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Marine and Petroleum Geology 77 (2016) 693e702

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Marine and Petroleum Geology

journal homepage: www.elsevier .com/locate/marpetgeo

Stable isotope geochemistry of the nitrogen-rich gas from lowerCambrian shale in the Yangtze Gorges area, South China

Yang Liu a, Jinchuan Zhang a, b, *, Jun Ren a, Ziyi Liu a, Huang Huang a, Xuan Tang a

a School of Energy Resources, China University of Geosciences, 29 Xueyuan Road, Beijing 100083, Chinab Key Laboratory of Shale Gas Exploration and Evaluation, Ministry of Land and Resources, China University of Geosciences, 29 Xueyuan Road, Beijing100083, China

a r t i c l e i n f o

Article history:Received 5 April 2016Received in revised form20 July 2016Accepted 20 July 2016Available online 22 July 2016

Keywords:Shale gasCarbon isotopeHydrogen isotopeNitrogen isotopeYangtze Gorges area

* Corresponding author. School of Energy Resourcsciences, 29 Xueyuan Road, Beijing 100083, China.

E-mail address: zhangjc@cugb.edu.cn (J. Zhang).

http://dx.doi.org/10.1016/j.marpetgeo.2016.07.0200264-8172/© 2016 Elsevier Ltd. All rights reserved.

a b s t r a c t

The organic-rich marine shales from the lower Cambrian Niutitang formation in the Yangtze Gorges areaare characterized by their high TOC values (0.54e5.13%), types I and II organic matter, and high Ro values(2.4%e2.6%). Geochemical parameters of 19 gas samples from the lower Cambrian shale formation fromwell ZD-1 were analyzed in this study. The gases are dominated by methane, with small amounts ofethane and no propane or butane. The d13C(CH4) values range from �31.7‰ to �27.9‰, the d13C(C2H6)values range from �35.2‰ to �33.4‰, and the d2H(CH4) values range from �214‰ to �187‰.Furthermore, carbon isotopic compositions of the alkane gases from the lower Cambrian shale arecharacterized by d13C1 > d13C2; these results indicate that the gases are of thermogenic origin and are oil-derived. Moreover, methane with heavy carbon isotopic and light hydrogen isotopic values may beproduced when water reacts with inorganic carbon. The isotopic reversal of alkane gases was probablydue to destructive redox reactions at maximum burial resulting in Rayleigh-type fractionation and/ormixing by gases from the same source but at a different maturity. Nitrogen occurs in the analyzed gasesat concentrations from 8.15 to 96.52%, and the d15N(N2) values vary from 3.2 to 7.4‰. The d15N(N2) valuesand high illite content suggest that nitrogen was mainly generated during thermal transformation oforganic matter and/or from NH4-rich illites of the clayey facies of the lower Cambrian shale. Carbondioxide concentrations vary from 1.13 to 4.96%, and the d13C(CO2) values range from �8.9 to�13.6‰. Thissuggests that carbon dioxide was mainly generated during thermogenic processes of transformation oforganic matter. Moreover, the major reason for the change in nitrogen and methane contents in thestudied well (ZD-1) is probably due to the development of micro cracks in core samples with highcontents of nitrogen. A high nitrogen content may be caused by the strong adsorption capacity of ni-trogen, which leads to the retention of nitrogen and the diffusion of methane in shale.

© 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Natural gas storage in shale differs significantly from conven-tional gas reservoirs and may be composed of free gas withinfractures and intergranular pores, absorbed gas within organicmatter and on inorganic minerals, or as dissolved gas in oil andwater (Curtis, 2002; Ross and Bustin, 2007; Gasparik et al., 2012,2014; Zhang et al., 2012). Productive shale gas formations inNorth America are generally characterized by TOC >2.0%, type IIkerogen, and Ro (vitrinite reflectance) > 1.3% (Zumberge et al.,

es, China University of Geo-

2012). However, the formations of shale gas in China are charac-terized by old formation, deep burial, high thermal maturity, and amulti-stage structure (Dai et al., 2014b; Zou et al., 2015). Shale gassystems can be divided into two distinct types, namely, biogenicand thermogenic (Claypool, 1998), although there can also bemixtures of the two types of gas (Jarvie et al., 2007). Distinguishingbetweenmicrobial and thermogenic CH4 is essential for natural gasexploration and production strategies and improved resource es-timates (Osborn and McIntosh, 2010). Investigations into the originand formation mechanism of highly mature and over-mature shalegas and the effect of intense tectonism on the deformation andalteration of shale gas reservoirs are more critical for Chinese shalegas exploration.

Stable carbon and hydrogen isotopic compositions of methane

Y. Liu et al. / Marine and Petroleum Geology 77 (2016) 693e702694

are key attributes that promote understanding of the origin ofnatural gases in surface ecosystems, clathrates in marine sedi-ments, deep rocks or oil, and gas reservoirs (Schoell, 1980, 1988;Jenden et al., 1993; Whiticar, 1999; Etiope et al., 2009). In addi-tion, the chemical composition and carbon isotopic composition of

Fig. 1. Map showing the geology of the

Table 1Total organic carbon (TOC) content, mineral content and vitrinite reflectance (Ro) of sam

Sample Strata Depth TOC (%) Ro (%) Shale composition (wt%

Quartz Feldspar

ZD-01 21 263.4 0.95 2.4 32 12ZD-02 21 265.9 0.83 2.5 35 10ZD-03 21 268.6 1.07 2.5 42 13ZD-08 21 282.2 0.54 2.5 40 4ZD-10 21 288.6 1.64 2.6 46 6ZD-13 21 306.4 2.07 2.6 51 9ZD-18 21 334.7 4.43 2.5 54 8ZD-20 21 351.4 5.13 2.6 56 8

hydrocarbons have beenwidely applied to the geochemical study ofconventional natural gases (Schoell, 1980, 1983, 1988; Xu, 1994;Berner and Faber, 1996; Tang et al., 2000). Numerous studies haveindicated that the geochemical characteristics of shale gas havesome special characteristics compared with that of conventional

study area and the well location.

ples.

)

Calcite Dolomite Pyrite Illite Chlorite Illite/Smectite

9 0 4 27 7 98 1 8 25 6 76 3 5 22 4 515 4 10 18 3 612 4 7 19 3 310 5 6 15 1 310 7 4 16 0 116 5 4 10 0 1

Y. Liu et al. / Marine and Petroleum Geology 77 (2016) 693e702 695

natural gas. Variations in the gas isotope composition in conven-tional gas reservoirs are mainly determined by the kerogen isotopecomposition, maturity, and charging efficiency. However, for shalegas, residual oil in shale may convert to gas, and water may beinvolved in this conversion. Products from different chemical pro-cesses cause the isotope composition to deviate from conventionalmodels (Liu et al., 2016). Moreover, the d13C of gaseous alkanesmostly shows a reversal in highly mature shale gases such as theBarnett, Haynesville, Fayetteville, Woodford, Marcellus, Horn River,Utica, and Longmaxi shales (Burruss and Laughrey, 2010; Tilleyet al., 2011; Tilley and Muehlenbachs, 2013; Zumberge et al.,2012; Xia et al., 2013; Dai et al., 2014b).

Nitrogen is one of the most common non-hydrocarbons innatural gas, and many previous studies have focused on thecomposition and origin of nitrogen in natural gas using isotopegeochemical methods (Dai, 1992; Hoering and Moore, 1958; Littkeet al., 1995; Stahl, 1977). Most conventional natural gases containonly a few percent nitrogen; however, high nitrogen contents in thenatural gases from lower Cambrian shale have been observed inmany areas in South China. Therefore, the increasing attention paidto non-hydrocarbons in shale gas will help to solve a series ofproblems in geological theories and reduce the investment risksencountered with high contents non-hydrocarbons in explorationand production.

