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Accepted Manuscript Petroleum Geoscience

Geology of the Chang 7 Member oil shale of the Yanchang Formation of the Ordos Basin in central North China

Bai Yunlai & Ma Yuhua

DOI: https://doi.org/10.1144/petgeo2018-091

Received 28 July 2018 Revised 23 December 2018 Accepted 17 February 2019

© 2019 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London for GSL and EAGE. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

Supplementary material at https://doi.org/10.6084/m9.figshare.c.4411703

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Geology of the Chang 7 Member oil shale of the Yanchang Formation of the Ordos Basin in central North China

Bai Yunlai & Ma Yuhua*

Northwest Branch of Research Institute of Petroleum Exploration and Development (NWGI), PetroChina. Lanzhou, 730020, Gansu, China *Corresponding author, e-mail [email protected]

Abstract We present a review of the Chang 7 Member oil shale, which occurs in the middle –late Triassic

Yanchang Formation of the Ordos Basin in central north China. The oil shale has a thickness of 28 m

(average), an area of around 30,000 km2 and a Ladinian age. It is mainly brown-black to black in colour with a laminar structure. It is characterized by average values of 18wt% TOC (total organic carbon), 8wt% oil yield, and 8.35 MJ/kg calorific value, 400kg/t hydrocarbon productivity, and kerogen of type I-II1, showing a medium quality. On average it comprises 49% clay minerals, 29% quartz, 16% feldspar, and iron oxides, closed to the average mineral composition of global shale. The total SiO2 and Al2O3 comprise

63.69wt% of the whole rock, indicating a medium ash type. The Sr/Ba is 0.33, the V/Ni is 7.8, the U/Th is

4.8, and the FeO/Fe2O3 is 0.5, indicating formation in a strongly reducing, fresh water or low salinity sedimentary environment. Multilayered intermediateMANUSCRIPT- acid tuff is developed in the basin, which may have promoted the formation of the oil shale. The Ordos Basin was formed during the northward subduction of the Qinling oceanic plate during Ladinian to Norian in a back-arc basin context. The oil shale of the Ordos

Basin has a large potential for hydrocarbon generation.

Supplementary material: The table of oil shale geochemical composition, proximate and organic matter

analyses frome the Chang7 Member oil shale, the Ordos Basin, Central northChina.

Received 31 August 2017; revised 09 January 2019; accepted

IntroductionACCEPTED

In addition to oil, natural gas, coal, coal seam gas, uranium and groundwater resources, a large tonnage of oil shale is also present in the Ordos Basin (Figure1) , which is one of the largest oil and gas–bearing basins, ranked second in oil and gas resources, and first in annual production, in China ( Pan 1934; Yang

1991, 2002; Wang et al. 1992; Guan et al. 1995; Zhang et al. 1995; Yang and Pei 1996; Li 2000; Chen

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2002; Liu and Liu 2005; Lu et al. 2006; Yang et al. 2006, 2016; Bai et al. 2006, 2009, 2010a,b, 2011; Liu et al. 2009; Qian 2009; Wang et al. 2016; Deng et al. 2017). The oil shale has attracted considerable interest in recent years in the context of global oil depletion and the search for unconventional energy resources. The commercial potential of the oil shale has been assessed and the resource may become more important as other energy reserves decline. Recent research undertaken by the Northwest Branch of the

Research Institute of Petroleum Exploration and Development (NWGI), PetroChina, and others, has shown that the Ordos Basin contains more than 2000×108 tonnes of „shale oil‟ predicted resources, of which the

Chang 7 Member shale oil comprises more than 1000×108 tonnes, i.e. 50% of that occurring at depths of

0-2000 m in the entire basin. The Chang 7 Member oil shale, also called the Zhangjiatan shale in the northern Ordos Basin, has an area of around 30,000 km2, an average thickness of 28 m and an average oil yield of 8 wt %. It is both a high quality source rock and an oil shale with a high residual organic matter content and is the most important oil shale seam in the basin (Yang 2002; Duan et al. 2004; Yang et al.

2006, 2006; Bai et al. 2006; Zhang et al. 2008a). It is located in the Yanchang Formation (Fig.2) , which was originally deposited overe an area of ~400,000 km2 during the middle to late Triassic, with a current area of 250,000 km2 and current thicknesses of 184-2060 m (Yang 2002; Bai et al. 2009). The Yanchang Formation is a well-documented lithostratigraphicMANUSCRIPT unit, first discovered in the northern Shaanxi in 1926 by M L Fuller and F G Clapp, who described it as a set of stratigraphic units including grey or green sandstone and shale (Fuller & Clapp 1926, Pan 1934; Yang 2002). Its reservoirs produce 90% of the oil in the Ordos Basin. It is a group of terrestrial sequences of middle-late Triassic age, and is composed of fluvial, delta plain and lake facies. The overlying strata of theYanchang Formationare is the

Wayaobao Formation, which was a set coal-bearing stratum and originally belonged to the upper of the

Yanchang Formation, but it was separated from the Yanchang Formation later.They contain 10 reservoir members, each including both reservoir sands and (oil) shales (Figure 2). The units are numbered 1to 10 from topACCEPTED of the Wayaobao Formation to bottom of the Yanchang Formation, i.e. opposite to the normal geological convention. Chang 1 now was contained in the Wayaobao Formation, but it is still called

Chang1 rather than“Wa1” customarily. Among them, Chang 6 and 8 Member are the two main oil reservoirs.

The Chang 7 Member oil shale is a high quality source rock which supplies oil for the reservoirs of the

Yanchang Formation, especially, the Chang 6 and Chang 8 Member reservoirs, and it even supplies oil to

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reservoirs of the younger middle Jurasic Yanan Formation (Figure 2). Yang et al (2016) discovered that the oil derived from the Chang 7 Member oil shale also fills internal nano-sized pores and pore-throats, to form “Shale Oil”, equivalent to tight oil occurring naturally in oil-bearing shales, which is is different from

“Oil shalederived oils” ( “oil shale” is an organic-rich fine-grained sedimentary rock containing immature kerogen, a solid mixture of organic chemical compounds, from which liquid hydrocarbons, can be produced by low-temperature pyrolysis). The Chang 7 Member oil shale and its host stratum, the

Yanchang Formation have been investigated by the drilling of at least 57 boreholes in the basin interior

(Figure 1). Early in the last century, outcrops of the Chang 7 Member oil shale were investigated by Pan

Zhongxiang (Pan 1934) who studied its distribution, occurrence and quality. In the 1960s, the Shaanxi

Industrial Bureau commenced opencast mining of the oil shale which outcrops in the Tongchuan area in the southern margin of the basin. They refined the oil shale using a cracking process but the annual shale oil production was only 80-90 tonnes and production was terminated because of water shortages and technological shortcomings (Bai et al. 2009). During 2000-2008, several private enterprises attempted to initiate oil shale production using the Fushun Furnace, a small, low-temperature, pyrolysis furnace, but all of them failed. Since 2005, given the heightened imperative for seeking alternative energy sources, outcrops and drilling data for the oil shales in the regionMANUSCRIPT were re-investigated (Liu & Liu 2005; Lu et al. 2006; Liu et al. 2006; 2009; Qian 2009; Shu 2012; Bai et al. 2009, 2010a, 2010b, 2011). Supported by

PetroChina, Professor Bai Yunlai conducted a high-quality study (Bai et al. 2010a). However, in general, research on the oil shale of the region remains sporadic and unsystematic. This has hindered further exploration and exploitation of the oil shale resource; moreover, most of the research has been published in

Chinese, making comparisons with comparable deposits elsewhere more difficult.

