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Natural gas potential of Neoproterozoic and lowerPalaeozoic marine shales in the Upper YangtzePlatform, South China: geological and organicgeochemical characterizationJingqiang Tana, Brian Horsfielda, Nicolaj Mahlstedta, Jinchuan Zhangb, Christopher J.Borehamc, Dorothee Hipplerd, Ger van Graase & Bruce Alastair Tochere

a Section of Organic Geochemistry, GFZ- German Research Centre for Geosciences,Potsdam, Germanyb School of Energy Resources, China University of Geosciences (Beijing), Beijing, Chinac Geoscience Australia, Canberra, Australiad Institute of Applied Geosciences, Technical University of Berlin, Berlin, Germanye Statoil, Oslo, NorwayPublished online: 09 Feb 2015.

To cite this article: Jingqiang Tan, Brian Horsfield, Nicolaj Mahlstedt, Jinchuan Zhang, Christopher J. Boreham, DorotheeHippler, Ger van Graas & Bruce Alastair Tocher (2015): Natural gas potential of Neoproterozoic and lower Palaeozoic marineshales in the Upper Yangtze Platform, South China: geological and organic geochemical characterization, InternationalGeology Review, DOI: 10.1080/00206814.2015.1004200

To link to this article: http://dx.doi.org/10.1080/00206814.2015.1004200

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Natural gas potential of Neoproterozoic and lower Palaeozoic marine shales in the UpperYangtze Platform, South China: geological and organic geochemical characterization

Jingqiang Tana*, Brian Horsfielda, Nicolaj Mahlstedta, Jinchuan Zhangb, Christopher J. Borehamc, Dorothee Hipplerd,Ger van Graase and Bruce Alastair Tochere

aSection of Organic Geochemistry, GFZ- German Research Centre for Geosciences, Potsdam, Germany; bSchool of Energy Resources,China University of Geosciences (Beijing), Beijing, China; cGeoscience Australia, Canberra, Australia; dInstitute of Applied

Geosciences, Technical University of Berlin, Berlin, Germany; eStatoil, Oslo, Norway

(Received 16 August 2014; accepted 31 December 2014)

In this article, we describe the geological features of the Ediacaran (upper Sinian), lower Cambrian and lower Silurian shaleintervals in the Upper Yangtze Platform, South China, and report on the gas potential of 53 samples from these majormarine shale formations. Reflected light microscopy, total organic carbon (TOC) measurement, Rock-Eval, carbon isotoperatio analysis, thermovaporization gas chromatography (Tvap-GC), and open pyrolysis gas chromatography (open py-GC)were used to characterize the organic matter. Measured TOC in this research is normally >2% and averages 5%. TOCcontents are roughly positively correlated with increasing geological age, i.e. lower Silurian shales exhibit generally lowerTOC contents than lower Cambrian shales, which in turn commonly have lower TOC contents than Ediacaran shales.Kerogen has evolved to the metagenesis stage, which was demonstrated by the abundant pyrobitumen on microphotographs,the high calculated vitrinite reflectance (Ro = 3%) via bitumen reflectance (Rb), as well as δ

13 C of gas (methane) inclusions.Pyrolysates from Tvap-GC and open py-GC are quantitatively low and only light hydrocarbons were detected. The lowerSilurian shale generally exhibits higher generation of hydrocarbon than the lower Cambrian and Ediacaran shale. Cooles’method and Claypool’s equations were used to reconstruct the original TOC and Rock-Eval parameters of these overmaturesamples. Excellent original hydrocarbon generation was revealed in that the original TOC (TOCo) is between 5% and 23%,and original S1+S2 (S1o+S2o) is ranging from 29 to 215 mg HC/g rock.

Keywords: shale gas; unconventional resource; organic geochemistry; hydrocarbon potential; marine shale; upper YangtzePlatform; South China

1. Introduction

The remarkable success of shale gas exploitation in theUSA has led to shale systems becoming increasinglyimportant exploration targets worldwide. In China, differ-ent black shale formations ranging from Precambrian toCenozoic in age are known (Figure 1A–C). Technically,recoverable shale gas reserves are estimated to range from10 to 45 × 1012 m3 (Zhang et al. 2009; Dong et al. 2010;Liu et al. 2010; EIA 2011, 2013; Zou et al. 2011).Exploration activities are already underway in southern(Sichuan Basin, Yangtze Platform) and northern (e.g.Ordos Basin, Songliao Basin) parts of the country.

The Upper Yangtze Platform (UYP), where the SichuanBasin is located, is one of the largest conventional naturalgas provinces of China, accounting for 22–43% of theannually produced gas over the last 20 years (NationalBureau of Statistics and Energy Bureau 1991-2010). Therecoverable reserves of conventional gas have been esti-mated at around 1.7 × 1012 m3, with annual yields exceed-ing 23 × 109 m3 in the year 2010 (Ma et al. 2010). Inaddition to conventional petroleum, the lower Silurian andlower Cambrian shale formations have been regarded as

two prolific unconventional shale gas reservoirs with reco-verable reserves around 19.6 × 1012 m3 (EIA 2011).Although only two formations were reported in that evalua-tion and shale gas reserves outside the Sichuan Basin werenot considered, reported unconventional reserves are sig-nificantly higher than the total reserves of conventionalpetroleum (Ma et al. 2010; EIA 2011). Moreover, theEdiacaran and upper Permian marine shale, the UpperTriassic transitional shale (from marine to lacustrineenvironment) and the Lower Jurassic lacustrine shale alllocally possess shale gas potential in the UYP (Zouet al. 2010; Chen 2011; Guo et al. 2011; He andZhu 2012; EIA 2013; Li et al. 2013; Wang andWang 2013; Wu et al. 2013; Wang et al. 2013b; Zhenget al. 2013b; Chen et al. 2014; Liu et al. 2014).

Studies on shale gas plays in the USA have shown thatthe type, abundance, and maturity of organic matter, aswell as the grain size and mineralogy, determine thesource, reservoir, and seal rock properties of gas shales(Schoell 1980; Martini et al. 1998; Curtis 2002;Bowker 2007; Wang and Carr 2013). Prolific shale playsare normally recognized by certain minimal technical

*Corresponding author. Email: jtan8@central.uh.edu

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thresholds, which should be fulfilled. These include butare not limited to shale thickness, organic matter richness,maturity, mineralogy, porosity, permeability, pore pressure,and gas-in-place (Curtis 2002; Bowker 2007; Jarvieet al. 2007; Strąpoć et al. 2010; Miceli Romero andPhilp 2012). Zhao et al. (2007) stated that shale thickness,depth, total organic carbon (TOC), and thermal maturityare the most important geological factors for commercialgas production. The minimum net thickness for a shale gasplay to be successfully developed amounts to approxi-mately 10 m, for example for the Fayetteville, Antrim,and Ohio shales, while the depth can be lower than 200 min the case of the Antrim shale, containing biogenic gas, orup to 5 km in the case of the Haynesville shale(Bowker 2007; Hammes et al. 2011). Meanwhile, explora-tion targets are usually thick, widespread, gas-saturated,fine-grained, organic-rich units with (1) significant Type IIorganic matter in a thermally overmature state, (2) richorganic matter of any type that is thermally immature andsubject to extensive biogenic decomposition, and/or (3)significant Type III organic matter in thermally mature toovermature states (Martini et al. 2003; Hamblin 2006).

Some of these general principles have already beenapplied to Chinese gas shales. In the UYP, the Ediacaran,lower Cambrian, and lower Silurian black shale formationsdeposited in marine environments are widespread.

Generally speaking, these black shales are thick succes-sions (from 10 m to >100 m), have TOC content >2% andhigh quartz content, and are variable but usually have highlevels of thermal maturity. Burial depths of these forma-tions can be shallow, such as at outcrop in the marginalregions of the Sichuan Basin and in the southeast UYP, ordeep, reaching >5 km in some depressions (Chenget al. 2009; Zhang et al. 2009, 2013; Wang et al. 2009b;Zou et al. 2010, 2011; Nie et al. 2011; Huang et al. 2012;Song et al. 2013; Tan et al. 2013, 2014a, 2014b; Zhenget al. 2013a; Fan et al. 2014; Guo and Zhang 2014; Luoet al. 2014; Zhou et al. 2014). In a strict sense, however,issues remain on the composition of organic matter, thermalevolution and maturity, and thus the original hydrocarbonpotential and hydrocarbon generation properties have notbeen clearly clarified.