The objective of this study is to determine the origin of hydro-carbon and non-hydrocarbon gases based on the molecular and

Table 2Rock-Eval Pyrolysis parameters of solid residue of shale samples.

Sample Strata Depth TOC (%) Ro (%)

ZD-08 21 282.2 0.54 2.5ZD-13 21 306.4 2.07 2.6ZD-18 21 334.7 4.43 2.5ZD-20 21 351.4 5.13 2.6

S0, hydrocarbon released from source rock under 90 �C, mg/g; S1, hydrocarbon releasedunder 300e600 �C; Tmax, the temperature at maximum release of hydrocarbons occurs

Table 3Geochemical characteristics of natural gases from the Lower Cambrian shale in the stud

Sample Strata Depth(m) Gas composition (%) Measured gas content (m

CH4 C2H6 C3H8 CO2 N2

ZD-01 21 263.4 27.53 n.d. n.d. 2.25 70.34 0.07ZD-02 21 265.9 61.02 n.d. n.d. 1.80 37.19 0.11ZD-03 21 268.6 58.42 n.d. n.d. 2.81 38.77 0.16ZD-04 21 271.2 16.02 n.d. n.d. 4.96 79.03 0.13ZD-06 21 275.6 0.99 n.d. n.d. 2.49 96.52 0.01ZD-07 21 277.9 15.80 0.11 n.d. 3.63 80.47 0.06ZD-08 21 282.2 21.29 n.d. n.d. 2.91 75.81 0.07ZD-09 21 285.1 9.58 n.d. n.d. 3.99 86.43 0.06ZD-10 21 288.6 69.20 0.27 n.d. 2.20 28.32 0.05ZD-12 21 299.6 70.30 n.d. n.d. 3.12 26.58 0.14ZD-13 21 306.4 50.96 n.d. n.d. 3.71 45.33 0.13ZD-14 21 311.5 33.02 n.d. n.d. 3.54 63.44 0.07ZD-15 21 317.4 27.49 n.d. n.d. 2.12 70.39 0.06ZD-16 21 324.7 77.59 0.69 n.d. 1.83 19.91 0.34ZD-17 21 328.0 69.61 1.60 n.d. 2.05 26.74 0.16ZD-18 21 334.7 83.24 2.13 n.d. 1.90 12.73 0.43ZD-19 21 351.4 86.30 1.70 n.d. 1.13 10.87 0.20ZD-20 21 263.4 89.23 0.72 n.d. 1.91 8.15 0.57ZD-22 21 265.9 71.88 0.81 n.d. 1.68 25.64 0.15

Notes.1. n.d. means not detected.2. n.m. means not measured.3. All the gas compositions (CH4, C2H6, N2, CO2) have made correction to eliminate the i

stable carbon isotope compositions of gaseous hydrocarbons (CH4and C2H6) and CO2, stable hydrogen isotope compositions ofmethane, stable nitrogen isotope compositions of gaseous nitrogen,and stable isotope compositions of noble gases in natural gases. Byinvestigating the N2 content changes, the genetic types of nitrogenwere classified, which are expected to provide geochemical evi-dence to explain nitrogen enrichment in natural gases from thelower Cambrian shale in the Yangtze Gorges area.

2. Geological setting

The Yangtze platform is located between the Qin-Ling orogen tothe North and the Cathaysia suture to the Southeast (Zhu et al.,2003; Jiang et al., 2011). The marine black shales are widelydistributed on the Yangtze Platform, particularly in the LowerPaleozoic formations. The Niutitang shale from the Lower Cambrianand the Longmaxi shale from the Lower Silurian have been regar-ded as prolific shale gas formations (Tang et al., 2014). They aregenerally characterized by large distribution, large thickness, highTOC content, high over maturity, and high quartz content (e.g.,Zhang et al., 2008a,b; Zou et al., 2010; Tian et al., 2013; Tan et al.,2013). Until recently, shale gas was produced from these forma-tions in the Sichuan Basin.

The Yangtze Gorges area is located 25 km to the northwest ofYichang city in central Hubei province (Fig. 1). The study areacontains nine formations: Liantuo, Nantuo, Doushantuo, Dengying,

S0 (mg/g) S1 (mg/g) S2 (mg/g) Tmax(�C)

0.0065 0.04 0.09 5350.0088 0.06 0.13 5360.0064 0.04 0.48 5320.0053 0.05 0.62 536

from source rock under 300 �C, mg/g; S2, hydrocarbon released from source rockduring Rock-Eval pyrolysis.

y area.

3/t) d13C(‰)VPDB d2H(‰)VSMOW d15N(‰)VAIR 3He/4He (10�8) R/Ra

CH4 C2H6 CO2 CH4 N2

�28.8 n.d. �12.5 �202 5.7 3.12 0.02�30.2 n.d. �10.2 �196 5.2 n.m. n.m.�29.4 n.d. �11.7 �198 4.6 n.m. n.m.�29.9 n.d. �12.8 �207 6.8 3.36 0.02n.d. n.d. �9.6 �213 7.4 n.m. n.m.�31.5 n.d. �8.9 �210 6.5 n.m. n.m.�28.6 n.d. �13.6 �211 6.5 n.m. n.m.�30.8 n.d. �12.3 �214 6.2 2.76 0.02�28.7 n.d. �10.5 �193 5.1 n.m. n.m.�27.9 n.d. �9.8 �191 4.3 n.m. n.m.�31.4 n.d. �10.2 �193 4.7 n.m. n.m.�30.7 n.d. �12.4 �201 5.1 n.m. n.m.�28.8 n.d. �9.8 �197 5.7 n.m. n.m.�29.8 �34.8 �11.6 �189 5.3 n.m. n.m.�30.1 �35.2 �10.4 �192 4.8 n.m. n.m.�31.7 �33.4 �11.9 �191 3.2 2.18 0.02�30.3 �34.5 �10.7 �192 4.8 2.24 0.02�29.2 �33.6 �12.0 �192 3.8 1.86 0.01�28.7 �34.2 �11.8 �187 4.1 n.m. n.m.

nfluence of the air.

Fig. 3. Plot of d13CCH4 vs. d2HCH4 of gases from the Lower Cambrian shale in the studyarea, implying a thermogenic origin (After Schoell, 1980; Whiticar, 1999).

Y. Liu et al. / Marine and Petroleum Geology 77 (2016) 693e702696

Yanjiahe, Shuijingtuo, Shipai, Tianheban, and Shilongdong forma-tions in ascending order (e.g., Zhang, 1981; Zhao et al., 1985; Chen,1987; Zhou and Xu, 1987; Zhao et al., 1988; Ding et al., 1996). Inseveral recent studies, the Yanjiahe and Shuijingtuo formationshave collectively been referred to as the Niutitang Formation(Wang et al., 2008; Jiang et al., 2012). The lower Cambrian marineblack shale in this area is considered a high-quality hydrocarbonsource rock (Zhang et al., 2007; Cheng et al., 2013) that wasdeposited in a period of extensive transgression in the earlyCambrian (Goldberg et al., 2007; Li et al., 2015a,b; Wang et al.,2012).

In recent years, significant attention has been paid to the Niu-titang organic-rich shale in the Sichuan basin and the peripheralarea for the high shale gas potential (Zhang et al., 2008a,b; Zouet al., 2014; Wang et al., 2015). In early 2015, the vertical drill(well ZD-1) in the lower Cambrian shale found shale gas for the firsttime in the Yangtze Gorges area, which marked a major break-through in the exploration of shale gas in this area.