Previous research focused primarily on the characteristics of the oil shale, especially its occurrence, volume and quality. However little research has been conducted to address such key questions as: How and when didACCEPTED the oil shale form? Is it the Triassic or middle -late Triassic in age? How did the basin accumulate such abundant oil and oil shale resources? What was the tectonic setting? What are the properties of the basin? Is it marine? Is it an intracratonic basin or a foreland basin (Li 2000; Yang 2002; Yang &Zhang

2005; Bai et al. 2006; Wang et al. 2017)? The present review seeks to address these questions.

The structure of the present study is to summarize pre-existing research, describe the characteristics of the Chang 7 Member oil shale, and to discuss the environment and processes of formation. The overall aim

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is to provide a basis for future investigations of the Ordos Basin oil shale and to facilitate comparisons with similar deposits elsewhere.

Geological context

Tectonic setting

The Ordos Lake was located in the southwestern part of the North China plate and at the northern edge of the Qinling orogenic belt (Figure 1, 3) (Yang 1991; Zhang et al. 1995; Yang & Pei 1996; Yang 2002; Yang et al. 2006; Bai et al. 2006; Felix et al. 2012 ). The Qinling Ocean plate subducted beneath the North

China plate in the middle-late Triassic, forming a volcanic-magmatic arc and back-arc basin on the northern side of the volcanic-magmatic arc (Figure 3) (Wan 2004; Chen 2010). Thick piedmont facies (up to 2400 m) are distributed in the southwest Ordos Basin and are called the Kongtongshan conglomerates

(the lower left part of Figure 1 highlights its location). They are regarded as the remnants of the foredeep deposits of the back-arc foreland basin, most of which were destroyed subsequently. These suggest that the

Ordos lake environment was in fact a back-arc foreland basin, with a similar structural mechanism to the

Karoo Basin in South Africa (Smith 1990).

Sedimentary fill

The Ordos Basin has a sedimentary fill with a maximumMANUSCRIPT thickness of about 12,800 m, that accumulated in varying tectonic settings and different climatic regimes since the Proterozoic era (Figure 2). Broadly

Ordos area has experienced 5 sedimentary cycles, including (1) the Meso-Neo Proterozoic, (2) the early

Palaeozoic, (3) the late Palaeozoic, (4) the middle to late Triassic and (5) the Jurassic to the early

Cretaceous, and just in the last 3 phases the oil shale was formed. In the Meso-Neo Proterozoic and the early Palaeozoic, the marine facies carbonate sedimentary rocks accumulated on the Ordos area. In the late

Palaeozoic 600-1400 m thick deltaic and fluvial facies sandstone, mudstone and coal and oil shale seams formed in a paralic environment at first under humid and later dry hot conditions (Figure 2). From the middleACCEPTED Triassic to Jurassic fluvial, deltaic and lacustrine facies sandstone, mudstone and oil shale accumulated in terrestrial environments in a damp hot phase, in general attaining 20-3000 m in thickness in the middle to late Triassic, and 184-2060 m in the Jurassic. There is also an unconformable blanket of

Cretaceous sandstone and mudstone (terrestrial facies), which covers the entire basin, varying from 600 m to 3000 m in the thickness (Figure 2). Recent day, the southern Ordos area is covered by nearly 100 m of

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Quaternary loess and the northern area is beneath the Mu Us Desert (Figure 2) (Liu 1986; Bureau of

Geology and Mineral Resources of Shaanxi Province (BGMRSP) 1989, 1998; Yang 1991, 2002; Yang &

Pei 1996; Zhang et al. 2005; Yang & Zhang 2005; Yang & Liu 2006; Yang et al. 2006; Yang et al. 2016 a, b; Bai et al. 2006, 2013, 2014). Based on analyses of previous works on vitrinite reflectance, fluid inclusions and apatite fission tracks in the basin, Ren (1991) reconstructed the thermal history of the Ordos

Basin, depicting a temperature gradient of 2.2-3.0℃/100 m from the Palaeozoic to the early Mesozoic, while increased to 3.3-4.5 ℃/100 m in the late Mesozoic, gradually decreasing to 2.8℃/100 m during the the Cenozoic. Research on the relation between the thermal history and oil-gas accumulation of the Ordos

Basin suggests that (1) the low temperature gradient and low thermal maturation of gas resource rocks were favourable for the preservation of organic matter from the Palaeozoic to the early Mesozoic; (2) the higher temperature gradient in the Cretaceous (150 -125Ma) was responsible for generating and migrating gas from the Paleozoic coal series and carbonates (Jia et al.2006); (3) the higher temperature gradient during the Cretaceous was also responsible for maturing and migrating Triassic and Jurassic oils; (4) the decrease of temperature gradient during the Cenozoic, was favourable to the preservation of oil-gas fields.

Both the late generation of hydrocarbons and the lack of faults in the Ordos Basin are key factors in preserving the hydrocarbon accumulations. MANUSCRIPT Burial history analysis show that the strongest uplift and erosion event took place at the end of the later Cretaceous，and three weaker uplift and erosion events took place at the end of the late Triassic, the middle Jurassic and the late Jurassic (Chen et al. 2006 ).

Characteristics of the oil shales and shale in different strata in the Ordos Basin

Since the late Paleozoic, multiple oil shale (shale) seams developed in different strata of the Ordos Basin, in the late Carboniferous -early Permian Taiyuan Formation, the middle-late Triassic (Ladinian to Norian)

Yanchang Formation, the late Triassic (Rhaetian) Wayaobao Formation, the middle Jurassic (Aalenian to

Bajocian)ACCEPTED Yanan Formation, and the middle Jurassic (Bathonian to Callovian) Anding Formation (Bai et al.

2009, 2010a).

The oil shales of the Taiyuan Formation were formed in a paralic environment. Because of deep burial, it is mature to over-mature, with the vitrinite reflectance of the shale verying from 0.9-2.5% Ro, most of the kerogen was converted to gas.The oil shales and coals mainly cropout along the edge of the basin, with a

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burial depth greater than 600 m in the eastern part, near Hancheng, and attain a maximum burial depth of

~3000 m in the mid-western part, qingshen2 well (Figure 1). The oil shale generally has a low oil yield

(only 2.8 wt %), and thin seams (~2m), forming a relatively low grade resource (Bai et al. 2009).

The Jurassic oil shale seams were mainly formed in a lake-delta environment, and are interbedded with coal seams. The oil shale seams are thin, local, and therefore are of low economic value (Bai et al. 2009).

The Chang 9, 7, 4+5 and 1 Member oil shale (shale) occurred in the Yanchang Formation and the

Wayaobao Formation.

The Chang 9 Member oil shale, also called „the Lijiapan shale‟ in the Ordos Basin, is present at the top of the Chang 9 Member of the Yanchang Formation. The oil shale is mainly distributed in the north-central basin, in Yanan, Zhidan and Ansai counties. It has an area of 4336 km2, about one seventh of the Chang 7

Member oil shale, with a limited thickness of about 6 m. It is characterized by a relatively large burial depth, and a relatively low abundance of organic matter (~ 4.5 wt %, on average) (Zhang et al.2008b; Zhou et al.2008). Its organic matter type is different from that of the Chang 7 Member oil shale: the sapropel content of the former is less than in the latter. A deep or semi- deep lake was formed during the interval of accumulation of the Chang 9 Member, which was supplied with large amounts of terrigenous material, a small amount of algal parent material. Framboidal MANUSCRIPTpyrite content is low. Although indicating an overall euxinic environment, the low the framboidal pyrite in the Chang 9 Member oil shale indicates a weakly oxidizing -reducing environment.