Because of the importance of these marine shale for-mations, we have undertaken a series of investigations ontheir petroleum physical properties, mineralogy, lithofa-cies, petrophysics, rock mechanics, and methane sorptioncapacity using core and outcrop studies, and the resultshave been summarized and published in Marine andPetroleum Geology (Tan et al. 2013), Energy & Fuels(Tan et al. 2014a), and Fuel (Tan et al. 2014b). In orderto explore their natural gas potential more systematicallyand comprehensively, the objectives of this article are to

Figure 1. Black shale distribution map (A–C), modified after Zhang et al. (2009); Zou et al. (2010), and China tectonic framework andlocations of the Upper Yangtze platform (UYP) and the Sichuan Basin (D), modified after Zhang et al. (2011).

TR-P, Tarim Platform; YZ-P, Yangtze Platform; NC-P, North China Platform; JG-F, Junggar fold system; TS-F, Tianshan fold system;EK-F, East Kunlun fold system; QI-F, Qilian fold system; SG-F, Songpan-Ganzi fold system; GN-F, Gangdise-Nyainqentanglha foldsystem; SJ-F, Sanjiang fold system; HY-F, Himalayan fold system; MD-F, Mongolia-Daxinganlin fold system; JH-F, Ji-He fold system;ND-F, Nadanhada fold system; QL-F, Qinling fold system; SC-F, South China fold system; SE-F, Southeast China fold system; UpperYP, Upper Yangtze Platform; Middle YP, Middle Yangtze Platform; Lower YP, Lower Yangtze Platform.

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(1) describe the depositional and evolutional history of theUYP and the present geological features of these marineshale formations, and (2) investigate their organic geo-chemical properties, for example organic richness, thermalmaturity, kerogen type, as well as their present and origi-nal hydrocarbon potential.

The construction of organic geochemical properties forthe shale intervals under study is especially difficult becausethe organic matter is overmature (Ro �2.0%). Therefore, aseries of organic screening methods, for example, TOC,Rock-Eval, microcopies, carbon isotope ratio, thermovapor-ization gas chromatography (Tvap-GC), and open-systempyrolysis gas chromatography (open py-GC), were per-formed on representative samples from the central area ofthe UYP. In conjunction with an extensive data compilation,the organic geochemical properties are then revealed indetail. Finally, original organic abundance and hydrocarbongenerative potential were reconstructed via the mass bal-ance models developed by Cooles et al. (1986) and viaClaypool’s equations (Peters et al. 2005).

2. Geological characterization

2.1. Tectonic and sedimentary history

The present-day tectonic framework of China is a mosaiccomposed of three platforms and fifteen-fold systems, andthe UYP is located in the western zone of the YangtzePlatform (Zhang et al. 2011) (Figure 1D). In the lateProterozoic (early Sinian), the basement and boundaries ofthe Yangtze Craton were created in the course of theChengjiang tectonic movement (Editorial Committee ofthe Sichuan Oil & Gas field 1989; Guo 1996). The earliestsedimentary rocks within the UYP, consisting of blackshales and dolomites, were deposited during a major trans-gression in the Ediacaran (late Sinian). The Tongwan tec-tonic movement at the end of the Proterozoic caused adisconformity between the Ediacaran and lower Cambrianstrata (Ma et al. 2007). The second major marine transgres-sion occurred in the early Cambrian. Sedimentary succes-sions, consisting of shale, siltstone, limestone, anddolomite, were laterally deposited extensively in open torestricted marine environments. Between the middleCambrian and Ordovician, depositional environments weregenerally maintained and geographically similar in ancientcontinent/islands that were located in the west, while thelittoral, restricted, and open marine platform and deep mar-ine shelf were developed southeastwards. Carbonates weretherefore deposited in most of the regions, in addition to thedeposition of minor amounts of clastic material occurringalong the western margin. During the early Silurian, thethird major transgression occurred southwestwards, result-ing in the widespread deposition of black shales that con-tributed significantly to the charging of oil and gas fields inthe eastern Sichuan Basin. However, the late Caledonian

orogeny impacted on the whole area at the end of theSilurian. Structural highs and lows in the central and south-ern UYP were markedly formed, which affected sourcerock sedimentation and preservation. As a result of thelate Caledonian orogeny and the subsequent Liujiang andYunnan orogeny, most of the regions in the UYP were incontinental environments during the Devonian andCarboniferous. Consequently, Devonian and Carboniferousterrestrial/continental sedimentary successions were con-fined locally to the east and south of the area, whereas thelower Palaeozoic strata underwent prolonged erosion. Thefourth major marine transgression commenced during theearly Permian and was interrupted only shortly by theDongwu uplift in middle Permian times. Open to restrictedmarine environments re-established again in late Permiantimes and persisted until the Middle Triassic. Coal-bearingshales and carbonates were periodically deposited and pro-vided important source and reservoir rocks (EditorialCommittee of the Sichuan Oil & Gas field 1989;Guo 1996; Ma et al. 2006; Hao et al. 2008; Wanget al. 2013a; Zheng et al. 2013a).

The Indosinian movement in the Middle–Late Triassic isa pivotal event to the tectonic and environmental evolution.The entire UYP was escalated during the course of the inten-sive compression between the South China Plate, the NorthChina Plate, and the Pacific Ocean Plate. As a result, marinesedimentation diminished while terrigenous sedimentationprevailed. A series of structural highs and lows were thencreated during the Yanshanian orogeny in the middle–lateMesozoic and the Himalayan orogeny in the Cenozoic, indi-cating that the tectonic and stratigraphic architectures of theUYP were almost formed. In the Sichuan Basin, thousands ofmetres of clastic strata were deposited, providing importantpetroleum source, reservoir and seal rocks, whereas theregions surrounding the basin in the UYP experiencedremarkable erosions (Editorial Committee of the SichuanOil & Gas field 1989; Korsch et al. 1991; Dai et al. 1992;Guo 1996; Ma et al. 2007, 2010; Hao et al. 2008).

2.2. Petroleum systems of the UYP

In the UYP, marine sediments are characterized by thickcarbonates and shale formations, and terrigenous sedimentsare characterized by fluvio-deltaic strata, coal measures, andlacustrine carbonates or shales (Editorial Committee of theSichuan Oil & Gas field 1989). As far as conventionalpetroleum is concerned, the regional source rocks are thelower Cambrian, lower Silurian, and Lower Jurassic mud-stones and shales, the lower Permian carbonates, and theupper Permian and Upper Triassic coal-bearing shales(Huang et al. 1997). Principal reservoirs comprise theChangxing (upper Permian) and Feixianguan (LowerTriassic) carbonates and the Xujiahe (Upper Triassic) andJurassic terrigenous clastics. They account for over 70% ofthe total conventional reserves in the Sichuan Basin (Dai

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et al. 2009; Ma et al. 2010). Significant seal rocks includeanhydrite, halite, and gypsiferous dolomite, which devel-oped in a major marine regression during Early to MiddleTriassic times (Ma et al. 2006; Hao et al. 2008). The ear-liest thermogenic hydrocarbon generation period has beenreported to be during the Silurian, when Precambrian andlower Cambrian source rocks were buried almost 2 km,ostensibly leading to the formation of small petroleumaccumulations in the east and south Sichuan Basin(Huang et al. 1997). However, the major periods of hydro-carbon generation, migration, and accumulation are thoughtto have occurred between the Triassic and Cretaceous,when the most important source rocks were buried succes-sively to more than 2 km, and reservoir seals became

effective (Editorial Committee of the Sichuan Oil & Gasfield 1989; Dai et al. 1992; Guo 1996; Ma et al. 2010)(Figure 2).

More than a hundred conventional petroleum fields orstructures have been found in the UYP, but most of themare gas fields located in the Sichuan Basin. Within thisbasin, commercial-scale oil fields are confined to the cen-tral area. Gas fields are volumetrically more important andhave preponderance in the south, while most giant gasfields (reserves > 30 × 1012 m3) are situated in the eastof the basin (Editorial Committee of the Sichuan Oil &Gas field 1989; Zhu et al. 2006; Ma et al. 2010). Thebasin is therefore normally divided into four petroleumregions according to the prevalent petroleum type (oil or

Figure 2. Generalized stratigraphic column and petroleum system event chart in the Sichuan Basin, modified after Editorial Committeeof the Sichuan Oil & Gas field (1989).