3. Samples and methods

In this study, 19 shale core samples were collected from shalegas well ZD-1 in the Yangtze Gorges area. The TOC values of theseshale core samples range from 0.54% to 5.13%, and the vitrinitereflectance values (Ro) range from 2.4% to 2.6% (Table 1). Thesecores were placed as quickly as possible inside a hermeticallysealed canister immersed in a water bath at the reservoir temper-ature, and the volumes of gas released inside the canister wereperiodically measured using a graduated cylinder at atmosphericpressure (Liu et al., 2016). The gas samples were collected from thegas released in the desorption process of the above-mentioned 19core samples. The molecular composition of the gas was analyzedat the Key Laboratory of Marine Reservoir Evolution and Hydro-carbon Abundance Mechanism, China University of Geosciences,Beijing. Stable isotopes analyses were performed at the NuclearIndustry Beijing Geological Research Analysis and Test ResearchCentre.

The molecular compositions of natural gases (CH4, C2H6, CO2,and N2) were determined using an Agilent 6890N gas

Fig. 2. Natural gas interpretative “Bernard” diagram combining the molecular andisotopic compositional information shown that natural gases from the lower Cambrianshale in the study area are slightly outside the thermogenic gas outline, probably dueto their relatively high gas dryness coefficient and heavy methane carbon isotopicvalues (After Bernard et al., 1978; Whiticar, 1999).

chromatograph (GC) equipped with a flame ionization detector anda thermal conductivity detector. The GC oven temperature wasinitially set at 30 �C for 10min and then increased to 180 �C at 10 �C/min and held at this temperature for 20e30 min. Analytical pre-cision was estimated to be ±1%. For all of the gas compositions,oxygen-free corrections and the corresponding correction for ni-trogen were performed (Ni et al., 2013). Nitrogen was corrected forthe presence of traces of oxygen using the area ratio of nitrogen tooxygen peaks measured for air (Jenden et al., 2015).

Stable carbon, hydrogen, and nitrogen isotope analyses wereperformed using a Finnigan MAT-253 instrument. The stable car-bon and hydrogen isotope data are presented in d-notation relativeto V-PDB and V-SMOW standards, respectively. The measurementprecision was estimated to be ±0.5‰ and ±3‰, respectively (Niet al., 2013; Dai et al., 2014a, 2014b). The results of stable nitro-gen isotope analysis are presented in d-notation (d15N, ‰) relativeto an air nitrogen standard. Analytical precision is estimated to be±0.4‰. Gaseous nitrogen was separated chromatographically forstable nitrogen isotope analysis and was transmitted to a mass

Fig. 4. Plot of d13C2 vs. C2H6 of coal-derived and oil-derived gases shown that gasesfrom the Lower Cambrian shale in the study area are oil-derived gases (modified afterDai et al., 2014a).

Y. Liu et al. / Marine and Petroleum Geology 77 (2016) 693e702 697

spectrometer via an on-line system (Kotarba and Nagao., 2008).Noble gases (He, Ar) were isolated from other major compo-

nents such as hydrocarbons, carbon dioxide, and nitrogen usingtwo Ti-Zr getters heated at approximately 800 �C. The noble gasfraction was analyzed with a VG 5400 sector-type mass spec-trometer (Fisons Instruments). Air standards were measuredrepeatedly between sample analyses to correct the mass discrimi-nation of the mass spectrometer (Xu et al., 1995a, 1995b). Experi-mental uncertainties for the noble gas concentrations wereestimated to be approximately 10%.

4. Results and discussion

4.1. Origin of alkane gases

Table 3 shows the compositional and stable carbon andhydrogen isotopic data for 19 gas samples from lower Cambrianshale in the Yangtze Gorges area. The content of methane andethane varies from 0.99% to 89.23% and from 0.11% to 2.13%,respectively. The d13CCH4 values range from �31.7‰ to �27.9‰(average �29.8‰), while the d13CC2H6 values range from �35.2‰to �33.4‰ (average �34.3‰). The d2HCH4 values rangefrom �214‰ to �187‰ (average �198‰).

However, small amounts of ethane without propane and butanewere detected in this study. The complete lack of propane or butanein this shale has a close relationship with the extremely highmaturity of the lower Cambrian shale. This phenomenon is alsoreflected in the Rock-Eval pyrolysis data, which is illustrated inTable 2, samples have quite low values of S2 (the hydrocarbon yieldby cracking of the remaining organic matter).

The natural gas composition and the stable isotopes have beenwidely used in the identification of natural gas origin (e.g., Whiticaret al., 1986; Berner and Faber, 1988; Chung et al., 1988; Schoell,1980; Whiticar, 1999; Strapo�c al., 2007, 2008). Both molecularand isotopic compositions (Figs. 2 and 3) show that the analyzedgases are genetically related to thermogenic processes. Althoughnatural gases from the lower Cambrian shale in the study area areslightly outside the thermogenic gas outline, this is probably due totheir relatively high gas dryness coefficient and heavy methanecarbon isotopic values caused by the extremely highmaturity of theshale.

Fig. 5. a, Plot of d13C1-d13C2 indicating the gases from the Lower Cambrian shale in the stu2014a); b, plot between the d13C1 and d13C2 values, the trend line “S” is based on the data2014b).

Thermogenic gas could be divided into coal-derived gas fromterrestrial humic organic matter and oil-derived gases mainly frommarine sapropelic organic matter (Ni et al., 2013). Multiple studiesof the gases in Chinese basins suggest a cut-off point in the d13C ofethane for differentiating the origin of thermogenic gases. Ganget al. (1997) indicated that d13C(C2H6) > �29‰ for coal-derivedgas and < �29‰ for oil-derived gas according to hydrous pyroly-sis of carbonates, mudstones, oil shales, and coals from differentlocations in the world. Dai et al. (2005) proposed a d13C(C2H6)value > �27.5‰ for coal-derived gas and a d13C(C2H6)value < �29‰ for oil-derived gas. However, based on a compre-hensive study on the natural gases from 600 gas wells in China, Daiet al. (2014a) proposed a d13C(C2H6) > �27.5‰ cutoff for coal-derived gas and a d13C(C2H6) < �29‰ cutoff for oil-derived gas.As shown in Fig. 4, the gases from lower Cambrian shale in thestudy area are oil-derived gases.

4.2. Isotopic rollovers and reversals

Reversed stable carbon trends have been found in conventionalreservoirs in the U.S. (Jenden and Drazan, 1993; Burruss andLaughrey, 2010), China (Xia et al., 1999; Dai et al., 2004), and Can-ada (Tilley et al., 2011; Tilley and Muehlenbachs, 2013). They havealso been identified in shale gas fields in Utica Shale of the Appa-lachian basin (Burruss and Laughrey, 2010), Barnett Shale of theFort Worth basin (Zumberge et al., 2012; Xia et al., 2013), Fayette-ville Shale of the Arkoma basin (Zumberge et al., 2012), Longmaxishale of the southern Sichuan Basin (Dai et al., 2014b), and lowerPermian shale of the southern North China basin (Liu et al., 2016).

A complete and partial reversed trend in the carbon isotopes ofn-alkanes could be due to (1) mixing between biogenic and abio-genic gases, (2) mixing between different types of gases, (3) mixingbetween gases from the same type of source rocks with differentmaturities, (4) mixing of gases from the same rock interval ofvarying maturities, and (5) the influence of secondary cracking orthermochemical sulfur reduction in carbonates (e.g., Xia et al.,1999; Dai et al., 2004; Burruss and Laughrey, 2010; Tilley et al.,2011; Zumberge et al., 2012).

The complete reversed trend of shale gas can be also caused bythe Rayleigh fractionation of ethane and propane involved in redoxreactions with transition metals and water at late stages of

dy area are mixed gases with negative carbon isotopic series (modified after Dai et al.,from typical marine shale gas in China, Canada and America (modified after Dai et al.,

Fig. 7. Genetic characterization of analysed natural gases using d13C(CH4) versusd13C(CO2) (modified after Gutsalo and Plotnikov, 1981; Kotarba, 2001).