The Chang 7 Member oil shale is widely distributed in the region, with an area of around 30,000 km 2 and a thickness of 28 m (average). It developed in an anoxic, deep lake environment (about 60 m deep,

Yang et al. 2016 b ), and is rich in framboidal pyrite; there is a relatively small amount of clay minerals and abundant algal material (Ji et al. 2007). Although some of the oil from the oil shales have been migrated into oil reservoirs of the oilfields, the residual organic matter content is still very high, about 18 wt % TOCACCEPTED (see below), and the in situ oil shale resources account for more than 50% of the total oil shale resources of the basin (Wang et al. 1992; Guan et al. 1995; Liu & Liu 2005; Liu et al. 2006; Lu et al. 2006,

2009; Bai et al. 2009, 2010, 2011).

In addition to the Chang 7 and 9 Member oil shales, a shale seam is present in the Chang 4+5 Member of the Yanchang Formation with a wide distribution and a distinct response in wireline logs, and is known as “thin neck section”, forming a regional marker. It was deposited in a shallow lake delta environment,

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and there is no kerogen in the shales, which therefore lacks the basic conditions for forming oil shale or hydrocarbon source rocks (Fu et al. 2012).

A thin oil shale seam is present in the Chang 1 Member of the Wayaobao Formation, formed in the limnic and delta environment, and interbedded with coal seams, and covers a limited area and is thin

(Wang et al. 2007). The oil shale with coal had been mined, mainly used as fuel. In summary, the Chang 7

Member oil shale has the real exploration significance, and is quite different to the others.

The Yanchang Formation, host rock of the Chang 7 Member oil shale

The middle-late Triassic Yanchang Formation (Ty) mainly comprises carnation, celadon fine to coarse grain arkose, with the interbeds of black shale, oil shale and andesitic to dacitic tuff (Figure 4). It is an important oil-bearing formation.

The lower part of the Yanchang Formation consists of carnation and celadon medium-coarse grain arkose, fine sandstone, sandwiched with siltite, argillaceous siltite, mudstone, oil shale (Change 9 Member oil shale）and tuff; followed by oil shale（Change 7 Member oil shale), black shale, interbeded with argillaceous siltite and tuff (Figure 4).

The upper part of the Yanchang Formation is gray, celadon fine to medium grain arkose, black shale, mudstone, siltite and interbeding of celadon sandstoneMANUSCRIPT and black silty mudstone (Figure 4). The Yanchang Formation has conformable contacts with the overlying stratum (the Wayaobao

Formation) and underlying stratum (the Ermaying Formation) and can be readily distinguished by its celadon, gray, black colour. The base of the Yanchang Formation is marked by the disappearance of the crimson mudstone, which located the top of the Ermaying Formation, also known as the Zhifang

Formation (T2). The top of the Yanchang Formation, or the base of the Wayaobo Formation is marked by occurrence of rhythmic layers of sandstone and mudstone containing coal seams or very thin coal seams

(Figure 4). The ACCEPTED Yanchang Formation contains abundant Fossils (e, g., phytoliths, palynoflora, estheria, bivalves, insect, acritarchs and fish), and framboidal pyrite (Bai et al. 2006; Liu 1986) and formed in fluviatile-delta

-lake faces during the middle to late Triassic (BGMRSP 1998；Bai et al. 2006, 2009; Deng et al. 2017). The age of the Yanchang Formation were regarded as Late Triassic (Yang 2002, Bai et al. 2006, Wang et al.

2017), has recently been determined as the middle to late Triassic (Deng et al. 2017).

The Yanchang Formation is a lithostratigraphic unit. According to the formal definition, it lacks coals.

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The coal-bearing section, of the Chnag 1 Member, is therefore assigned to the Wayaobao Formation

(BGMRSP 1998).

The Yanchang Formation experienced three lake transgressions, corresponding respectively to the Chang

9 Member, Chang 7 Member, and Chang 4+5 Member (in terms of sedimentary cycles, Ty1, Ty2, Ty3, respectively) (Figures 2 and 4).

In the Chang 10 Member, deposition comprised alluvial plain facies, deltaic plain and shallow lake facies. The lake facies covers a relatively small area. During deposition of the Chang 9 Member, the lacustrine area was significantly enlarged with development of shallow lake and delta facies, while in some regions a deeper lake facies formed. The alluvial plain and alluvial fan facies became restricted. The Chang

9 Member represents the first lake transgression in the area. In the Chang 8 Member, although the lake area was wide, it was narrower and shallower than that of the Chang 9 Member. The deltaic sandstone deposited during this stage is one of the main reservoirs in the basin. This completes the first lake transgressive-regressive cycle (Ty1) (Deng et al. 2011).

In the Chang 7 Member, lacustrine facies dominated and the deeper lake facies reached its maximum extent of ~ 30000km2 and possibly water depth of about 60m, representing a major lacustrin transgression. This was fellowed by the Chang 6 where the lake MANUSCRIPTshallowed and deltaic sand bodies developed to form another main reservoir. This represents the second lake transgressive-regressive cycle (Ty2) (Deng et al.

2011).

In the early stages of Chang 4+5, the lake began to narrow considerably but was extended again in the middle of the interval. A mudstone-rich lake facies developed that formed the regional cap rocks.

Subsequently, deltaic plain sand bodies developed again. In the Chang 3 and Chang 2 Member sedimentary intervals, the lake remained narrow, and the deeper lake facies began to disappear. This represents the third lake transgressive-regressive cycle (Ty3) (Deng et al. 2011). In theACCEPTED Wayaobao Formation, or the Chang 1 Member , the lake disappeared completely and there was the extensive swamp development, with the depositing of some coal seams, some interbedding of oil shale, large amounts of charcoal debris and numerous plant fossils.

Geological and geochemical characteristics of the Chang 7 Member oil shale

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Spatial distribution

The Chang 7 Member oil shale is present in a large scale, with an almost orientated N-S asymmetric

syncline (Figures1 and 5).

The oil shales with the Yanchang Formation, have been uplifted and eroded in the eastern, southern and western parts (arc distribution) and have subsided in the mid-western parts (included qingshen2 well,

Huangxian county, Huachi county, Qingyang county and Xifeng city, Figures 1 and 5). The deepest burial is in Huanxian county in Gansu Province (Figures 5). In the western part of the Ordos Basin (included tiantan

1 well, west Huanxian county, Zhenyuan county and qingshen2 well) (Figures 1and 5), the oil shale seams and its host rock are steeply uplifted and dips to the east while in the eastern part (included Zhidan county,Yanan city, Fuxian county, Yanchang county ) is gently uplifted and dips to the west (Figures1 and

5). The structural contours in Figure 5 indicate the burial depth of the oil shales, which also reflects the structural characteristics of the oil shale layers. Outcrops of both the oil shale and strata are mainly distributed in the east and south, in Yijun County, Tongchuan City, Yaoqu town, and Binxian County in northern Shaanxi (Figures1).

Basic sequenceof Chang 7 Member The basic sequence of Channg 7 Member consists MANUSCRIPTof three parts: (1) oil shale, shale and mudstone, (2) sandstone, siltite and (3) tuff (Figure 4). The lower part of Chang7 Member consists of oil shale and tuff，

with the interbeded fine sandstone, siltite. The upper part consists of mudstone, shale and tuff, sandwiched

with siltite and fine sandstone. The stratigraphic characteristics of the oil shales are clearly resolved in a

well wireline logging, the oil shale being characterized by high natural Gamma Ray (GR) and Resistivity

of Induction in Lateral and Deep (RILD) logs, low ρ (density) and Spontaneous Potential ( SP ) logs

(Figure 6) (Yang et al. 2005, Wang et al. 2007).