Note: The lower Cambrian shale formation has several local names, usually Jiulaodong or Qiongzhusi in the Sichuan Basin and Niutitangin the Chongqing area and Guizhou province.

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gas) and scale. They are the western gas field region,central oil-gas field region, eastern giant gas field region,and southern gas field region.

2.3. Black shale intervals under investigation

A total of eight major black shale formations can beidentified in the UYP. The Doushantuo Fm. (Ediacaran),Qiongzhusi/Niutitang Fm. (lower Cambrian), Wufeng Fm.(Ordovician), Longmaxi Fm. (lower Silurian), Luofu Fm.(Devonian), and Longtan Fm. (upper Permian) are themajor marine/transitional shale formations, whereas theXujiahe Fm. (Upper Triassic) and Ziliujin Fm. (LowerJurassic) are the principal lacustrine shale formations(Figure 2) (Editorial Committee of the Sichuan Oil &Gas field 1989; Liang et al. 2008; Dong et al. 2010; Zouet al. 2010). In this article, we primarily focus on thelower Silurian and lower Cambrian black shale intervals,

and to some extent on the Ediacaran black shale formationthat occurs in the upper part of Precambrian strata.

The lower Silurian shale thickens in the southeastSichuan Basin, Chongqing area, and west Hubei province(50–200 m), but it has lower thickness in most otherplaces in the UYP (Wang et al. 2009b; Dong et al.2010) (Figure 3). This distribution can be traced back toa regression of open marine conditions resulting fromuplifts of tectonic blocks in the southern and westernUYP, and thus the lower Silurian shale was deposited ina restricted marine environment (Zheng et al. 2013a; Tanet al. 2014a). In the Sichuan Basin, the lower Siluriansuccession can be subdivided into the LongmaxiFormation and Xiaoheba Formation. The bottomLongmaxi Formation is abundant in Graptolite, for exam-ple, Glyptograptus persculptus, Pristiograptus cyphus,and Demirastrites triangulates, and consists of blackshale, green sandy shale, siltstone, and argillaceous

Figure 3. Isopach maps of the lower Silurian (A) and lower Cambrian (B) shale formations in the Yangtze Platform, modified afterLiang et al. (2008); Zou et al. (2011), and the locations of investigated profiles and wells.

Sampled lower Silurian profiles or wells are 1 –YY1#well, 2 –Guankou, and 3 – Lujiao. Sampled lower Cambrian profiles are 4 – Jinggang, 5– Rongxi, 6 – Cenggong, and 7 –Heishapo. Data compiled lower Silurian profiles or wells are 10 – Zhenba-Guanyin, 11 – Shizhu-Qiliao, 12 –Shizhu-Dafengao, 13 –Dingshan 1# well, 14 –Xishui-liangchun 5# well, and 15 – Changxin 1# well. Data compiled lower Cambrian profilesor wells are 16 – Nanjiang-Shatang, 17 – Guangyuan, 18 – Gaoke 1# well, 19 – Wei 001-2# well, 20 – Fanshen 1# well, and 21 – Majiang–Yangtiao. In addition, two sampled Ediacaran profiles denoted in the bottom plot are 8 – Maoshi and 9 – Lijiatuo.

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limestone (Chen et al. 2013; Qiu et al. 2013; Wanget al. 2014). The top Xiaoheba Formation contains fossilssuch as Graptolites, Trilobites, Brachiopods, etc., andcomprises grey-green siltstone, fine stone, shale, and bio-logical limestone lenses with distinct ripples and biologi-cal traces (Editorial Committee of the Sichuan Oil & Gasfield 1989). The thickness of the complete lower Siluriansuccession ranges between 100 and 500 m in the SichuanBasin, but exceeds 700 m in the southeast Sichuan andChongqing area. In the Longmaxi Formation, organicmatter contents decrease while siliceous minerals increasefrom the bottom upwards. It has been recognized as one ofthe principal contributors to large- and medium-sized gasfields, for example the Puguang and Hechuan giant gasfields (Editorial Committee of the Sichuan Oil & Gasfield 1989; Huang et al. 1997; Wang et al. 2009b; Maet al. 2010). In this investigation, the analysed lowerSilurian samples are dominantly siliceous shale/mudstoneand consist of eight outcrops (Sample ID 01–08, Table 1)collected from the Lujiao and Guankou profiles and sixcores (Sample ID 09–14) sampled from the YY1 shale gaswell (Figures 3 and 4) with depths ranging from the topdown at 136, 207, 267, 276, 285, and 321 m.

The lower Cambrian black shale usually exhibits grossthicknesses of 50–200 m in most of the regions in theYangtze Platform, but it is thinner in the central SichuanBasin and the western and southern marginal areas of theUYP (Figure 3). This feature can be related to the marinetransgression during the early Cambrian. In the UYP, thepresence of thick black shale packages indicate marineplatform to marine shelf depositional environments,whereas the presence of thin black shale packages indi-cates a littoral depositional environment (Donget al. 2010; Wang et al. 2013a; Luo et al. 2014). Thelower Cambrian interval disconformably overlays theuppermost Ediacaran carbonates (Figure 2), with the stra-tigraphic boundary consisting of phosphate-containingsiliceous dolomite, phosphorites, sandy dolomite, and cal-careous shale. Three subdivisions can be recognized onthe basis of lithology, biota, and texture. The bottomcomprises Protospongia containing black shale or carbo-naceous shale with pyrite, phosphate, and siderite nodules,and exhibits a distinct laminated bedding. The middle isgrey-black sandy or silty shale, accompanied by calcar-eous sandstone, fine sandstone and dolomite, and rich infossils, for example containing trilobites, sponge spicules,or brachiopods. The top consists of juxtaposed carbonate,siltstone, and shale, containing various trilobite species,such as Eoredlichia, Hebediscus, and Yunanocephalus(Editorial Committee of the Sichuan Oil & Gasfield 1989). Organic richness, strata colour, and laminationvary gradually from the bottom upwards (Chenget al. 2009; Wang et al. 2009b). Black shale occurs pri-marily in the middle or bottom intervals of the lowerCambrian formation. It is characterized by high gamma Ta

ble1.

Sam

pleinform

ation,

totalorganiccarbon

content,andRock-Evaldata.

Unit

Profile/area

OriginalID

IDDepth

(m)

TOC(%

)S1(m

g/g)

S2(m

g/g)

Tmax

(°C)

HI(m

gHC/g

TOC)

OI(m

gCO2/g

TOC)

Low

erSilu

rian–L

ongm

axi

Lujiao,

Cho

ngqing

G00

9507

13.34

0.02

0.28

439

814

G00

9508

23.76

0.05

0.43

598

1116

G00

9509

31.68

0.03

0.41

331

2465

G00

9510

41.84

0.02

0.31

336

1720

G00

9511

50.93

0.02

0.35

599

3869

Guank

ou,Cho

ngqing

G00

9512

63.89

0.02

0.33

601

814

G00

9513

74.00

0.04

0.48

600

128

G00

9514

83.64

0.02

0.33

599

923

YY1well,Cho

ngqing

G00

9524

913

61.96

0.04

0.37

409

1935

G00

9525

1020

71.47

0.05

0.42

405

2931

G00

9526

1126

72.31

0.04

0.43

600

1922

G00

9527

1227

61.69

0.02

0.33

400

2020

G00

9528

1328

51.68

0.02

0.26

409

1522

G00

9529

1432

12.21

0.03

0.34

313

1545

Low

erCam

brian–Niutitang

Ron

gxi,Cho

ngqing

G00

9515

150.44

0.02

0.38

291

8624

0G00

9516

1610

.20

0.02

0.37

593

419

G00

9517

176.02

0.02

0.33

365

58

(Con

tinued)

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Table1.

(Con

tinued).