Fig. 6. d15N(N2) values suggest that nitrogen was mainly generated during thermaltransformation of organic matter and/or from NH4-rich illites of the clayey facies of thelower Cambrian shale (modified after Zhu et al., 2000a,b).

Y. Liu et al. / Marine and Petroleum Geology 77 (2016) 693e702698

catagenesis at temperatures of 250e300 �C. Furthermore, thereversal in the 2H composition of methane can be explained bymixing with a late stage, super-mature methane that has isotopi-cally exchanged with formation water at these temperatures(Burruss and Laughrey, 2010).

As shown in Fig. 5a, all data points fall in the zone of mixed gaseswith negative carbon isotopic series. In addition, as shown inFig. 5b, the trend line “S” is d13C1 and d13C2 values based on the datafrom typical marine shale gas in China, Canada, and the UnitedStates (Rodriguez and Philp, 2010; Zumberge et al., 2012; Tilley andMuehlenbachs, 2013; Dai et al., 2014b). The first turning point wasconsidered to be the beginning of secondary cracking, and mixingof primary and secondary cracking gases may cause an isotopereversal (Rodriguez and Philp, 2010; Hao and Zou, 2013; Xia et al.,2013; Tilley et al., 2011; Tilley and Muehlenbachs, 2013). The sec-ond turning point may indicate a “post-reversal” stage of extremelyhigh maturity (Dai et al., 2014b). In this study, all data points fallnear the end of the trend line “S” after the second turning point inthe zone of d13C1 > d13C2. This indicated that d13C1 > d13C2 in thestudy area is closely related to the extremely high maturity(Ro ¼ 2.4%e2.6%) of the lower Cambrian shale.

The maximum burial depth reached approximately 10 km oflower Cambrian strata in the study area (Wo et al., 2007). Moreover,numerous studies have shown that the uplifting time of theHuangling granite developed from the Late Jurassic to EarlyCretaceous (Wang et al., 2003; Liu et al., 2009; Shen et al., 2009; Xuet al., 2010; Li and Shan, 2011; Hu et al., 2012). Due to the influenceof magmatic and hydrothermal activities, the lower Cambriansource rocks have obtained a maximum temperature of250e450 �C (Ji et al., 2014).

As indicated above, the isotopic reversal of alkane gases fromthe lower Cambrian shale in the study area was probably due todestructive redox reactions at maximum burial, resulting in Ray-leigh type fractionation and/or being mixed by gases from the samesource but at different maturity. Moreover, compared with thegases from Longmaxi shale in the southern Sichuan Basin (Dai et al.,2014b), themethanewith heavy carbon isotopic and light hydrogenisotopic values in the study area (Fig. 3) may be produced throughthe formation of water reacting with inorganic carbon.

4.3. Origin of nitrogen

As shown in Table 3, the 3He/4He and R/Ra values are1.86 � 10�8e3.36 � 10�8 and 0.01e0.02, respectively. Nitrogenoccurs in the analyzed gases in concentrations from 8.15 to 96.52%,and the d15N(N2) values vary from 3.2 to 7.4‰. Usually, the R/Ra ofcrustal helium ranges from 0.01 to 0.1 (Wang, 1989), while that ofmantle derived helium is often > 0.1 (Jenden et al., 1993). Low3He/4He and R/Ra ratios indicate that the gases from the lowerCambrian shale in the study area are essentially of crustal origin.

Nitrogen present in most of the natural gases has variableconcentrations, and its origin can be determined on the basis ofstable isotope analysis (e.g., Stahl, 1977; Gerling et al., 1997). Theorigin of nitrogen in natural gases can be organic or inorganicprocesses in crust and upper mantle. Nitrogen can be part of theatmosphere or trapped in rock during sedimentation, or it mayinfiltrate the rock complex with surface water. It may also derivefrom ammonium in clay minerals (Mingram et al., 2005) or fromN2-bearing fluid inclusions (Lüders et al., 2005, 2012). Furthermore,molecular nitrogen is produced in great quantities during thethermogenic transformation of organic matter both from humicand sapropelic organic matter (Kotarba, 1988; Krooss et al., 1995,2005; Littke et al., 1995; Zhu et al., 2000a,b). The process of mo-lecular nitrogen production from organic matter was also docu-mented by anhydrous pyrolysis (Gerling et al., 1997) and hydrous

pyrolysis (Kotarba and Lewan, 2013) experiments.In this study, the content of clay minerals in the lower Cambrian

shale is 11e43% (26% average) and is dominated by illite (10e27%).The d15N(N2) values and high illite content suggest that nitrogenwas mainly generated during thermal transformation of organic

Y. Liu et al. / Marine and Petroleum Geology 77 (2016) 693e702 699

matter and/or from NH4-rich illites of the clayey facies of the lowerCambrian shale (Fig. 6).

4.4. Origin of carbon dioxide

The carbon dioxide concentration in the analyzed gases from thelower Cambrian shale in the study area varies from 1.13 to 4.96%,and the d13C values of the carbon dioxide range from �8.9to �13.6‰ (Table 3). Various studies have been carried out toidentify biogenic and abiogenic CO2 (Moore et al., 1977) Thed13C(CO2) values ranged from �4.5‰ to �6.0‰ in basalt inclusionsfrom the Pacific Mid-Ocean Ridge (Moore et al., 1977); Gould et al.(1981) found that themagmatic CO2 had a d13C values of�7‰± 2‰despite its variety; the d13C values of the crustal abiogenic CO2commonly ranged from þ1‰ to �3‰ (Shangguan and Gao, 1990).Comprehensive study of more than 1000 gas samples from 16 gasfields in China as well as international data indicated that the d13Cvalues of biogenic CO2 mainly ranging from �10‰ to �30‰, whilethose of abiogenic CO2 are more than �8‰, mainly ranging

Fig. 9. N2 versus CH4 concentrations (a) and measured gas content (b)

Fig. 8. d13C(CO2) versus R/Ra for analysed natural gases (modified after Zhang et al.,2008a,b).

from �8‰ to 3‰ (Dai, 2000; Dai et al., 2014b). The abiogenic CO2derived from the decomposition of carbonate sediments usuallyhas a d13C value of 0‰ ± 3‰. (Dai et al., 2014b).

A plot of d13C(CO2) versus d13C(CH4) (Fig. 7) and a plot ofd13C(CO2) versus R/Ra (Fig. 8) suggest that the analyzed carbondioxide was mainly generated during thermogenic processes oftransformation of organic matter. This is distinctly different fromcarbon dioxide that occurs in natural gas from the Longmaxi shalein southern Sichuan Basin (Dai et al., 2014b), which is identified asabiogenic derived from the decomposition of carbonate sediments(Figs. 7 and 8).

4.5. Geological implications

High nitrogen contents in the natural gas from lower Cambrianshale have been observed in many areas in southern China. A fewstudies (Chen, 2005; Nie et al., 2012) suggest that the nitrogenderived from the atmosphere and migrated by infiltration of sur-face water into the strata. However, as discussed in the previoussection, nitrogen occurring in the natural gases from the lowerCambrian shale in the study area was mainly generated duringthermal transformation of organic matter and/or from NH4-richillites of the clayey facies of the lower Cambrian shale.