Age BiostratigraphACCEPTEDic age The biostratigraphy is based on phytoliths. The Danaeopsis-Bernoullia assemblage, with a Carnian-Norian

age (BGMRSP 1989, 1998; Si 1956), occurs in the upper part of the Yanchang Formation, suggesting the

upper part of the Yanchang Formation is late Triassic age (Carnian-Norian age). The

Annalepis-Tongchuanophyllum assemblage, with a Ladinian age (Si 1956), occurring in the lower part of

the Yanchang Formation below the Chang 7 member, indicates an middle Triassic age for the lower part

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of the Yanchang Formation. The Chang 7 Member oil shale is therefore Ladinian age, i.e. middle Triassic age.

Zircon SHRIMP U-Pb ages

Zircon SHRIMP U-Pb ages have recently been published for the lowermost tuff units (K0) of the Chang 7

Member oil shale (stratigraphic horizon K0 see Figure 4; Wang et al. 2014; Xie 2007). These ages range from 239.7-241.3 Ma, which are equivalent to the Ladinian as indicated by the phytoliths.

In summary, the Yanchang Formation is the middle-late Triassic age (Ladinian-Norian age), not just the late Triassic age (Wang et al.2017). The Chang 7 Member oil shale is the middle Triassic age (Ladinian age) .

Thickness

Based on outcrops (Figures 1) and logging data (Figures 6), the thickness of the oil shale ranges from 0 -

61 m, with an average of ~28 m (Figure 5). The areas with a thickness greater than 20 m are elongated approximately northwest-southeast and include Huanxian, Huchi, Qingyang, and Zhengning counties and

Tongchuan city (Figure 5). The oil shale is thin at the edge of the basin and thickest in the central part where it is more than 40 m thick near Huanxian County, greater than 20 m thick to the northwest of Tongchuan city (Figure 5). MANUSCRIPT Petrological and geochemical characteristics

Petrological characteristics

The oil shales have a dark, greasy lustre with a maroon-coloured surface resulting from oxidation (Figure

7). The fresh oil shales have a flakey, banded structure, uneven-conchoidal fractures, low hardness, and light brown streak.

The main components of the oil shale by average are 49 % clays, 29% quartz, 16% feldspars, and iron oxides. The composition falls within the muddy shale area in the shale classification scheme of Kuila & Prasad ACCEPTED2012 (Figure 8). Carbonate minerals are rare. Clay minerals comprise mainly mixed-layer illite and smectite, followed by illite and chlorite, and are partially affected by sericitization. The clastic minerals are mainly quartz, followed by feldspars (Bai et al. 2009, 2010a). Iron oxides and organic matter fill the pore spaces between the clay minerals (Figure 9, left). The diameter of the detrital mineral grains varies from

0.03-0.06 mm ( i.e. silt) occasionally up to 0.15 mm, Sand-size mineral grains are angular, subangular and rounded, and consist of quartz and feldspar (Figure 9, right), and indicate a proximal provenance trait.

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Chemical composition characteristics

The average chemical composition of the oil shale is shown inTable 1. Compared with “North American shale composite” (NASC) (Gromet et al .1984), the oil shale has higher P2O5 and Fe2O3; lower CaO, SiO2, and MgO; slightly lower Na2O and K2O; and similar Al2O3, TiO2.

The concentrations of CaO, SiO2, and MgO in the oil shale are relatively low, which indicates that limited terrigenous matter input to the lake. The concentrations of P2O5 and Fe2O3 in the oil shale is relatively high, if primary, indicateding that the nutrient content of the lake water was relatively high, which may have been associated with volcanism to the south of the lake: numerous tuff layers are present in the oil shale seams.

M (M = 100×MgO /Al2O3) values of the shale could reflect the salinity of the lake water and the provenance; in general, M < 1 for fresh water environments, 1 < M < 10 for transitional environments, 10

< M < 500 for marine environments, and M>500 for epicontinental seas or lagoons (Liu 1984). M = 6.1for the oil shale indicates a transitional brackish water environment. However numerous specimens of

Leiosphaeridia and Micrhystridium are preserved, which indicates that the lake was primarily freshwater

(Ji et al. 2006). The Sr/Ba ratios cited below also support this conclusion. The sum of SiO2 and Al2O3 reaches 63.69 % of theMANUSCRIPT whole rock chemical composition, indicating a continental deposition. This cooresponds to a siliceous ash on combustion (the criteria for siliceous ash-type oil shale are SiO2 (40-70 wt % ), Al2O3 (8–50 wt % ), Fe2O3 (less than 20 wt % ), CaO ( 1.20 wt % ) ( Zhao et al.1991). The oil shales are slightly lower in SiO2 and Al2O3 than that of the Tertiary oil shales of the Fushun Basin, which consist of 61.59 wt % SiO2 and 23.36 wt % Al2O3 (Yuan et al. 1979 ;

The office of the National Committee of Mineral Reserves 1987), indicated the latter have more obvious continental deposition ( Zhao et al.1991).

Oil shale fusibility can be expressed by (SiO2 + Al2O3) / (Fe2O3 + CaO + MgO) values, which is less than 5 ACCEPTEDfor fusible ash, 5-9 for medium fusion ash, more than 9 for refractory ash (Zhao et al. 1991). Because (SiO2+Al2O3) / (Fe2O3+CaO+MgO) values of the oil shales is 5.87, it belongs to a medium fusion ash.

Trace elements characteristics

The average trace element concentrations of the oil shale are given in Table 1. Both Mn and Ni have enrichment coefficients (relative to NASC according to Grome et al.,1984, the same below) of less than 0.5;

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Ba, Zr, Rb, Cr, Co and Th have coefficients from 0.5-1; Sr, V, Zn have coefficients from 1-1.5; Pb has a coefficient of 1.7, and Cu has a coefficient 3.02. Both Mo and U are very strongly enriched. The strong enrichment of U, Mo, Cu and Pb, if primary, shows that the lake was rich in organic nutrients. The eutrophic lake water would have enhancing the productivity, promoting algal booms, and at the same time resulting in anoxia of the water. The enrichment of U, Mo, Pb, Cu is a positive relationship with TOC

( Zhang et al.2008a).

The Sr/Ba ratio of a shale, if primary, is proportional to the salinity of water; Sr/Ba >1 indicates a marine or saline lake environment, 0.5

(Liu 1984). The Sr/Ba ratio of 0.33 in the oil shale indicates that the lake was a fresh water environment.

The Mn content of lake water is positively correlated with water depth. The Mn abundance is about 10 ppm for lake shore, about 60 ppm for shallow lakes, and about 400 ppm for semi-deep lakes-deep lakes

(Liu 1984). The 313 ppm Mn of the Chang 7 Member oil shale indicates a semi-deep - deep lake.

The geochemical behavior of the variable valence elements V and U is closely related to the sedimentary redox environment. In a reducing environment, V and U have a low valency, are less soluble and are readily enriched, so that the ratios of V/Ni, V/Cr, U/Th are often used as redox indicators (Lewan & Maynard 1982). The oil shale has a V/Ni ratio of MANUSCRIPT7.8, U/Th ratio of 4.8, indicating a strongly reducing environment.

The Sr/Cu ratio is climatically related. Sr/Cu ratio of 1.3-5.0 indicates a warm and humid climate; a ratio value greater than 5 indicates a hot, dry climate; and a ratio less than 1.3 indicates a cold, humid climate

(Liu 1984). The Sr/Cu ratio of the oil shale is about 2, indicating a warm, humid climate.

Redox conditions in the original water settings controlled the concentrations of some major and trace elements in sediments and sedimentary rocks. Thus, the concentration of them could be used to reconstruct the redox of the original water (Liu 1984; Tribovillard et al. 2006). Because of fine particles, compacting constructACCEPTED, and very low porosity of oil shale, the concentration and ratios of some major and trace elements are very small change in the diagenetic alteration, and could be used to indicating sedimentary environment (Liu 1984).