Unit

Profile/area

OriginalID

IDDepth

(m)

TOC(%

)S1(m

g/g)

S2(m

g/g)

Tmax

(°C)

HI(m

gHC/g

TOC)

OI(m

gCO2/g

TOC)

Jing

gang

,Cho

ngqq

ing

G00

9518

187.44

0.05

0.62

281

86

G00

9519

198.15

0.03

0.43

565

515

G00

9520

209.41

0.02

0.35

565

44

Cengg

ong,

Guizhou

G00

9521

215.04

0.03

0.31

600

66

G00

9522

225.79

0.04

0.39

598

715

G00

9523

232.85

0.01

0.23

501

848

Heishapo,

Guizhou

He25d

240.44

0.03

0.09

445

2012

0He25e1

250.52

0.04

0.11

441

2176

He25e2

260.39

0.02

0.30

404

7710

5He29

277.79

0.05

0.44

413

624

He30

285.80

0.03

0.34

522

651

He31

297.69

0.01

0.20

411

310

Ediacaran

Maoshi,Guizhou

Ma32-2

304.42

0.06

0.12

317

318

Ma25

312.59

0.02

0.31

403

1226

Ma26

323.30

0.08

0.63

421

1980

Ma32-1

335.05

0.04

0.39

312

819

Ma34

341.89

0.04

0.39

403

2161

Ma38

351.43

0.02

0.38

582

2718

Lijiatuo

,Hun

anLi3

360.90

0.02

0.05

409

630

Li7b

372.10

0.02

0.06

446

326

Li8

380.43

0.01

0.27

400

6328

8Li9a

396.15

0.14

0.09

406

123

Li12b

400.90

0.04

0.05

609

649

Li19a

415.90

0.13

0.15

439

318

Li20b

421.20

0.02

0.03

413

324

Li21c

431.00

0.04

0.21

474

2119

9Li22

441.73

0.03

0.05

451

334

Li25a

4512

.70

0.46

0.26

283

210

Li26a

467.60

0.08

0.09

414

128

Li27

4713

.50

0.23

0.12

422

127

Li28

4819

.00

0.55

0.32

412

26

Li29

497.90

0.31

0.16

290

24

Li30

501.30

0.12

0.14

449

1129

Li33

5111.40

0.31

0.09

450

116

Li35

524.90

0.05

0.10

467

241

Li37

535.30

0.11

0.08

513

239

Note:

The

locatio

nsof

theprofi

lesandwellsaredeno

tedin

Figure3.

TOCistotalorganiccarbon

(wt.%);S1isexistin

ghy

drocarbon(m

ghy

drocarbo

n/grock);S2isgeneratedhy

drocarbo

nfrom

kerogen

(mghy

drocarbo

n/grock);HIishy

drog

enindex(m

ghy

drocarbo

n/gTOC);andOIisox

ygen

index(m

gCO2/g

TOC).

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responses with values between 70 and 150 API – inferringhigh TOC contents of the shale (Cheng et al. 2009). Thelower Cambrian shale has been recognized as the sourcerock of the Weiyuan gas field, which possesses the oldestreservoir rock (Ediacaran dolomite) of all Chinese petro-leum fields (Editorial Committee of the Sichuan Oil & Gasfield 1989; Xu et al. 1989; Huang et al. 1997; Dai 2003;Ma et al. 2010). In the research reported here, a total of 15lower Cambrian samples comprising siliceous or

carbonaceous shale/mudstone and phosphorite were inves-tigated. They were collected from the Rongxi andJinggang profiles (Sample ID 15–20, Table 1) in theChongqing area, the Cenggong profile in the Guizhouprovince (Sample ID 21–23), and the Heishapo profile inthe Hunan province (Sample ID 24–29) (Figures 3 and 4).

The Ediacaran black shale has a more restricted dis-tribution than the lower Silurian and lower Cambrian shaleand locally crop out to the southeast of the Sichuan Basin,

Figure 4. Outcrop photographs and selected hand specimens. Profiles: (A) lower Silurian shale in the Guankou profile, Chongqing area(ID:2); (B) lower Cambrian black (bottom) and Siliceous (upper) shale in the Jinggang profile, Chongqing area (ID4); (C) lowerCambrian black shale (left) in the Cenggong profile, Guizhou province (ID:6); and (D) lower Cambrian siliceous shale (right) in theCenggong profile, Guizhou province (ID:6). ID numbers of these profiles are indicated in Figure 3. Samples: (E) lower Siluriancalcareous shale sample from the Lujiao profile, Chongqing area (ID:1); (F) distinct horizontal laminations in the lower Cambrianshale, sampled from the Rongxi profile, Chongqing area (ID:6); (G) lower Silurian fossiliferous (graptolite) shale sample from theGuankou profile, Chongqing area (ID:21) (Tan et al. 2014a); (H) lower Silurian mudstone with big pyrite accumulation or segregation,sample from the YY1 well (ID:35); (I) fractured lower Silurian mudstone filled with quartz, sampled from the YY1 well (ID:5) (Tanet al. 2014a); (J) lower Cambrian carbonaceous shale sample from the Cenggong profile, Guizhou area (ID:9); (K) Ediacaran phosphaticshale sample from the Maoshi profile, Guizhou province (ID: 12); (L) Ediacaran black shale from the Lijiatuo profile, Hunan province(ID: 26). ID numbers of these samples are indicated in Table 1.

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north Guizhou province, and west Hunan and Hubei pro-vinces. A total of 24 Ediacaran samples (Sample ID 30–53, Table 1) collected from two profiles in Guizhou andHunan provinces (Figures 3 and 4) were investigated;here, the lithofacies comprised mudstone, siliceous shale,chert, and phosphorite (Table 1).

3. Methods for organic geochemical characterization

3.1. Sample preparation

Samples were broken by a hammer to make small frag-ments and/or cut into small rectangular strips by a water-cooled saw to make sections for light microscopy. Quartzand calcite minerals were selected from roughly crushedfragments for fluid inclusion analysis. The remaining mate-rials were then ground to 200–250 μm for measuring TOC,Rock-Eval, Tvap-GC, and open py-GC.

3.2. Electronic microscopy

Prepared thin and polished sections were investigated underplane-polarized, cross-polarized, and reflected white lightusing a Zeiss Axiophot Electronic Microscope System,which was equipped with a Carl Zeiss Axiocam digitalcamera and Carl Zeiss Axiovision 2.0 software, and iscapable of taking high-resolution photomicrographs withmagnifying lenses of 10×, 20×, 30×, 40×, and 50×.

3.3. Carbon isotope ratio of fluid inclusions

A total of four samples that had fluid inclusions were ana-lysed for stable isotope ratios using an Elemental Analyser(EA)-IRMS system. This system comprises a crusher, a GC-column, an EA, a ConFloIII interface, and a ThermoDeltaplusXL mass spectrometer, and allows the simulta-neous measurement of the stable isotope ratios of gas mix-tures, for example, N2, CH4, and/or CO2 (Lüders et al. 2012).

3.4. Rock-Eval and TOC measurements

In order to characterize the organic matter richness andthermally extractable total hydrocarbons in the rock, allsamples were measured for TOC and Rock-Eval (Espitaliéet al. 1977). Applied Petroleum Technology AS (Norway)measured these parameters using a Leco SC-632 and aRock-Eval 6 device (Behar et al. 2001).

3.5. Tvap-GC and open py-GC

For Tvap-GC, roughly 20–30 mg of finely ground powderwas weighed into glass capillaries, which were then sealedwith an H2-flame after reducing the internal volume of thetube from ca. 40 ml to ca. 15 ml with pre-cleaned quartzsand. The sealed tubes were then introduced into a

Quantum MSSV-2 Thermal Analyser pyrolysis oven unitinterfaced with an Agilent 6890A GC apparatus equippedwith an HP-Ultra 1 column of 50 m length, 0.32 mminternal diameter and 0.52 µm thickness and an FID. Theexternal surfaces of each tube were purged for 5 min at300°C, during which time volatiles were also mobilizedwithin the tube and thereafter cracked open by a pistondevice to transfer generated products from within thevessel by helium carrier gas with a flow rate of 30 ml/min into a liquid nitrogen-cooled trap. The trapped hydro-carbons were released to the column by ballistic heating ofthe trap to 300°C, and then thermovaporization productswere recorded. Py-GC was performed on the remainingpowder by heating from 300–600°C using a 50°C/minheating rate (3 min, isothermal). Pyrolysis products wereagain collected in the cryogenic trap and further analysedusing the same GC conditions as for Tvap-GC. The GCoven temperature was programmed from 30 to 320°C at5°C/min. Quantification of individual compounds and totalswas conducted by external standardization with n-butane.Reproducibility of measured product concentration wasgenerally better than 4% (Schenk et al. 1997). Because ofthe lower response factor of methane compared to theheavier gases, the methane content was multiplied by 1.1.