Shale gas usually consists of methane, small amounts of heavyhydrocarbons (C2eC6), and nonhydrocarbon gases (CO2, N2). In thisstudy, the contents of nitrogen and methane vary greatly amongthe gas samples (Table 3), and there is an obvious negative corre-lation between N2 and CH4 concentrations (Fig. 9a). Furthermore,the negative correlation between N2 concentration and themeasured gas content was also shown in this study (Fig. 9b). Asshown in Table 4, the porosity and permeability of core sampleswith a high content of nitrogen are from 1.8% to 3.2% and from0.0958 � 10�3 mm2 to 0.2367 � 10�3 mm2, respectively, which weresignificantly higher than those of samples with a low nitrogencontent (the porosity and permeability are from 0.8% to 2.2% andfrom 0.0056 � 10�3 mm2 to 0.0087 � 10�3 mm2, respectively). Liet al. (2015a,b) indicated that the shale adsorption capacity ishigher for N2 than for CH4, and CH4 has the lowest capacity. Liuet al. (2016) observed that CH4 had a higher diffusion ability thanN2 according to the shale desorption experiments. Therefore, themajor reason for the change in nitrogen and methane contents inthe studied well (ZD-1) is probably the development of microcracks in core samples with high nitrogen contents. The high ni-trogen content may be caused by the strong adsorption capacity of

of natural gases from the Lower Cambrian shale in the study area.

Table 4Porosity and permeability of the core samples from the lower Cambrian shale in the study area.

Sample Strata Depth(m) Length (cm) Diameter (cm) Effective porosity (%) Effective permeability ( � 10�3 mm2)

ZD-01 21 263.4 2.5 2.5 2.3 0.0958ZD-02 21 265.9 2.5 2.5 2.2 0.0087ZD-03 21 268.6 2.5 2.5 2.1 0.0093ZD-04 21 271.2 2.5 2.5 2.7 0.1088ZD-06 21 275.6 2.5 2.5 2.6 0.1857ZD-07 21 277.9 2.5 2.5 2.2 0.1365ZD-08 21 282.2 2.5 2.5 2.4 0.1617ZD-09 21 285.1 2.5 2.5 3.2 0.2367ZD-10 21 288.6 2.5 2.5 1.6 0.0111ZD-12 21 299.6 2.5 2.5 2.4 0.0253ZD-13 21 306.4 2.5 2.5 2.6 0.0198ZD-14 21 311.5 2.5 2.5 1.8 0.0994ZD-15 21 317.4 2.5 2.5 3.2 0.1066ZD-16 21 324.7 2.5 2.5 1.9 0.0069ZD-17 21 328.0 2.5 2.5 1.8 0.0091ZD-18 21 334.7 2.5 2.5 1.1 0.0062ZD-19 21 351.4 2.5 2.5 1.2 0.0057ZD-20 21 263.4 2.5 2.5 0.8 0.0056ZD-22 21 265.9 2.5 2.5 2.2 0.0087

Y. Liu et al. / Marine and Petroleum Geology 77 (2016) 693e702700

nitrogen, which leads to the retention of nitrogen and the diffusionof methane in shale.

Previous studies suggested that the study area underwentseveral stages of uplift and subsidence throughout the Neo-proterozoic to Cretaceous times (Zhang,1986; Dai, 1996; Jiang et al.,2002; Xu et al., 2004; Ge et al., 2010). Complex tectonic movementsand faults accelerate the diffusion of methane in shale gas. Inaddition, the aquifers near the lower Cambrian strata in the studyarea have been observed bymany studies (Wei et al., 2010; Liu et al.,2011). The water circulation in the aquifers and unconformityprovide other paths for the diffusion of methane in shale gas.

5. Conclusions

(1) Stable isotopic data of carbon and hydrogen of gases from thelower Cambrian shale in the Yangtze Gorges area show thatthe gases are thermogenic in origin and oil-derived gases.Moreover, methane with heavy carbon isotopic and lighthydrogen isotopic values may be produced through forma-tion water reacting with inorganic carbon.

(2) The carbon isotopic compositions of all of the alkane gasesfrom the lower Cambrian shale in the Yangtze Gorges areaare characterized by d13C1 > d13C2, which was probably dueto destructive redox reactions at maximum burial resultingin Rayleigh-type fractionation and/or being mixed by gasesfrom the same source but at a different maturity.

(3) d15N(N2) and N2 concentration suggests that nitrogen wasmainly generated during thermal transformation of organicmatter and/or from NH4-rich illites of the clayey facies of thelower Cambrian shale. Carbon dioxide was mainly generatedduring thermogenic processes of transformation of organicmatter. This is distinctly different from carbon dioxideoccurring in the natural gas from the Longmaxi shale in thesouthern Sichuan Basin, which is identified as abiogenicderived from the decomposition of carbonate sediments.

(4) The high nitrogen content may be caused by the strongadsorption capacity of nitrogen, which leads to the retentionof nitrogen and the diffusion of methane in shale. A complexfaulting system and water circulation in the aquifers or un-conformity provide paths for the diffusion of methane inshale gas.

Acknowledgments

This work was supported by the Key Program of National Nat-ural Science Foundation of China (41102088), the FundamentalResearch Funds for the Central Universities (Grant No. 2012047),and China Geological Survey (12120115007201).

References

Bernard, B.B., Brooks, J.M., Sackett, W.M., 1978. Light hydrocarbons in recent Texascontinental shelf and slope sediments. J. Geophys. Res. 83, 4053e4061.

Berner, U., Faber, E., 1988. Maturity related mixing model for methane, ethane andpropane, based on carbon isotopes. Org. Geochem. 13, 67e72.

Berner, U., Faber, E., 1996. Empirical carbon isotope/maturity relationships for gasesfrom algal kerogens and terrigenous organic matter, based on dry, open-systempyrolysis. Org. Geochem. 24, 947e955.

Burruss, R.C., Laughrey, C.D., 2010. Carbon and hydrogen isotopic reversals in deepbasin gas: evidence for limits to the stability of hydrocarbons. Org. Geochem. 41,1285e1296.

Chen, P., 1987. The sinian system. In: Wang, X. (Ed.), Stratigraphic ExcursionGuidebook in the Yangtze Gorge Area. Yichang Institute of Geology and MineralResources, Chinese Academy of Geological Sciences, Geological PublishingHouse, Beijing, China, pp. 2e7.

Chen, A.D., 2005. Nitrogen as an index of oil-gas preservation conditions in marinestrata. Pet. Geol. Exp. 27, 85e89 (in Chinese with English abstract).

Cheng, L.X., Wang, Y.J., Chen, H.D., Wang, Y., Zhong, Y.J., 2013. Sedimentary andburial environment of black shales of Sinian to early Palaeozoic in upperYangtze region. Acta Petrol. Sin. 29, 2906e2912 (in Chinese with Englishabstract).

Chung, H.M., Gormly, J.R., Squires, R.M., 1988. Origin of gaseous hydrocarbons insubsurface environments: theoretical considerations of carbon isotope distri-bution. Chem. Geol. 71, 91e103.

Claypool, G.E., 1998. Kerogen conversion in fractured shale petroleum systems. Am.Assoc. Pet. Geol. Bull. 82 (Suppl. 5).

Curtis, J.B., 2002. Fractured shale-gas systems. Am. Assoc. Pet. Geol. Bull. 86,1921e1938.

Dai, J.X., 1992. Identification and distinction of various alkane gases. Sci. China. Ser.B Chem. 35, 1246e1257 (in Chinese).

Dai, S.W., 1996. Discussion on the regional structural features of Jianghan basinsince Indosinian movement. J. Geomech. 2, 80e84 (in Chinese with Englishabstract).

Dai, J.X., 2000. Geological Basis and Main Controlling Factors of Coal Formed Largeand Medium Sized Gas Fields in China. Petroleum Industry Press, pp. 69e70.

Dai, J.X., Qin, S.F., Tao, S.Z., Zhu, G., Mi, J.K., 2005. Development trends of natural gasindustry and the significant progress on natural gas geological theories inChina. Nat. Gas Geosci. 16, 127e142 (in Chinese with English abstract).

Dai, J.X., Xia, X.Y., Qin, S.F., Zhao, J.Z., 2004. Origins of partially reversed alkane d13Cvalues for biogenic gases in China. Org. Geochem. 35, 405e411.