Rare earth elements characteristics

The amount of REE (Rare Earth Elements) of the oil shales is slightly higher than the average amount of

REE (146.4 ppm ) in the upper crust, and slightly lower than that (197 ppm ) in NASC (Grome et al.1984;

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cf.Table 1 and Figure10). Fu & Qi (1995) show that the amount of both REE and TOC in the deposits of the warm, damp, climate environments is, generally, higher than that in arid and cold climate environments.

The amount of REE is relatively high in the oil shale, which shows that the warm and damp climate prevailed during the middle Triassic, favouring biological productivity.

The REE distribution patterns of the oil shales are characteristically rich in LREE (Light Rare Earth

Elements) and have a weakly negative Eu anomaly, similar to that of the upper crust (Fu & Qi 1995), which suggests the degree of differentiation of REE is relatively high, and the deposition rate is relatively low in the lake, which favoured enrichment in organic matter (Fu & Qi 1995).

In sedimentary systems the Ce anomaly may reflect changes of the redox conditions in water, Ce anom = lg [3Ce n/ (2 La n＋Nd n )] (The subscript n is standardized values for NASC). Ce anom>-0.1 reflects a reducing water body, and Ce anom<-0.1 reflects an oxidised water body (Fu & Qi 1995). The oil shale has

Ce anomaly greater than -0.1 (Ma et al. 2016).

The oil shales have very similar REE characteristics to chondrite distribution patterns among the different samples (Figure10). The coherence of the REE distribution patterns indicates a consistent provenance. Organic geochemistry characteristics MANUSCRIPT The oil shale has a high residual organic matter content, with an average TOC content of 18wt% (Table 2).

The main component (kerogen) of the organic matter has reached maturity, with Ro of 0. 9 -1. 15 %

(Tmax= 445-455 ℃), a residual chloroform bitumen “A” (Chloroform bitumen is a soluble organic matter in rocks that can be dissolved in chloroform, composed of saturated hydrocarbon, aromatic hydrocarbon, gum and asphaltene. Generally, chloroform bitumen “A” is the ratio of the extracted bitumen mass to the mass of rock sample.) content of 0.1-0.4 wt %，a hydrocarbons content of 0.3-0.6 wt%，and pyrolytic hydrocarbonACCEPTED-generation potertial (S1+S2) content of about 70 mg HC/g rock (Table 2 ). The yield of the oil shale is up to 400 mgHC/g rock. IH is two interval values (bimodal), 200-300 mg HC/g TOC, and 600-650

HC/g TOC; IO is also two interval values, less than 5 and 50-100mg CO2 /g TOC (Yang & Zhang 2005;

Ma et al. 2016), which suggest that the kerogens come from a variety of sources.The residual “Chloroform bitumen A” conversion rates (A/TOC) are 3.14-9.84 and the hydrocarbon conversion rates (HC/TOC) are

2.11-5.77 (Yang & Zhang 2005). The hydrocarbon-expulsion efficiency reaches an average of 72 % (Mu et

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al. 2001; Yang & Zhang 2005; Zhang et al. 2006, 2008a). The kerogens mainly consist of amorphous lipids, with a few Hystrichosphaera and spores, and are characterized by a uniform, monotonous biological component (Mu et al. 2001; Yang & Zhang 2005; Ji et al. 2007). They lack aryl isoprenoid alkane complexes, which shows that the kerogens are mainly derived from algal material of lacustrine origin, of the I-II1 type (Mu et al. 2001; Yang & Zhang 2005; Ji et al. 2007; Ma et al. 2016). The high residual organic matter contents, good types of kerogens with 0.9%-1.05 R0 but low of (S1+S2) (Table 2) indicate that the oil shales (source rocks) underwent strong hydrocarbon expulsion and a low ratio of saturated hydrocarbon to aromatic hydrocarbon: SH/AH 0.86-3.0) also suggests this (Yang & Zhang 2005).

The Chang 7 Member oil shale kerogen and “Chloroform bitumen” are enriched in the light carbon isotope 12C. The kerogen and “Chloroform bitumen” have a limited range of δ13C values, which are

-28.5 ～- 30.00‰and -32.2～ -33.00‰ (Yang & Zhang 2005), respectively, which shows that the kerogen formed in a terrestrial, fresh to low salinity water body.

Gas chromatography shows that the saturated hydrocarbon chromatogram is of unimodal type, and the main carbon peak is nC16 - nC19, showing an odd-even equilibrium with 0.95-1.21 of OEP. Pr/Ph is

0.56–1.17, Pr /nC17 is 0.11–0.33, and Pr /nC18 is 0.16–0.40, which also indicates a reducing environment. The low Pr/Ph, lower Pr /nC17 and Pr /nC18 ratios indicateMANUSCRIPT that the sedimentary environment was a deep reducing water body, and the source of the organic material was primarily lower aquatic organisms, in addition, it has reached the peak of the oil source mature phase (Yang & Zhang 2005; Zhang et al.2006;Ji

& Xu 2007; Ji et al.2007).

Hopane is composed primarily of C30αβ. The content of gammacerane and tricyclic terpane is low, and the content of Ts is high. Sterane is given priority to with regular Sterane, with preponderant C29, slightly low C28, low C22, and a high content of diasteranes. Both a low content of gammacerane and a high content of diasteranes indicate that the oil shale formed in a low salinity sedimentary environment (Yang & Zhang 2005). ACCEPTED Quality

Oil yield and calorific value are the most common parameters for evaluating oil shales (Yuan et al. 1979;

Smith 1980; The office of the National Committee of Mineral Reserves 1987; Zhao & Liu 1992; Guan et al.1995; Zhao et al. 1991; Dyni 2006a, b; Liu et al. 2006, 2009). The oil yield of the oil shale was measured by the Gray-King low-temperature dry distillation assay method following the Chinese standard

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methods (GB/T 1341-2007) (AQSIQ and SAC 2007), and the calorific value of the oil shale was measured by isothermal Oxidation Bomb Calorimetry following the Chinese standard methods GB/T

213-2008(AQSIQ and SAC 2008).

Based on our own and previously published data, the oil shale has an average oil yield of 8 wt%, a calorific value of 8.35MJ/kg (Net calorific value at constant volume), and an apparent specific gravity of

1.79 (Table 2).

The grade of oil shale can be divided into 3 types by oil yield of oil shale (dry basis), which is, respectively, low (3.5 wt % 10 wt % ) ( Liu et al.2009). The oil shale is medium quality.

The calorific value is useful for determining the quality of oil shale that is burned directly in a power plant to produce electricity. The calorific value of a given oil shale is a useful and fundamental property of the rock, although it does not provide information on the amounts of shale oil or combustible gas that would be yielded by retorting (destructive distillation). The oil shale is high grade compared with other

Chinese oil shale deposits, which have, respectively, average calorific values of 5.7 MJ/kg (Fushun),

7.3MJ/kg (Maoming), 7.0 MJ/kg (Yaojie), 3.6 MJ/kg (Nongan), 4.2MJ/kg (Dongsheng), 6.6MJ/kg (Huadian) , 4.2-5.0MJ/kg (Guyang) (Zhao et al. 1991;MANUSCRIPT Liu et al.2009), but it is low grade compared with the high-grade kukersite oil shale of Estonia, which fuels several electric power plants, has a calorific value of about 10.03 to 12.62MJ/kg on a dry –weight basis (Dyni 2006a,b). The higher calorific value are linked to the higher oil yields, TOC and lower Ad (ash content, dry basis)) in the oil shale (Figure 11 a, b, c).

The oil shale averages 69 wt % ash yield (dry basis): a high ash type (Zhao et al. 1991; Liu et al.2009).

