-hopanes. d 20(S)/(20S + 20R) = C29ααα,20S/C29 ααα, (20S + 20R) steranes. e ββ/(ββ + αα) = C29-regular sterane (20αββR+20αββS)/(20αααS+20αααR+20αββR+20αββS) isomer ratio. f %C27 =%C27ααα/C27–C29ααα steranes. g %C28 =%C28ααα/C27–C29ααα steranes. h %C29=%C29ααα/C27–C29ααα steranes.

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