Dai, J.X., Gong, D.Y., Ni, Y.Y., Huang, S.P., Wu, W., 2014a. Stable carbon isotopes ofcoal-derived gases sourced from the Mesozoic coal measures in China. Org.Geochem. 74, 123e142.

Dai, J.X., Zou, C.N., Liao, S.M., Dong, D.Z., Ni, Y.Y., Huang, W.H., Gong, D.Y., Huang, S.P.,Hu, G.Y., 2014b. Geochemistry of the extremely high thermal maturity

Y. Liu et al. / Marine and Petroleum Geology 77 (2016) 693e702 701

Longmaxi shale gas, southern Sichuan Basin. Org. Geochem. 74, 3e12.Ding, L., Li, Y., Hu, X., Xiao, Y., Su, C., Huang, J., 1996. Sinian Miaohe Biota of China.

Geological Publishing House, Beijing, p. 261.Etiope, G., Feyzullayev, A., Baciu, C.L., 2009. Terrestrial methane seeps and mud

volcanoes: a global perspective of gas origin. Mar. Pet. Geol. 26, 333e344.Gang, W., Gao, G., Hao, S., Zhu, L., 1997. Carbon isotope of ethane applied in the

analyses of genetic types of natural gas. Pet. Explor. Dev. 19, 164e167 (in Chi-nese with English abstract).

Gasparik, M., Ghanizadeh, A., Bertier, P., Gensterblum, Y., Bouw, S., Krooss, B.M.,2012. High-pressure methane sorption isotherms of black shales from TheNetherlands. Energy Fuels 26, 4995e5004.

Gasparik, M., Bertier, P., Gensterblum, Y., Ghanizadeh, A., Krooss, B.M., Littke, R.,2014. Geological controls on the methane storage capacity in organic-richshales. Int. J. Coal Geol. 123, 34e51.

Ge, X.H., Wang, M.P., Liu, J.L., 2010. Redefining the Sichuan Movement and the ageand background of Qingzang plateau's first uplift: the implication of Huanglinganticline and its enlightenment. Earth Sci. Front. 17, 206e217 (in Chinese withEnglish abstract).

Gerling, P., Idiz, E., Everlien, G., Sohns, E., 1997. New aspects on the origin of nitrogenin natural gas in Northern Germany. Geol. Jahrb. D. 103, 65e84.

Goldberg, T., Strauss, H., Guo, Q., Liu, C., 2007. Reconstructing marine redox con-ditions for the early Cambrian Yangtze platform: evidence from biogenicsulphur and organic carbon isotopes. Palaeogeogr. Palaeoclimatol. Palaeoecol.254, 175e193.

Gould, K.W., Hart, G.H., Smith, J.W., 1981. Carbon dioxide in the southern coalfields-a factor in the evaluation of natural gas potential. Proc. Australas. Inst. Min.Metall. 279, 41e42.

Gutsalo, L.K., Plotnikov, A.M., 1981. Carbon isotopic composition in the CH4-CO2system as a criterion for the origin of methane and carbon dioxide in Earthnatural gases. Dokl. Akad. Nauk. SSSR 259, 470e473 [in Russian].

Hao, F., Zou, H., 2013. Cause of shale gas geochemical anomalies and mechanismsfor gas enrichment and depletion in high-maturity shales. Mar. Pet. Geol. 44,1e12.

Hoering, T.C., Moore, H., 1958. The isotopic composition of the nitrogen in naturalgases and associated crude oils. Geochim. Cosmochim. Acta 13, 225e232.

Hu, J.M., Chen, H., Qu, H.J., Wu, G.L., Yang, J.X., Zhang, Z.Y., 2012. Mesozoic de-formations of the Dabashan in the southern Qinling orogen, central China.J. Asian Earth Sci. 47, 171e184.

Jarvie, D.M., Hill, R.J., Ruble, T.E., Pollastro, R.M., 2007. Unconventional shale-gassystems: the Mississippian Barnett Shale of north-central Texas as one modelfor thermogenic shale-gas assessment. Am. Assoc. Pet. Geol. Bull. 91, 523e533.

Jenden, P.D., Drazan, D.J., 1993. Mixing of thermogenic natural gases in northernAppalachian basin. Am. Assoc. Pet. Geol. Bull. 77, 980e998.

Jenden, P.D., Hilton, D.R., Kaplan, I.R., Craig, H., 1993. Abiogenic hydrocarbons andmantle helium in oil and gas fields. In: Howell, D.G. (Ed.), The Future of EnergyGases: United States Geological Survey, pp. 31e57. Professional Paper 1570.

Jenden, P.D., Titley, P.A., Worden, R.H., 2015. Enrichment of nitrogen and 13C ofmethane in natural gases from the Khuff Formation, Saudi Arabia, caused bythermochemical sulfate reduction. Org. Geochem. 82, 54e68.

Ji, W., Song, Y., Jiang, Z., Wang, X., Bai, Y., Xing, J., 2014. Geological controls andestimation algorithms of lacustrine shale gas adsorption capacity: a case studyof the Triassic strata in the southeastern ordos basin, China. Int. J. Coal Geol. 134,61e73.

Jiang, L.S., Chen, T.L., Zhou, Z.Y., 2002. Several principal geological problems inHuangling. Hubei Geol. Miner. Resour. 16, 8e13 (in Chinese with Englishabstract).

Jiang, G., Shi, X., Zhang, S., Wang, Y., Xiao, S., 2011. Stratigraphy and paleogeographyof the Ediacaran Doushantuo Formation (ca. 635e551 Ma) in South China.Gondwana Res. 19, 831e849.

Jiang, G.Q., Wang, X.Q., Shi, X.Y., Xiao, S.H., Zhang, S.H., Dong, J., 2012. The origin ofdecoupled carbonate and organic carbon isotope signatures in the earlyCambrian (ca. 542-520 Ma) Yangtze platform. Earth Planet. Sci. Lett. 317,96e110.

Kotarba, M., 1988. Geochemical criteria for origin of natural gases accumulated inthe Upper Carboniferous coal-seam-bearing formations in Walbrzych Coal Ba-sin. Zesz. Nauk. Akad. G�orn-Hutn. Acad. Miningand Metall. Bull. No. 1199 Geol.Geol. z 42, 1e119 [in Polish with English abstract].

Kotarba, M.J., 2001. Composition and origin of coalbed gases in the upper silesianand lublin basins. Pol. Org. Geochem. 32, 163e180.

Kotarba, M.J., Lewan, M.D., 2013. Sources of natural gases in Middle Cambrianreservoirs in Polish and Lithuanian Baltic Basin as determined by stable isotopesand hydrous pyrolysis of Lower Palaeozoic source rocks. Chem. Geol. 345,62e76.

Kotarba, M.J., Nagao, K., 2008. Composition and origin of natural gases accumulatedin the Polish and Ukrainian parts of the Carpathian region: gaseous hydrocar-bons, noble gases, carbon dioxide and nitrogen. Chem. Geol. 255, 426e438.

Krooss, B.M., Littke, R., Müller, B., Frielingsdorf, J., Schwochau, K., Idiz, E.F., 1995.Generation of nitrogen and methane from sedimentary organic matter: impli-cations on the dynamics of natural gas accumulations. Chem. Geol. 126,291e318.

Krooss, B.M., Friberg, L., Gensterblum, Y., Hollenstein, J., Prinz, D., Littke, R., 2005.Investigation of the pyrolytic liberation of molecular nitrogen from Paleozoicsedimentary rocks. Int. J. Earth Sci. 94, 1023e1038.