3.6. Reconstructing the original TOC and hydrocarbonpotential

Cooles’ method (Cooles et al. 1986) and Claypool’s equa-tions (Peters et al. 2005) were used to reconstruct originalTOC and Rock-Eval parameters of the overmature sam-ples from the UYP, and the original hydrocarbon genera-tive potential is revealed as input parameter for massbalance calculations.

4. Results and discussions

4.1. Organic matter richness

Based on thin section and polished section microscopy on30 samples, the matrix was seen to be composed of quartzand clay minerals, with abundant pyrite and organic mate-rial (Figure 5). In the thin section photomicrographs(sample 6 and G011315), dark coloured amorphous organicmatter clusters can be observed within the quartz and clayor shale matrix, as well as along the crinkle to wavylamination. In polished section photomicrographs (sample2 and 20), organic matter appears as grey aggregates show-ing high reflectance (approximately 4%). Organic matter ofthe examined sample accounts for 20–30% of the photo-micrograph area (sample 20). Its high level of reflectancepoints to the particles being spent kerogen or pyrobitumen(Tissot and Welte 1984; Horsfield and Rullkötter 1994).

The TOC (wt. %) and Rock-Eval data are listed inTable 1. Figure 6 illustrates that the TOC content falls

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Figure 5. Organic matter in section photomicrographs of selected and representative shale samples. (A) – Organic matter (pyrobitumen)and pyrite framboids (polished section with reflected light) (ID:2); (B) – disseminated pyrobitumen in shale matrix, with light reflectanceof the bitumen is over 4.0 % (polished section, reflected light) (ID:20); (C) – unlaminated siliceous shale showing organic matter (black),platy clay or mica minerals and subrounded to angular detrital quartz grains (white) (thin section, plane polarized light) (ID:6); (D) –siliceous shale showing graded bedding with crinkle to wavy laminated organic matter in the bottom and lower organic content in themore granular top (thin section, plane polarized light) (ID:23). ID numbers are indicated in Table 1.

Figure 6. Histograms of TOC content in relation to stratigraphic age.

The TOC content almost positively correlates with increasing geological age. Sample IDs are denoted in Table 1.

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between 1% and 19%, with TOC content almost positivelycorrelated with increasing geological age, i.e. theLongmaxi shales (lower Silurian) exhibit generally lowerTOC contents than that of the Niutitang shales (lowerCambrian), which commonly exhibit lower TOC contentsthan that of the Ediacaran shales. The TOC content of theLongmaxi shales mainly range from 1–3% showing thelowest average value of 2.5% of all investigated samples.The TOC values of the Niutitang shales range from0.4–10.2 % with an average value of 5.2%. Particularly,the TOC content values of phosphatic samples from theNiutitang interval are <1%. The highest TOC concentra-tion can be found for the Ediacaran shales for which halfof the samples exhibit >4% TOC, with maximum contentsexceeding 10%, at least in one profile located in thewestern Hunan Province.

The organic matter richness of the measured samplescan thus be described as moderate to very high, which isan important parameter for evaluating either source rock ina conventional petroleum system (Tissot and Welte 1984;Peters and Cassa 1994; Hunt 1995; Peters et al. 2005) orhydrocarbon potential and reservoir quality in unconven-tional petroleum systems (Curtis 2002; Montgomeryet al. 2005; Jarvie et al. 2007; Kinley et al. 2008; Rossand Bustin 2008; Rodriguez and Philp 2010;Hackley 2012). Clearly, the TOC content of the originalimmature equivalents of these samples was much higherthan the present values, the factor being dependent onoriginal kerogen type. In addition, high TOC may indicatean anoxic environment. The abundance of organic matterthus reflects the fact that the UYP sea floor was at times ofdeposition likely anoxic, for if the sea bottom had beenoxic, aerobic bacteria would have thrived and consumed

much of the organic material (Tissot and Welte 1984;Loucks and Ruppel 2007).

Published TOC results of marine shale in the UYP(Liang et al. 2008; Wang et al. 2009a, 2013a; Donget al. 2010; Guo and Liu 2013; Han et al. 2013; Guoand Zhang 2014; Luo et al. 2014) support the abovefindings. High TOC concentration is demonstrated by thehundreds of samples taken from eight profiles/wells withinthe UYP (Figures 3 and 7). Most of the profiles exhibithigher TOC concentrations in the lower part, where blackshale is present, and lower TOC concentrations in theupper part, where carbonates, mud, and/or siltstone dom-inate (Editorial Committee of the Sichuan Oil & Gasfield 1989; Liang et al. 2008; Cheng et al. 2009; Wanget al. 2014). In addition, the published isopach mapsshow that the thickness of shale units with TOC valuesexceeding 2% can reach 20–200 m in most of the regionsof southeast Sichuan, Chongqing, north Guizhou, and westHunan and Hubei (Dong et al. 2010; Guo and Zhang 2014).

4.2. Thermal history and maturity

The Ediacaran and lower Palaeozoic source rocks did notcommence to generate considerable thermogenic hydro-carbon until the late Permian to Early Triassic, followingsubsidence and re-deposition post the prolonged upliftcaused by the late Caledonian and Yunnan orogenybetween the late Silurian and Carboniferous (Huanget al. 1997; Zhang et al. 2007a, 2007b; Wang et al.2009a). The principal period of hydrocarbon generationfrom kerogen (primary cracking) of the lower Palaeozoicshales was suggested to have occurred between the LateTriassic and Early Jurassic. During this period, these

Figure 7. TOC content versus stratigraphic depth for eight sections studied on the UYP (Liang et al. 2008).

(A)–(D) are lower Silurian profiles/wells (IDs: 10-13); (E)–(H) are lower Cambrian profiles/wells (IDs: 14–17). The locations are denotedin Figure 3.

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intervals were buried between 2 and 4 km, which indicatesa geo-temperature range of 120–165°C. In the course ofthe continuous burial and kerogen conversion, secondarycracking processes started to form light hydrocarbon pro-ducts since the Early Jurassic. By the end of the Jurassic,the lower Palaeozoic shales endured maximum burial of6 km and the highest temperature of 220°C (Tanet al. 2013). In this regard, the present organic matter inthe investigated Ediacaran and lower Palaeozoic sourcerocks should have been overmature, having evolved farinto the metagenesis stage (Tissot and Welte 1984;Horsfield and Rullkötter 1994).

Using microscopic images, dark organic clusters canbe observed under transmitted light on thin sections andbright-grey organic aggregates with intense bitumenreflectance (as much as 4%) can be seen under reflectedlight on polished sections (Figure 5). Rock-Eval pyrolysisof whole rocks also indicates the high thermal maturity.Investigated samples exhibit, even though with richorganic matter, very low S2 (<0.65 mg HC /g rock) andconsequently low HI values (<100 mg HC/g TOC)(Table 1), which in turn leads to Tmax values being unreli-able to indicate thermal maturity levels. Vitrinite reflec-tance (Ro) is the most widely used key measure for thermalmaturity. Nevertheless, it is not suitable for the Ediacaranand lower Palaeozoic source rocks since vitrinite in itsstrict sense does not exist in sediments deposited beforeDevonian times. However, methods have been set up tocompare equivalent parameters with Ro, for instance, bitu-men reflectance (Jacob 1967, 1985; Buchardt andLewan 1990; Pawlewicz and King 1992; Bertrand 1993)and carbon isotope ratios (Stahl 1974; Stahl and Carey1975; Schoell 1980; James 1983; Laughrey andBaldassare 1998; Prinzhofer et al. 2000).