The higher ash yield is linked to the lower calorific value and oil yield (Figure11 b, d). Considering the above data of the oil shale fusibility, it is a medium fusion, high ash type. The ACCEPTEDdata analysis indicates that there is an obvious positive correlation between the oil yields and Cad (carbon, air dry basis) (Figure10 e). The higher total sulfur content, the greater the potential environmental pollution in oil shale utilization. Oil shale can be divided into five levels, ultra-low sulphur (≤1.0 wt % ), low sulfur oil shale (1.0 -1.5 wt % ), medium sulfur (1.5 - 2.5 wt % ), rich sulfur (2.5 - 4.0 wt % ) and high sulfur (> 4.0 wt % ) (The office of the National Committee of Mineral Reserves 1987). The total sulfur is

4.69 wt %: indicating a high sulfur oil shale.

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Oil shale can be divided on moisture content into high moisture content (Mt of 20 -30 wt %), medium moisture content (Mt of 10 -20 wt % ), low moisture content (Mt of less than 10 wt %) (The office of the

National Committee of Mineral Reserves 1987).The oil shale has Mt of 3.37 wt %: a low moisture content oil shale.

The oil shale has an average density of 1.77 kg/m3，which is quite high, related to the higher silicon and aluminum components, and means a lower oil yield per tonne.

The oil shale has an average Vdaf (volatile, dry ash-free basis) of 68 wt %，which is quite high also, reflects the relatively high metamorphic grade, and relatively high organic matter content in the shale (Liu et al. 2009).

The average TOC of the oil shale is high (Table 2). The correlation is very obvious between the TOC and oil yield in the outcrop oil shale samples (Figure 11 f), but there is no obvious correlation between

TOC and (S1+S2).

The average content of Cad (Carbon, air dry basis) and Had (Hygrogen ,air dry basis) in the oil shale are, respectively, 19.08 wt % , and 2.13 wt % (Table 2), so, an average of 1.4 H/C is got. Ma ei al (2016) pointed out that the oil shale has an average 1.34 H/C, 0.1 O/C. Therefore the organic matter of the oil shale belongs to Type I and II1. Tissot & Welte (1978)MANUSCRIPT state that the kerogen of Type I has greater than 1.5 H/C, less than 0.1 O/C and the precursors of the kerogen are mainly from marine or continental deep-water lakes algae, bacteria; and the kerogen of II type has the value of 1.0 -1.5 H/C, of 0.1 - 0.2 O/C and the precursors of the kerogen are mainly from continental deep-bathyal lake spores and pollen, plankton, microorganisms and other mixed organic matter; and the kerogen of Ⅲ type kerogen has the value less than 1.0 H/C, higher than 0.2 O/C, the precursors of the kerogen are mainly from terrestrial higher plant.

Based on content of Cad and Had, H/Cand O/C in the oil shale, the organic matter is mainly derived from lacustrine algae, spores and pollen. Thus, “Carbon” in the organic matter of the oil shale is unlikely to have beenACCEPTED derived from seawater or carbonate minerals , with a probable lake water origin.

Origin

Classification of the Ordos Basin oil shale

Oil shales can be classified by their depositional environment (e.g.large lake, shallow marine, deltaic and lagoonal/small lake settings) (Carman & Bayes 1961; Surdam & Wolfbauer 1975; Yuan et al. 1979;

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Macauley 1981; Bruce 1982; Francis &Miknis 1983; Hutton 1987; Brendow 2003; Altun et al. 2006; Dyni

2006a,b; Ots 2007; lu et al.2006; Durham 2010). Oil shales of great lakes have large thicknesses and areas, and are of good quality. A typical example is the Green River oil shale in the northwestern United States, which is black in colour with a thickness of several hundred meters, and with an oil yield generally less than 15 wt % (Surdam & Wolfbauer 1975; Smith 1980; Bruce 1982; Dyni 2006a, b).

Shallow sea and continental shelf oil shales are generally much thinner than the large lake deposits and are associated with carbonates, siliceous and phosphatic facies. They do not exceed 2-3 m in thickness and are distributed over very large areas, up to thousands of km2 (Hutton 1987). They are black to light brown in color with a high oil yield (~20 wt %). A typical example is the Kukersite oil shale of Ordovician age in

Estonia, which is in a single calcareous layer, 2.5-3 m in thickness, with an average oil yield of 20 wt %.

Most of the organic matter is derived from green algae (Hutton 1987).

Oil shales deposited in lagoonal or small lake environments are rarely extensive and are often associated

Althrought havng a high oil yield, they are thin, and are unlikely candidates for commercial exploitation. A typical example is the Yaojie oil shale of Jurassic age in northwest China, which is black in color, 4-11m thick, with an oil yield of 4.6-8.9 wt %, and most of the organic matter is derived from macrophytes (Bai et al. 2010a). MANUSCRIPT The Chang 7 Member oil shale formed in a larger scale lake setting. The “Ordos lake” itself covering an area of 400,000 km2 with a maximum water depth of about 60 m (Yang et al. 2016b ) during the middle

Triassic, resembling the Green River oil shale (Surdam & Wolfbauer1975; Dyni 2006a, b; Smith 1980;

Bruce 1982). The oil shale covers an area of around 30,000 km2, and has an average thickness of 28 m and an average oil yield of 8 wt %.

The Chang 7 Member oil shale clay mineral content of 49% is similar to the composition of the Darden

Gulch oil shale seam of the Green River, which has a clay mineral content of 40-70%. However, it differs from theACCEPTED Kukersite oil shale in Estonia which has a clay mineral content of only 13.9%, and a carbonate mineral content of 56.1% (Hutton 1987).

The relatively low concentration of CaO, SiO2 and MgO, and relatively high concentration of P2O5 and

Fe2O3 and MgO /Al2O3 show that the lake was a coastal lake, lacked significant terrigenous matter inputs, and that the lake water had a high nutrient content. The coherence of the REE distribution patterns among the different samples indicates a consistent provenance. The Pr/Ph, Pr /nC17, Pr /nC18 ratios also indicate

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the biological source material is dominated by lower aquatic organisms (Yang & Zhang 2005; Ji & Xu,

2007; Ji et al.2007).

The oil shale formed in a reducing environment. Its surface is maroon after oxidation, indicating enrichment in Fe2＋, and thus a deep-water reducing environment. Pb, Cu, Mo and U are stronly enriched, the the ratios of V/Ni, U/Th, FeO/ Fe2O3, Pr/Ph, Pr /nC17, Pr /nC18 also indicate that lake was a strongly reducing environment.

The lake formed oil shale may be a freshwater to brackish water. The Sr/Ba ratio indicates that the lake was a freshwater lake, but M value of the oil shale indicates a transitional brackish water environment.

Both the low content of gammacerane and high content of diasteranes indicates that the oil shale formed in a low salinity sedimentary environment also (Yang & Zhang 2005).

The Sr/Cu ratio indicates a warm, humid climate.

Recent research shows that the sapropel group in the kerogens in Chang 7 Member oil shale contains abundant leiosphaeridia, which is multicellular macro red algae and/or chlorophytes, root in the lacustrine macroscopic algae, fomed in a freshwater environment, different in the Proterozoic and Paleozoic

Leiosphaeridia which is commonly thought as the marine unicellular phytoplankton (Ji & Xu 2007; Ji et al. 2007). Although Leiosphaeridia is abundant in theMANUSCRIPT area, it is not only monotone in species but also conspicuous in echinulate process, suggest that some marine acanthomorphic acritarches survived in fresh water, and had experienced long-time evolution. Therefore, the sedimentary environment of Chang 7

Member oil shale is lacustrine environment, which turned into the climax of lake transgression in Change 7 sedimentary interval, indicating the supply of the large-scale lake water body come from rivers instead of the rise of sea level (Ji & Xu 2007; Ji et al. 2007).