Li, X.M., Shan, Y.H., 2011. Diverse exhumation of the Mesozoic tectonic belt withinthe Yangtze Plate, China, determined by apatite fission-track

thermochronology. Geosci. J. 15, 349e357.Li, J.J., Yan, X.T., Wang, W.M., Zhang, Y.N., Yin, J.X., Lu, S.F., Chen, F.W., Meng, Y.L.,

Zhang, X.W., Chen, X., Yan, Y.X., Zhu, J.X., 2015a. Key factors controlling the gasadsorption capacity of shale: a study based on parallel experiments. Appl.Geochem. 58, 88e96.

Li, Y., Fan, T., Zhang, J., Zhang, J., Wei, X., Hu, X., Zeng, W., Fu, W., 2015b. Geochemicalchanges in the early Cambrian interval of the Yangtze platform, South China:implications for hydrothermal influences and Paleocean redox conditions.J. Asian Earth Sci. 109, 100e123.

Littke, R., Krooss, B.M., Idiz, E.F., Frielingsdorf, J., 1995. Molecular nitrogen in naturalgas accumulations: generation from sedimentary organic matter at high tem-peratures. Am. Assoc. Pet. Geol. Bull. 79, 410e430.

Liu, P.J., Yin, C.Y., Gao, L.Z., Tang, F., Chen, S.M., 2009. New material of microfossilsfrom the ediacaran Doushantuo formation in the Zhangcunping area, Yichang,Hubei Province and its zircon SHRIMP U-Pb age. Chin. Sci. Bull. 54, 1058e1064(in Chinese with English abstract).

Liu, G., Wo, Y., Pan, W., Zhang, C., 2011. Fluid characteristics and hydrocarbonpreservation conditions in marine facies strata of middle-upper Yangtze region.Pet. Geol. Exp. 33, 17e21.

Liu, Y., Zhang, J.C., Tang, X., 2016. Predicting the proportion of free and adsorbed gasby isotopic geochemical data: a case study from lower Permian shale in thesouthern North China basin (SNCB). Int. J. Coal Geol. 156, 25e35.

Lüders, V., Reutel, C., Hoth, P., Banks, D.A., Mingram, B., Pettke, T., 2005. Fluid andgas migration in the North German Basin: fluid inclusion and stable isotopeconstraints. Int. J. Earth Sci. 94, 990e1009.

Lüders, V., Plessen, B., di Primio, R., 2012. Stable carbon isotopic ratios of CH4-CO2-bearing fluid inclusions in fracture-fill mineralization from the Lower SaxonyBasin (Germany)-a tool for tracing gas sources and maturity. Mar. Pet. Geol. 30,174e183.

Mingram, B., Hoth, P., Lüders, V., Harlov, D., 2005. The significance of fixedammonium in Palaeozoic sediments for the generation of nitrogen-rich naturalgases in the North German Basin. Int. J. Earth Sci. 94, 1010e1022.

Moore, J., Batchelder, N., Cunningham, C., 1977. CO2 filled vesicles in mid-oceanbasalt. J. Volcanol. Geotherm. Res. 2, 309e327.

Ni, Y.Y., Dai, J.X., Zhu, G.Y., Zhang, S.C., Zhang, D.J., Su, J., Tao, X.W., Liao, F.R., Wu, W.,Gong, D.Y., Liu, Q.Y., 2013. Stable hydrogen and carbon isotopic ratios of coal-derived and oil-derived gases: a case study in the Tarim basin, NW China. Int.J. Coal Geol. 116e117, 302e313.

Nie, H.K., Bao, S.J., Gao, B., Bian, R.K., Zhang, P.X., Ye, X., Chen, X.J., 2012. A study ofshale gas preservation conditions for the Lower Paleozoic in Sichuan Basin andits periphery. Earth Sci. Front. 19, 280e294.

Osborn, S.G., McIntosh, J.C., 2010. Chemical and isotopic tracers of the contributionof microbial gas in Devonian organic-rich shales and reservoir sandstones,northern Appalachian Basin. Appl. Geochem. 25, 456e471.

Rodriguez, N., Philp, R.P., 2010. Geochemical characterization of gases from themississippian Barnett shale, Fort Worth basin, Texas. Am. Assoc. Pet. Geol. Bull.94, 1641e1656.

Ross, D.J., Bustin, R.M., 2007. Shale gas potential of the lower Jurassic Gordondalemember, northeastern British Columbia, Canada. Bull. Can. Pet. Geol. 55, 51e75.

Schoell, M., 1980. The hydrogen and carbon isotopic composition of methane fromnatural gases of various origins. Geochim. Cosmochim. Acta 44, 649e661.

Schoell, M., 1983. Genetic characterization of natural gases. Am. Assoc. Pet. Geol.Bull. 67, 2225e2238.

Schoell, M., 1988. Multiple origins of methane in the Earth. Chem. Geol. 71, 1e10.Shangguan, Z., Gao, S., 1990. The CO2 discharges and earthquakes in western

Yunnan. Acta Seismol. Sin. 12, 186e193.Shen, C.B., Mei, L.F., Liu, Z.Q., Xu, S.H., 2009. Apatite and zircon fission track data,

evidences for the MesozoiceCenozoic uplift of Huangling dome, central China.J. Mineral. Petrol. 29, 54e60 (in Chinese with English abstract).

Stahl, W.J., 1977. Carbon and nitrogen isotopes in hydrocarbon research andexploration. Chem. Geol. 20, 121e149.

Strapo�c, D., Mastalerz, M., Eble, C., Schimmelmann, A., 2007. Characterization of theorigin of coalbed gases in southeastern Illinois Basin by compound-specificcarbon and hydrogen stable isotope ratios. Org. Geochem. 38, 267e287.

Strapo�c, D., Mastalerz, M., Schimmelmann, A., Drobniak, A., Hedges, S., 2008.Variability of geochemical properties in a microbially dominated coalbed gassystem from the eastern margin of the Illinois Basin, USA. Int. J. Coal Geol. 76,98e110.

Tan, J., Horsfield, B., Mahlstedt, N., Zhang, J., di Primio, R., Vu, T.A.T., Boreham, C.,Graas, G., Tocher, B., 2013. Physical properties of petroleum formed duringmaturation of Lower Cambrian shale in the upper Yangtze Platform, SouthChina, as inferred from Phase Kinetics modelling. Mar. Pet. Geol. 48, 47e56.

Tang, Y., Perry, J.K., Jenden, P.D., Schoell, M., 2000. Mathematical modeling of stablecarbon isotope ratios in natural gases. Geochim. Cosmochim. Acta 64,2673e2687.

Tang, Y., Li, L.Z., Jiang, S.X., 2014. A logging interpretation methodology of gascontent in shale reservoirs and its application. Nat. Gas. Ind. 34, 46e54 (inChinese with English abstract).

Tian, H., Pan, L., Xiao, X., Wilkins, R., Meng, Z., Huang, B., 2013. A preliminary studyon the pore characterization of Lower Silurian black shales in the ChuandongThrust Fold Belt, southwestern China using low pressure N2 adsorption and FE-SEM methods. Mar. Pet. Geol. 48, 8e19.

Tilley, B., Muehlenbachs, K., 2013. Isotope reversals and universal stages and trendsof gas maturation in sealed, self-contained petroleum systems. Chem. Geol. 339,194e204.

Y. Liu et al. / Marine and Petroleum Geology 77 (2016) 693e702702

Tilley, B., McLellan, S., Hiebert, S., Quartero, B., Veilleux, B., Muehlenbachs, K., 2011.Gas isotope reversals in fractured gas reservoirs of the western CanadianFoothills: mature shale gases in disguise. Am. Assoc. Pet. Geol. Bull. 95,1399e1422.

Wang, X., 1989. Geochemistry and Cosmochemistry of Noble Gas Isotope. SciencePress, Beijing, p. 112 (in Chinese).