For the Sichuan Basin, UYP, Feng and Chen (1988)reported a formula to calculate Ro via bitumen reflec-tance (Rb):

Ro ¼ 0:3195þ 0:6790Rb: (1)

This formula was deduced from thermal experimentson oil shale and was examined by applying the calculatedRo to the Permian source rocks in the Sichuan Basin. Anexcellent correlation of the real measured Ro and thecalculated Ro was found (Feng and Chen 1988).Measured Rb of the investigated lower Cambrian shalesis, as stated above, as high as 4%, which translates to a Ro

of about 3%.The isotopic behaviour of natural gas can be explained

by differences in the binding energy of molecules.Instantaneous methane should, in response to the kineticisotope effect, become increasingly enriched in the 13Cisotope with increasing thermal stress, i.e. with increasingmaturity of the source rock (Stahl 1974; Stahl andCarey 1975). Thus, δ13C of methane (δ13C1) can be used

to deduce Ro and reveal thermal maturity. In the currentwork, δ13C1 has been measured for two samples contain-ing methane inclusions. Measured δ13C1 values are−33.2‰ for the lower Silurian shale and −29.0‰ for thelower Cambrian shale. Figure 8 illustrates the correlationsbetween δ13C1 and Ro. It shows results of a series ofmarine source rocks from the Devonian throughCarboniferous strata in the Val Verde and DelawareBasins (Barnett shale, USA) (Stahl and Carey 1975) anda series of German Carboniferous coal samples(Schoell 1980). The UYP shale samples are obviously inagreement with the Barnett shale, and thermal maturitiesof the lower Silurian and lower Cambrian shale could beextrapolated to a Ro range of 2.5–3.0% and 3.0–3.5%,respectively (Figure 8). For the Ediacaran shale, the ther-mal maturity should be generally higher than that of thelower Silurian and lower Cambrian samples, and a Ro

range of 3.5–4.0% is assumed according to the modellingresults (Tan et al. 2013). The vitrinite reflectance valuespresent here are consistent with those deduced from Rb,which have been extensively published (Nie et al. 2011;Huang et al. 2012; Long et al. 2012; Yu et al. 2012; Hanet al. 2013; Guo and Zhang 2014; Zhou et al. 2014).

4.3. Kerogen type

The kerogen of the successful shale gas plays in NorthAmerica is dominated by Type II organic matter; never-theless, the thick, widespread, gas-saturated, fine-grainedunits with (1) rich organic matter of any type that isthermally immature and subject to extensive biogenic

R2 = 0.95

R2 = 0.87

–50

–45

–40

–35

–30

–25

–200 0.5 1 1.5 2 2.5 3 3.5 4

δ13C

1 (‰

)

Ro (%)

Barnett Shale

UYP lower Silurian shale

UYP lower Cambrian shale

German Carboniferous coal

Figure 8. Compilation of δ13C of methane (δ13C1) versus vitri-nite reflectance (Ro). Carbon isotopic composition of methaneand vitrinite reflectance are positively correlated for both marinesource rocks as well as terrestrial coal samples, with both arraysbeing considerably offset. This is reflected in δ13C1 values frommarine source rocks being more fractionated than δ13C1 valuesfrom terrestrial coal. Values for UYP samples are in agreementwith values measured for the Barnett shale (Stahl andCarey 1975) plotting on the extrapolation towards higher δ13C1

and higher vitrinite reflectance, respectively.

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decomposition, and/or (2) significant Type III organicmatter in thermally mature to overmature states can bepromising exploration targets as well (Martini et al. 2003;Hamblin 2006).

Kerogen types of the Ediacaran and lower Palaeozoicshale in the UYP have been speculated to be of Type I(Liu et al. 2000; Dai et al. 2003; Zhu et al. 2006; Guo andLiu 2013; Luo et al. 2014; Zhou et al. 2014), Type II (Yinet al. 2001; Liang et al. 2009; Liu et al. 2011), or Type Iand III origin (Huang 1986; Editorial Committee of theSichuan Oil & Gas field 1989; Zhang et al. 2008; Zouet al. 2010; Guo and Zhang 2014). In this study, the Rock-Eval results are characterized by extremely low HI valuesand a broad Tmax range (Table 1). The original structuresof the organic matter within these three intervals musthave been altered and lost during the maturation processfrom diagenesis to metagenesis. Identification of the ori-ginal kerogen is therefore fraught with difficulties. Wehave tried to identify clues based on the hydrocarbongeneration properties during maturation and the carbonisotope ratios of methane, kerogen, and chloroform bitu-men, which will be summarized in future reports.

4.4. Sorbed hydrocarbon composition as revealed byTvap-GC

Tvap-GC is a method of gas chromatography capable ofresolving the S1 peak of Rock-Eval pyrolysis, whichrepresents the bulk of retained hydrocarbons within therock. Because free gas is lost either during the sampling orpreparation process (crushing), gas measured using Tvap-GC primarily reflects the fraction of sorbed gas whichmight accumulate on the surfaces of minerals or organicmatter and/or retained gas, which may be stored in uncon-nected pores. Mahlstedt and Horsfield (personaldiscussion) have recently outlined how Tvap-GC can beused to screen samples based on their sorption capacity. Inthe current investigation, 14 samples were analysed usingTvap-GC to quantify the retained/sorbed hydrocarbonsthat were generated during the early geological history ofthe rocks (Table 2) (Tan et al. 2014b). Because the sam-ples were taken from outcrops or depths shallower than350 m, the measurable hydrocarbon amounts are less than10 mg/g TOC, i.e. extremely low. Generally, the Tvap-GCresults from the core samples are significantly higher thanthose from the outcrops because of low pressure andweathering influence. In addition, the detectable hydrocar-bons in overmature source rocks were expected to bedominated by light hydrocarbons. Hence, the measuredproducts primarily consist of methane (C1), ethane (C2),propane (C3), butane (C4), pentane (C5), and benzene.Interestingly, C1, C3, and C4 are major components ofthe core samples. C5 and benzene are present in highamounts in certain outcrops, whereas the levels of C2 arelow in the majority of the samples (Table 2). Ta

ble2.

Therm

ovaporizationgaschromatog

raph

yprod

uctsin

µg/gTOC

(Tan

etal.20

14b).

Unit

Low

erSilu

rian

shale

Low

erCam

brianshale

Ediacaran

shale

Sam

ple-ID

910

1112

1314

1617

1819

2039

4151

Depth

(m)

136

207

267

276

285

321

00

00

00

00

TOC

(%)

1.97

1.47

2.31

1.69

1.68

2.21

10.20

6.02

7.44

8.15

9.41

6.15

5.90

11.40

C1

50.66

68.50

13.81

64.56

52.98

75.25

0.79

7.96

7.47

8.98

3.91

8.44

9.71

17.87

C2

13.55

24.08

15.41

34.97

14.64

26.92

3.02

5.30

4.70

5.55

1.71

7.35

7.17

16.69

C3

34.47

95.44

45.97

292.01

51.61

180.14

2.36

6.26

7.96

4.66

2.55

15.41

12.78

6.19

C4

104.62

391.43

212.60

296.75

210.48

393.85

5.30

36.45

19.58

12.17

10.38

45.93

59.81

31.74

C5

6.19

44.76

35.97

39.70

39.82

111.13

2.71

11.03

57.07

173.87

2.62

126.65

149.83

9.65

Benzene

0.00

32.86

43.72

42.25

23.63

127.01

9.86

40.78

9.33

9.99

5.88

55.56

38.07

35.97

∑To

talidentified

209.49

657.07

367.49

770.24

393.15

914.30

24.05

107.77

106.12

215.21

27.06

259.35

277.37

118.11

∑Produ

cts

1707

.56

2551

.56

3397

.71

6516

.98

1610

.36

7600

.05

44.63

271.81

205.50

401.88

96.17

1189

.59

525.49

183.45

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4.5. Remaining hydrocarbon generative potential asrevealed by open py-GC

The low remaining hydrocarbon generative potential ofselected samples is illustrated in Figure 9. The HI of theUYP shales is much lower than that of the samplescontaining Types II and III kerogen from the USA,Germany, and New Zealand. Open py-GC was per-formed on 34 samples to investigate the remaininghydrocarbon generative potential on a molecular level.The pyrolysates are primarily composed of light hydro-carbons, i.e. methane (C1), ethane and ethene (C2),propane and propene (C3), butane and butene (C4),pentane and pentene (C5), and benzene (Figure 10 andTable 3). In general, the lower Silurian shales are cap-able of generating higher yields than the lowerCambrian and Ediacaran shales (Figures 10 and 11A–B), indicating somewhat lower maturities. C1 is by farthe dominant products, and C2 and benzene can bedetected in relatively high amounts in the pyrolysatesfrom the cores and certain outcrop samples (Figures11C–D). Individual hydrocarbon compounds generated

by the core samples display no significant variationswith depth (Figure 11E), whereas a generally positivecorrelation between the levels of C5 and TOC can beobserved (Figure 11F). In addition, the amount of totalhydrocarbon of per gramme TOC of the lower Siluriancore samples is higher than that of most outcrop sam-ples (Figure 11B).