The limited range of δ13C values of “chloroform bitumen” shows that the kerogen formed in deep, reducing, low salinity water body. Considering that the composition of the kerogen is monotonous, it is conjecturedACCEPTED that the water body of the Ordos Basin was indistinctly stratified (Yang et al. 2005). A low gammacerane content and absence of aryl isoprenoid compounds in the kerogen structure of the oil shale also indicates that the lake basin was not significantly delaminated (Zhang et al. 2008a). Both the low content of gammacerane and high content of diasteranes indicates that the oil shale formed in a low salinity sedimentary environment (Yang & Zhang 2005). The Pr/Ph, Pr /nC17, Pr /nC18 ratios also indicate a reducing deep-water environment within which the biological source material is dominated by lower

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aquatic organisms (Yang & Zhang 2005; Ji & Xu, 2007; Ji et al.2007).

To sum up，the Ordos Basin oil shale formed in a deep-water reducing environment with a warm, humid climate context.The lake may be a freshwater or brackish water and indistinctly stratified. The biological source material is dominated by lower aquatic organisms.

Volcanism in the Ordos area

The andesitic to dacitic tuff interbeds in the Chang7 Member oil shale seams and the Yanchang Formation

(Figure 7c) indicate its formation close to a volcanic arc, and that the lake was a relatively high energy environment. In addition, the sandstone types in the upper and lower host layers of the oil shale seams are mostly feldspar quartz sandstone and arkose, also indicating a relatively high energy environment. The

Ordos Basin was not a stable intracratonic basin (Yang 2002), and was subject to relatively energetic sedimentary processes. Moreover, the angular sandy debris grains suggest a proximal provenance (Figure

9b).

As stated above, the Ordos Lake was a reducing sedimentary environment; however, the atmospheric oxygen level was not low at the time of the oil shale formation and questions arise regarding the origin of the reducing lake environment. Multiple layers of andesitic acid tuff (Figures 4, 7c) are present in the Yanchang Formation and the oil shale seams, thereforeMANUSCRIPT it is possible that their deposition was to some extent responsible for the reducing conditions in the lake basin. There may have been catastrophic death of organisms because of the ash falls, which may be the main reason why organic matter was enriched in the lake. At the same time, the tuff layers also provided nutrients for the next cycle of oil shale formation

(Yang & Zhang 2005).

Marine facies or lacustrine facies?

It's problematic that recently one paper proposed that Chang 7 Member oil shale in the Ordos Basin was deposited in the marine intrusion (Wang et al.2017). Their evidence is a typical marine coelacanth fossil with aACCEPTED rounded tail was found in the late Triassic stratum in Huachi county area, a broken marine coelacanth fossil was discovered in Tongchuan city area about 20 year age by Liu et al(1999). The research shows these marine organisms actually belong to "the terrestrial organism with sea origin" rather than the marine organisms (Liu et al.1999; Wang 1995), and the terrestrial organism with sea origin is the survival of early marine creatures in the lake and does not represent the seawater intrusion. In combination with the geochemical evidence described above (0.33 of Sr/Ba), it is proposed that Chang 7 Member oil

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shale in the Ordos Basin was principally deposited in a fresh or brackish water body, neither marine environment nor salinized lake.

In fact, including the Ordos Basin, the North China plate suffered the Qinling ocean plate subduction at the middle-late Triassic, resulting in sea level declining, in such a tectonic setting, how did seawater over the island arc belt and invade into the area?

Conclusion

Oil shale resources are abundant in the Ordos Basin in central north China. There are multiple oil shale seams in the basin, but the Chang7 Member oil shale seam is the main oil shale seam (MOSS) with a thickness of 28 m and an area of around 30,000 km2. The oil shale is usually in layers developed in the top of the lower part of the Yanchang Formation in the middle Triassic (Ladinian) age. The Yanchang

Formation deposited in a great lake in the middle to late Triassic (Ladinian to Norian). The oil shale is mainly brown-black to black in colour, of a medium ash type with a TOC of 18 wt %, an oil yield of 8 wt %, a calorific value of 8.35 MJ/kg and a relatively high P2O5 and Fe2O3 content. It is strongly enriched in Mo, U, and LREE, and is kerogen type I- II1. Volcanism may have favored the formation of the oil shale.

The oil shale formed in a large, deep-to-moderately deep lake, the Ordos Lake, with a low input of terrigenous material but abundant algal growth. TheMANUSCRIPT water is fresh or brackish and strongly reducing. The tectonic context of the lake is a back-arc basin which was formed by the northward subduction of the

Qinling oceanic lithosphere beneath the southern margin of the Ordos Kratogen during the middle - late

Triassic (T2-3).

ACCEPTED

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Acknowledgements

We thank Mr.Yang Jie (Dean of NWGI), Prof. Yang Hua, Prof. Wang Daxing, Senior engineer Sun Liuyi, Senior engineer Mao Minglu, Senior engineer Bao Hongping, and Senior engineer Ren Junfeng for their help in work. We thank Dr P. A. F. Christie for his valuable modification advice and Professor Jan Bloemendal for polishing paper. We also thank Bruce Levell, Co-Editor Petroleum Geoscience and two experts in the field for many good revision suggestions. References

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Figure list

Fig.1. (A) Location map showing two cross sectiosns through the Ordos Baisn X-X and Y-Y, together with outcrops of the middle-late Triassic and the oil shale; ( B) Cross sections X-X and Y-Y through the Ordos Basin showing the spatial distribution of the Chang7 Member oil shale and its host strata (modified and supplemented after Bai et al., 2009, 2010a, b) (a)Location map of the Ordos Basin in China ; (b) Location map of Fig 5 on Fig 1A. Key Q Quaternary System; N Neogene System; E Paleogene System; Mz Mesozoic; K Cretaceous System; J Jurassic System; T Triassic System; C-P Carboniferous to Permian Systems; ∈-O Cambrian to Ordovician Systems; Pt Proterozoic; Ar Archean

Fig.2. Strtigraphic chart summarizing the history of the Ordos Basin which is divided into 5 phases including the Meso-Neo Proterozoic, the early Palaeozoic, the late Palaeozoic, the middle to late Triassic and the Jurassic to the early Cretaceous (modified and supplemented after Bai et al. 2009, 2010a) (The width of the bars in the far-right column represents the impoirtance of the resource).

Fig.3. Tectonic profiles and background during the middle-late Triassic (Ladinian to Norian) in the Ordos areas. Above sketch shows a back-arc foreland basin result from subduction of the Qinling Oceanic plate and below sketch shows the location of the Ordos Basin in regional structure (after Wan 2004; Chen 2010; Felix et al.2012). MANUSCRIPT

Fig.4. Stratigraphic column indicating the position of the oil shale rich seams within the Triassic section.

Fig.5. Thickness distribution and burial depth of the Chang7 Member oil shale (the location is shown in on Figure 1 (A) (modified and supplemented after Bai et al. 2009, 2010a ; Yang and Zhang 2005).

Fig.6. Logging and organic geochemical profile of the Chang7 Member oil shale in the Li 57 well which is located in the midwest on Figure 1 (A), near the southeast of Huanxian county (after Yang et al. 2005, Wang et al. 2007). The legend is the same as Fig.4.

Fig. 7. ACCEPTEDExamples of outcrops and specimens of the Chang 7 Member oil shale in the Ordos Basin Key- a Hejafang village oil shale (mining face of oil shale in 1960); b Bawangzhuang village oil shale (note the layer structure); c Jinsuoguan town oil shale (note the oil shale layers interbedded with thin layer of greyish-buff tuff); d Bawangzhuang oil shale specimen (note the maroon color of the surface of oil shale after weathering). Locate these outcrops on Fig. 1A.