Wang, E., Meng, Q.R., Burchfiel, B.C., Zhang, G.W., 2003. Mesozoic large-scale lateralextrusion, rotation, and uplift of the Tongbai-Dabie shan belt in east China.Geology 31, 307e310.

Wang, X.F., Erdtmann, B.D., Chen, X.H., Mao, X.D., 2008. Integrated sequence-, bio-and chemo-stratigraphy of the terminal Proterozoic to Lowermost Cambrian“black rock series” from central South China. Episodes 21, 178e189.

Wang, J., Chen, D., Yan, D., Wei, H., Xiang, L., 2012. Evolution from an anoxic to oxicdeep ocean during the Ediacaran-Cambrian transition and implications forbioradiation. Chem. Geol. 306e307, 129e138.

Wang, S., Zou, C., Dong, D., Wang, Y., Li, X., Huang, J., Guan, Q., 2015. Multiplecontrols on the paleoenvironment of the early Cambrian marine black shales inthe Sichuan Basin, SW China: geochemical and organic carbon isotopic evi-dence. Mar. Pet. Geol. 66, 660e672.

Wei, G., Jiao, G., Yang, W., Xie, Z., Li, D., Xie, W., Liu, M., Zeng, F., 2010. Hydrocarbonpooling conditions and exploration potential of Sinian-Lower Paleozoic gasreservoirs in the Sichuan Basin. Nat. Gas. Ind. 30, 5e9 (in Chinese with Englishabstract).

Whiticar, M.J., 1999. Carbon and hydrogen isotope systematic of microbial forma-tion and oxidation of methane. Chem. Geol. 161, 291e314.

Whiticar, M.J., Faber, E., Schoell, M., 1986. Biogenic methane production in marineand freshwater environments: CO2 reduction vs. acetate fermentation-isotopeevidence. Geochim. Cosmochim. Acta 50, 693e709.

Wo, Y.J., Zhou, Y., Xiao, K.H., 2007. The burial history and models for hydrocarbongeneration and evolution in the marine strata in southern China. Sediment.Geol. Tethyan Geol. 27, 94e100 (in Chinese with English abstract).

Xia, X., Zhao, L., Zhang, W., Li, J., 1999. Geochemical characteristics and source rockof Ordovician gas reservoir, Changqing Gasfield. Chin. Sci. Bull. 44, 1917e1920.

Xia, X., Chen, J., Braun, R., Tang, Y., 2013. Isotopic reversals with respect to maturitytrends due to mixing of primary and secondary products in source rocks. Chem.Geol. 339, 205e212.

Xu, Y., 1994. Genetic Theories of Natural Gases and Their Application. Science Press,Beijing, p. 413 (in Chinese).

Xu, S., Nakai, S., Wakita, H., Xu, Y., Wang, X., 1995a. Helium isotope compositions insedimentary basins in China. Appl. Geochem. 10, 643e656.

Xu, S., Nakai, S.I., Wakita, H., Wang, X., 1995b. Mantle-derived noble gases in naturalgases from Songliao Basin, China. Geochim. Cosmochim. Acta 59, 4675e4683.

Xu, Z.Y., Lin, G., Liu, C.Y., Wang, Y.J., Guo, F., 2004. A discussion on amalgamationcourse between the South China and North China blocks: evidences fromdeformational characters in the Jianghan superimposed basin. Chin. J. Geol. 39,284e295 (in Chinese with English abstract).

Xu, C.H., Zhou, Z.Y., Chang, Y., Guillot, F., 2010. Genesis of Daba arcuate structuralbelt related to adjacent basement upheavals: constraints from Fission-track and(U-Th)/He thermochronology. Sci. China Earth Sci. 53, 1634e1646.

Zhang, Z., 1981. Precambrian microfossils from the sinian of South China. Nature289, 792e793.

Zhang, H.D., 1986. Preliminary analysis of the formation and tectonic evolution ofHuangling anticline. J. Jianghan Pet. Inst. 10, 29e40 (in Chinese).

Zhang, B.M., Zhang, S.C., Bian, L.Z., Jin, Z.J., Wang, D.R., 2007. Developmental modesof the Neoproterozoic-lower Paleozoic marine hydrocarbon source rocks inChina. Chin. Sci. Bull. 52, 77e91 (in Chinese with English abstract).

Zhang, J.C., Nie, H.K., Xu, B., Jiang, S.C., Zhang, P.X., Wang, Z.Y., 2008a. Geologicalcondition of shale gas accumulation in sichuan basin. Nat. Gas. Ind. 28, 151e156(in Chinese with English abstract).

Zhang, T.W., Zhang, M.J., Bai, X.W., Li, L., 2008b. Origin and accumulation of carbondioxide in the huanghua depression, bohai bay basin, China. Am. Assoc. Pet.Geol. Bull. 92, 341e358.

Zhang, T.W., Ellis, G.S., Ruppel, S.C., Milliken, K., Yang, R.S., 2012. Effect of organic-matter type and thermal maturity on methane adsorption in shale-gas sys-tems. Org. Geochem. 47, 120e131.

Zhao, Z., Xing, Y., Ma, G., Chen, Y., 1985. Biostratigraphy of the Yangtze Gorge Area(1) Sinian. Geological Publishing House, Beijing, p. 164 (in Chinese with EnglishAbstract).

Zhao, Z., Xing, Y., Ding, Q., Liu, G., Zhao, Y., Zhang, S., Meng, X., Yin, C., Ning, B.,Han, P., 1988. The Sinian System of Hubei. China University of GeosciencesPress, Wuhan, p. 205.

Zhou, T., Xu, G., 1987. The Cambrian system. In: Wang, X. (Ed.), StratigraphicExcursion Guidebook in the Yangtze Gorge Area. Yichang Institute of Geologyand Mineral Resources, Chinese Academy of Geological Sciences, GeologicalPublishing House, Beijing, China, pp. 8e13.

Zhu, Y., Shi, B., Fang, Ch, 2000a. The isotopic composition of molecular nitrogen:implications on their origins in natural gas accumulations. Chem. Geol. 164,321e330.

Zhu, Y.N., Shi, B.Q., Fang, C.B., 2000b. The isotopic compositions of molecular ni-trogen: implications on their origins in natural gas accumulations. Chem. Geol.164, 321e330.

Zhu, M.Y., Zhang, J.M., Steiner, M., Yang, A.H., Li, G.X., Erdtmann, B.D., 2003. Sinian-Cambrian stratigraphic framework for shallow to deep-water environments ofthe Yangtze Platform: an integrated approach. Prog. Nat. Sci. 13, 951e960.

Zou, C.N., Dong, D.Z., Wang, S.J., Li, J.Z., Li, X.J., Wang, Y.M., Li, D.H., Cheng, K.M.,2010. Geological characteristics and resource potential of shale gas in China.Pet. Explor. Dev. 37, 641e653.

Zou, C.N., Wei, G.Q., Xu, C.C., Du, J.H., Xie, Z.Y., Wang, Z.C., Hou, L.H., Yang, C., Li, J.,Yang, W., 2014. Geochemistry of the sinian-cambrian gas system in the Sichuanbasin, China. Org. Geochem. 74, 13e21.

Zou, C.N., Yang, Z., Dai, J.X., Dong, D.Z., Zhang, B.M., Wang, Y.M., Deng, S.H.,Huang, J.L., Liu, K.Y., Yang, C., Wei, G.Y., 2015. The characteristics and signifi-cance of Conventional and unconventional Sinian-Silurian gas systems in theSichuan Basin, central China. Mar. Pet. Geol. 64, 386e402.

Zumberge, J., Ferworn, K., Brown, S., 2012. Isotopic reversal (‘rollover’) in shale gasesproduced from the Mississippian Barnett and Fayetteville formations. Mar. Pet.Geol. 31, 43e52.