The relatively high remaining generative potential ofthe lower Silurian shale can be principally attributed tothe progressive increase in thermal maturity with geolo-gical age, i.e. from the lower Silurian to lower Cambrianto Ediacaran strata, although the effects of the mineralogyand weathering on the kerogen decomposition cannot beruled out. The highest remaining hydrocarbon potentialper gramme of TOC is observed in the two samples withthe lowest TOC concentrations (<0.5%) (Figure 11B). Ithas been reported, however, that the primary heavy pro-ducts of whole rocks can be trapped on mineral surfacesor interlayers, which can then be cracked into lighthydrocarbon with temperature increases (Eisma andJurg 1967; Espitalié et al. 1980; Horsfield andDouglas 1980). The highest pyrolysis yields from organicmatter lean samples may therefore be attributed to thethermal decomposition of formerly retained bitumen, on aTOC normalized basis. Weathering may also impact theproperties of the organic matter in the shales. It has beenreported that the weathering process could result in thedecrease of TOC content, enrichment of 13C and varia-tion of kerogen structure (Leythaeuser 1973; Clayton andSwetland 1978; Clayton and King 1987; Littkeet al. 1991; Petsch et al. 2000; Wildman et al. 2004;Fischer et al. 2009; Marynowski et al. 2011). A decreasein TOC during weathering is consequently accompaniedby a pronounced reduction in the remaining hydrocarbongenerative potential, which may be responsible for therelatively low TOC content in core samples showinghigher generative potentials than most of the outcrops(Figure 11A).

4.6. Reconstruction of original hydrocarbon generativepotential

The algebraic procedures used in mass balance models forcalculating the transformation ratios (TRs), original TOC(TOCo), original petroleum potential (S1o+S2o), petroleumgeneration index (PGI), petroleum expulsion efficiency(PEE or ExEf), and amount of expelled petroleum(S1EX) have been developed and described in detail(Pelet 1985; Cooles et al. 1986; Santamaria andHorsfield 2004; Peters et al. 2005). These mass balancecalculations are important for constraining the quantity

0

150

300

450

600

750

900

295 330 365 400 435 470 505 540 575 610

Hyd

roge

n In

dex

(mg

HC

/g T

OC

)

Tmax (°C)Barnett shale, USA Bakken shale, USA

Posidonia shale, Germany Coal, Germany

Coal, New Zealand Upper Yangtze Platform marine shale, China

I

II

III

Figure 9. HI-Tmax plot of different source rocks, comprising theUYP shale, Barnett shale (Jarvie 2008), Bakken shale (Kuhnet al. 2010), Posidonia shale (Schenk and Horsfield 1998),German coal (Schenk and Horsfield 1998), and New Zealandcoal (Vu 2008).

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and quality of organic matter in the source rock beforematuration.

In this study, the input parameters consisted of themeasured Rock-Eval data (mean values of shale samplesfrom a given interval), the assumed HIo of immatureequivalents in the immature Australian samples(601–620 mg HC/g rock) and Green River shale(777 mg HC/g rock), and an assumed PIo (0.02), whichhas been suggested is reasonable for most immaturesource rocks (Peters et al. 2005). The calculated results(Table 4) indicate that the TRs are over 98%, which is inaccordance with the overmature kerogen and low remain-ing generative potential. However, the restored TOCo

(4.8–23.4%), S1o, and S2o (29–215 mg HC/g rock) indi-cate excellent original petroleum potential, and a mass ofhydrocarbons must have been generated and expelledupon kerogen maturation, as indicated by the fact thatboth the PGI and PEE are >99%. Cooles’ method andClaypool’s equations yield very similar values of TOCo,S1o, and S2o and high values of TR and PGI.Nevertheless, extremely low expulsion efficiencies (PEEor ExEf) of the Ediacaran shale were indicated whenClaypool’s method was applied. The explanation is thatClaypool’s method relies on PI and PIo to calculate theExEf, whereas the PI of the Ediacaran shale is abnormallyhigh (0.37) and that of the lower Silurian (0.08) and lower

Cambrian shale (0.09) is low. The high TOC Ediacaranshale samples commonly possess relatively high values ofS1 and low values of S2 (Table 1), and thus, the measuredPI values are abnormal, which may reflect a strongersorption capacity resulting from the high TOC content.The extrapolated amount of expelled petroleum (S1EX)ranges from 28.9 to 166.0 mg HC/g rock, or nearly88.0 mg HC/g rock on average.

5. Conclusions

The Ediacaran, lower Cambrian, and lower Silurian blackshale intervals were deposited during three major trans-gressions of the late Precambrian and early Palaeozoic.The sedimentary environments were dominated by opento restricted marine platforms and shelves with deep-water conditions. The lower Cambrian shale is distribu-ted throughout most of the UYP and is particularly thickin the southeast Sichuan Basin, Chongqing, northGuizhou, west Hunan, and west Hubei. The lowerSilurian shale, however, is primarily confined to thecentral UYP, where it underlies the southeast SichuanBasin and Chongqing area. The depths vary significantly.There are outcrops along the margins of the SichuanBasin and in the southeast UYP, whereas black shale isencountered at depths exceeding 5 km in certain

Figure 10. The dominant open py-GC pyrolysates of the lower Silurian, lower Cambrian, and Ediacaran shale samples. Considerabledifferences in open py-GC pyrolysate composition are found for the respective units. The lower Silurian sample shows relatively strongeryields than the lower Cambrian sample, which in turn has higher yields than the Ediacaran sample.

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Table3.

Open-system

pyrolysisgaschromatog

raph

yprod

uctsin

µg/gTOC.

Unit

Low

erSilu

rian

shale

Sam

pleID

12

34

56

78

910

1112

1314

Depth

(m)

00

00

00

00

136

207

267

276

285

321

TOC

(%)

3.34

3.76

1.68

1.84

0.93

3.89

43.64

1.97

1.47

2.31

1.69

1.68

2.21

C1

700.8

374.6

320.4

641.4

325.6

140.6

121.2

174.8

395.7

392.9

345.5

359.1

397.9

391.2

C2

327.6

183.5

120.6

141.1

166.4

165.4

48.5

53.2

174.0

245.9

200.8

222.8

175.3

160.8

C3

170.4

108.3

134.0

153.9

100.1

26.9

22.9

142.8

211.1

71.7

147.8

165.4

83.4

C4

47.8

36.6

129.0

66.5

9.3

9.2

101.3

231.9

101.8

160.0

146.5

235.7

C5

38.3

44.7

39.1

111.8

76.3

27.0

20.8

18.5

38.7

74.8

30.4

72.4

57.6

54.3

C6

18.8

33.6

14.7

19.0

12.3

15.6

14.9

18.9

Benzene

27.0

51.1

76.8

50.5

49.2

117.9

36.2

47.7

240.7

227.8

178.5

207.8

211.1

249.5

∑To

talidentified

1311.9

798.8

575.7

1078

.790

0.4

651.1

262.9

326.3

1107

.914

03.5

941.0

1185

.51168

.61193

.8∑

Produ

cts

1607

.616

88.0

1439

.516

13.5

1276

.328

65.0

377.0

469.7

1740

.829

84.1

2011.8

2252

.118

40.5

2774

.2

Unit

Low

erCam

brianshale

Ediacaran

shale

Sam

pleID

1516

1718

1920

2122

2326

2728

2931

3233

3435

3841

Depth

(m)

00

00

00

00

00

00

00

00

00

00

TOC

(%)