Fig.8. Shale mineral composition Triangular Diagram showing the Chang 7 Member oil shale

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characteristic in composition (modified and supplemented after Kuila & Prasad 2012) . Key- the square symbol shows the location of the average mineral composition of global shale regardless of the content of organic matter, which indicates the global shale generally has higher clay mineral content, but less quartz and feldspar content, and almost no calcite and dolomite content. Two ellipses all indicate the range of the Green river oil shale, of which the right ellipse are the distribution area of Parachute Greek oil shale, which symbolize by the black squares , and the left are the distribution area of Garden Gulch oil shale, which symbolize by the circles. The black rhombus is the location of the shales coming from all around the world and the triangle is the location of the Ordos Triassic oil shale.

Fig.9. The characteristics of the oil shale under a light microscope (after Bai et al., 2009; 2010a).Key a- remaining argillaceous texture, slab structure, weak sericitization (+); b - angular, subangular and rounded silt-sized mineral grains (feldspars) (+)

Fig.10. Chondrite-normalized REE distribution patterns of the Chang7 Member oil shale

Fig.11. The relations between key parameters of the Chang 7 Member oil shale

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Table 1. Major, Trace and Elements Rare-Earth Elements Analyses from the Chang7 Member oil shale Oxide Chang7 NASC2 Trace Chang7 NASC4 Rare-Earth Chang7 Chondrite6 NASC7 （wt%） Member Elements Member Elements Member oil shale (ppm) oil shale (ppm) oil shale (Average, ( Average, ( Average, N=54)1 N=43)3 N=8)5

SiO2 48.69 58.10 Mn 313.0 922.0 La 31.0 0.3 32.0

Al2 O3 14.40 15.40 Sr 197.0 142.0 Ce 56.0 1.0 73.0 TiO2 0.51 0.65 Ba 593.0 636.0 Pr 6.5 0.1 7.9 Fe2 O3 8.54 4.02 V 176.0 130.0 Nd 24.0 0.7 33.0 MgO 0.97 3.44 Zr 132.0 200.0 Sm 4.4 0.2 5.7 CaO 1.14 3.11 Rb 121.0 125.0 Eu 0.9 0.1 1.2 Na2 O 0.96 1.30 Cu 98.0 32.4 Gd 3.9 0.3 5.2 K2 O 2.72 3.24 Pb 34.5 20.0 Tb 0.6 0.1 0.85 Fe O 4.00 3.24 Zn 74.5 70.0 Dy 3.6 0.9 5.8 P2O5 0.30 0.17 Cr 65.2 125.0 Ho 0.8 0.1 1.0 Ni 22.5 58.0 Er 2.3 0.3 3.4 Co 17.1 26.0 Tm 0.4 0.1 0.5 Mo 59.1 3.1 Yb 2.5 0.2 3.1 U 31.9 3.0 Lu 0.4 0.1 0.48 Th 6.6 12.3 Y 23.0 1.9 24.0 ∑REE 160.5 160.5 197.0

Note：N number of samples Chang7 Member oil shale(N=54)1data compiled from Bai et al (2009), Zhang et al ( 2013,Wang et al (2016), Changqing oilfield company, PetroChina (2008), Sun et al ( 2015)andMiao et al (2005); NASC2 according to Gromet et al (1984). Chang7 Member oil shale (N=43)3data compiled from Bai et al ( 2009),Zhang et al (2013), Zhang et al (2008, Sun et al ( 2015, Miao et al ( 2005), Ma et al MANUSCRIPT(2016); NASC4 according to Grome et al (1984). Chang7 Member oil shale (N=8)5 data compiled from Ma et al (2016) and Bai et al ( 2009). Chondrite6 according to Taylor & Melennan ( 1985). NASC7according to Grome et al (1984). The analytical method: The analytical method for major elements is using X-ray fluorescence (XRF) in different laboratory following Chinese standards GB/T14506.14-2010 and GB/T14506.28-2010. The analytical method for microelement is using X-ray fluorescence (XRF) and Inductively-coupled plasma mass spectrometer (ICP-MS) following Chinese standards GB/T14506.30-2010.The analytical method for rare earth element is using X-ray fluorescence (XRF) and Inductively-coupled plasma mass spectrometer (ICP-MS) in different laboratory following Chinese standards GB/T14506.30-2010.

ACCEPTED

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Table 2. Proximate and Organic matter Analysis from the Chang7 Member oil shale Proximate Chang7 Member oil Organic matter abundance Chang7 Member oil Analysis1 shale2 Analysis3 shale4 items (Average, N=35) items (Average) Oil yield (wt%)/ 8.00 TOC(wt %) 17.76（N=72） Qnet,v,ar 8.35 Chloroform bitumen A (wt %) 0.4-1 (MJ/Kg) Ad (wt%) 69.24 S1(mg/g)（HC/ rock） 3.06(N=41) St,d (wt%) 4.69 S2(mg/g)（HC/ rock） 60.51(N=40)

Mt (wt%) 3.37 S3(mg/g)（CO2/ rock） 7.78(N=41) Vdaf (wt%) 68.16 S1+S2(mg/g )（HC/ rock） 70.00(N=76) Cad (wt%) 19.08 IH (mg/g) 407.80(N=434) Had (wt%) 2.13 IO (mg/g) 63.39(N=19) ARD (g/cm3) 1.77

Note: N number of samples Proximate Analysis1 Qnet, v, ar =Net calorific value at constant volume; Ad =ash content (dry basis); St,d =sulfur content(dry basis); Mt=Total moisture; Vdaf =volatile (dry ash-free basis); Cad =carbon (air dry basis); Had=hygrogen (air dry basis); ARD=apparent density. Chang7 Member oil shale2 data compiled from Bai et al (2009)， Changqing oilfield company, PetroChina (2008)， Ren (2007， Zhang et al (2006)， Lu et al (2006) and Zhang et al ( 2013). The analytical method for Oil yield is using Gray-King low-temperature distillation in different laboratory following Chinese standards GB-T1341-2007. The analytical method for ash yield is using fast ashing method in different laboratory following Chinese standards GB/T212-2008. The analytical method for calorific value is using the environmental isothermal automatic oxygen bomb calorimeter in different laboratory following Chinese standards GB/T213-2008. MANUSCRIPT3 Organic matter abundance Analysis : TOC( total organic carbon) –The content of residual organic matter in oil shale（%）. Chloroform bitumen “A” （%） is the ratio of the extracted bitumen mass to the mass of rock sample. S1 -The content of Soluble hydrocarbonr in oil shale（HC/ rock） (mg/g)。 S2 The content of Pyrolytic hydrocarbon in oil shale（HC/ rock）(mg/g)。S3 The content of

Pyrolytic carbon dioxide in oil shale（CO2/ rock）(mg/g)。S1+S2 Potential hydrocarbon generation amount（(mg/g（HC/ rock）, IH=QHC/COT×100; IO=QCO2 /COT×100（QHC is hydrocarbon from kerogen pyrolysis and extractable hydrocarbon components.QHC is hydrocarbon from kerogen pyrolysis and extractable hydrocarbon components; COT is total organic carbon; QCO2 is the number ofCO2.） The analytical method for Total Organic Carbon (TOC) is using the Carbon & Sulfur Determinator in different laboratory following Chinese standards GB/T19145-2003. ACCEPTEDThe analytical method for Chloroform bitumen A analysis is using Soxhlet extraction equipment in different laboratory following the enterprise standard of CN-PC SY/T5118-2005. The analytical method for Rock pyrolysis analysis is using therock-eval pyrolysis apparatus in different laboratory following Chinese standards GB/T18602-2001((Tmax(℃):425-450) Chang7 Member oil shale4 data compiled from Bai et al (2009), Changqing oilfield company, PetroChina (2008), Zhang et al(2013), Ma et al (2016), Ren (2007and Yang et al (2005, 2016).