0.45

10.2

6.02

7.44

8.15

9.41

2.85

5.79

5.04

0.39

7.79

5.81

7.69

2.59

3.3

5.05

1.89

1.43

0.43

6.7

C1

166.9

17.7

18.7

31.5

14.5

16.8

75.0

106.0

95.0

408.2

23.1

262.5

96.2

37.4

40.6

30.0

181.4

81.7

515.2

23.3

C2

164.0

8.4

15.9

39.7

15.3

12.0

70.9

52.2

48.2

670.3

11.7

35.9

36.4

28.3

91.4

20.2

149.1

80.7

510.4

21.9

C3

83.6

8.7

26.5

7.8

9.1

34.6

24.8

220.5

6.3

15.1

14.7

30.4

12.2

34.7

51.1

258.3

11.1

C4

55.6

6.3

17.8

3.7

3.5

20.5

7.1

9.8

156.2

4.6

14.7

7.3

8.7

54.7

7.5

58.4

170.9

4.9

C5

33.7

14.5

53.6

13.8

31.4

79.4

33.1

60.4

135.5

35.0

40.1

44.6

22.6

48.3

7.2

110.0

79.3

244.0

C6

Benzene

5.0

7.8

6.4

0.0

3.9

175.4

11.0

12.9

59.1

4.8

219.8

170.9

5.6

∑To

talidentified

503.9

31.0

72.0

175.4

55.2

76.7

421.1

243.9

251.0

1649

.785

.558

8.2

355.4

117.3

265.3

77.1

475.2

351.2

1698

.861

.2∑

Produ

cts

1339

.372

.916

5.8

464.5

95.7

102.4

1581

.538

2.2

389.8

3547

.914

3.4

1183

.678

7.0

204.2

520.7

184.6

2823

.961

2.4

2919

.315

8.8

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depressions. The mineralogy and organic matter vary inupward succession in both the lower Cambrian and lowerSilurian formations, such that high-TOC black shale istypically situated in the bottom parts, whereas the upperintervals consists of the low-TOC grey shale, siltstone, orcarbonate.

The TOC contents in the analysed samples are gen-erally >2% and average 5%, and they increase withgeological age. For example, the TOC levels in thelower Silurian shale are lower than those in the lowerCambrian shale, which are in turn lower than those inthe Ediacaran shale in general. The thermal maturity ofthe kerogen indicates metagenesis, based on the visibleorganic matter, bitumen reflectance and δ13C of

methane. On the basis of the δ13C of the methane andthe basin modelling results, extrapolated vitrinite reflec-tances (Ro) of 2.5–3.0% for the lower Silurian shale,3.0–3.5% for the lower Cambrian shale, and 3.5–4.0%for the Ediacaran shale were found. Tvap-GC and openpy-GC analyses indicate extremely low remaining gen-erated hydrocarbon and low remaining generative poten-tial, and the products are qualitatively dominated by thelight hydrocarbons. The original petroleum potential andoriginal organic status were calculated and reconstructedusing Cooles’ and Claypool’s equations. Excellent ori-ginal hydrocarbon potential was revealed, based on thevalues of TOCo between 5% and 23% and S1o+S2oranging from 29–215 mg HC/g rock.

Figure 11. Open py-GC products of selected samples. (A) Total amount of open py-GC products of per gram rock; (B) total amount ofopen py-GC products, values were normalized to TOC weight; (C) the amount of the identified light hydrocarbons per gramme rock; (D)the amount of the identified light hydrocarbons, values were normalized to TOC; (E) light hydrocarbons vs. depth of the core samples;and (F) the correlations between C5 and TOC.

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Table4.

Massbalancecalculations

usingthedifferentapproaches.

Unit

Measureddata

(meanvalue)

Assum

ption

Pelet’sequatio

n(Pelet19

85)

Coo

les’

metho

d(Coo

leset

al.19

86)

Clayp

ool’s

metho

d(Peterset

al.,20

06)

TOC

S1

S2

HI

PI

HI

PI O

TR

DCoratio

DCo

TOCo

S1 O

+S2 O

PGI

PEE

TR

TOCo

S1 O

+S2 O

S1E

XExE

f

Low

erSilu

rian

2.5

0.03

0.4

17.43

0.08

900

0.02

99.5

0.22

2.40

10.76

100.66

99.6

99.97

99.5

9.42

86.49

86.07

96.3

800

0.02

99.3

0.31

7.76

64.54

99.4

99.95

99.3

7.13

58.18

57.75

94.3

700

0.02

98.9

0.40

6.07

44.16

99.2

99.93

99.0

5.73

40.95

40.51

91.7

600

0.02

98.5

0.47

5.11

32.66

98.9

99.91

98.6

4.80

29.36

28.91

88.2

Low

erCam

brian

5.2

0.03

0.3

17.77

0.09

900

0.02

99.5

0.22

5.15

23.04

215.55

99.8

99.99

99.5

18.17

166.86

166.02

95.3

800

0.02

99.2

0.31

16.62

138.20

99.8

99.98

99.3

14.16

115.58

114.68

92.8

700

0.02

98.9

0.40

12.99

94.57

99.7

99.97

99.0

11.60

82.84

81.91

89.5

600

0.02

98.5

0.47

10.95

69.94

99.5

99.96

98.5

9.82

60.13

59.18

85.1

Ediacaran

5.1

0.12

0.2

9.17

0.37

900

0.02

99.7

0.22

4.98

22.32

208.82

99.9

99.94

99.8

17.97

164.98

164.37

85.8

800

0.02

99.6

0.31

16.10

133.89

99.9

99.91

99.6

13.99

114.20

113.54

78.2

700

0.02

99.5

0.40

12.59

91.62

99.8

99.87

99.5

11.45

81.82

81.13

68.4

600

0.02

99.2

0.47

10.61

67.76

99.7

99.82

99.2

9.70

59.37

58.67

55.4

(1)HIishydrogen

index(m

ghydrocarbon/gTOC);TRistransformationratio

(%);PIisproductio

nratio

(wt.ratio

);TOCistotalo

rganiccarbon

(wt.%);S1isexistin

ghy

drocarbo

n(m

ghy

drocarbo

n/grock);

S2isgeneratedhy

drocarbo

nfrom

kerogen(m

ghy

drocarbo

n/grock);S1E

Xisexpelledpetroleum

(mghy

drocarbo

n/grock);ExE

fisexpulsionefficiency(%

);DCratio

isdead

carbon/to

talo

rganiccarbon;

DCisdead

carbon

wt./rock

wt.(%

);PGIispetroleum

generatio

nindex(%

);PEEispetroleum

expulsionefficiency

(%).

(2)Pelet’sequatio

n:TR=(1200×(H

I o−HI))/(H

I o×(120

0−HI)),where

HI oishydrogen

indexof

immaturesample,HIishydrogen

indexof

maturesample,and1200

representsthereciprocal(tim

es1000)

of0.83,theassumed

proportio

nof

carbon

inRock-Evalpyrolysisproducts.

(3)Coo

les’

method:

DCo-ratio

=1−(C

2o+C1o)/TOCo=1−(S1 o

+S2 o)×0.83

/TOCo/100

0;DC

=TOC

−(C

2+C1)=TOC

−(S2+S1)

×0.83/10;

TOCo=(TOC

−(C

2+C1))/DCo=DC/DCo;

S1 o

+S2 o

=(C

2o+C1o)×10

/0.83=TOCo×(1

−DCratio

)×10

/0.83;

PGI=1−S2/(S2 o

+S1 o);PEE=((S2 o

+S1 o)−(S2−S1))/((S2 o

−S2)

+S1 o).

(4)Clayp

ool’s

metho

d:TR=1−HI×(120

0−(H

I o/((1

−PI o)))/(H

I o×(1200−(H

I/(1

−PI)));T

OCo=83

.33×HI×TOC/(HI o×(1

−TR)×(83.33

−TOC)+HI×TOC);S1 o

+S2 o

=HI o×TOCo/100

/(1−

PI o);S1EX

=10

00×(TOCo−TOC)/(83.33

−TOC);ExE

f=1−(1-f)×(PI/(1

−PI))/(f+(PIo/(1−PI o))×10

0.

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AcknowledgementsThe authors thank Ferdinand Perssen (GFZ-Potsdam) for techni-cal assistance performing experiments and Statoil for the permis-sion to publish.

Disclosure statementNo potential conflict of interest was reported by the authors

FundingThis work was supported by Statoil.

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