Hydrocarbon Generation Kinetics of Lacustrine Yanchang Shale in

Aug 19, 2014 - Key Laboratory of Marine Hydrocarbon Resources and ... School of Energy Resources, China University of Geosciences, Beijing 100083, ...
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Hydrocarbon Generation Kinetics of Lacustrine Yanchang Shale in Southeast Ordos Basin, North China Shuangbiao Han,*,†,‡,§,∥,⊥ Brian Horsfield,∥ Jinchuan Zhang,§ Qian Chen,§,⊥ Nicolaj Mahlstedt,∥ Rolando di Primio,∥ and Guolin Xiao‡ †

School of Geosciences, China University of Petroleum, Qingdao 266580, People’s Republic of China Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources, Qingdao 266071, People’s Republic of China § School of Energy Resources, China University of Geosciences, Beijing 100083, People’s Republic of China ∥ German Research Centre for Geosciences (GFZ), Potsdam 14473, Germany ⊥ Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, People’s Republic of China ‡

ABSTRACT: The upper Triassic Yanchang shale in Southeast Ordos Basin (SOB) is a main potential source rock for conventional petroleum fields and has been recently recognized as an important unconventional reservoir. Here, we report on the hydrocarbon potential of this lacustrine shale using bulk and quantitative pyrolysis techniques. The rock samples were taken from the Chang7 and Chang9 intervals of upper Triassic-aged cores. The analytical program included total organic carbon (TOC), Rock-Eval, pyrolysis gas chromatography (Py−GC), source rock analyzer (SRA), and microscale sealed vessel (MSSV) pyrolysis. Phase kinetic modeling was also employed on the basis of these data sets. The results were used to determine the petroleum-type organofacies, bulk hydrocarbon composition during maturation, bulk and compositional kinetics, and phase behavior of fluids generated in the Yanchang shales. The shales proved to contain type II2 kerogen with organic matter in high abundance and generate paraffinic−naphthenic−aromatic (PNA) low wax oils when mature, whereas samples with increasing maturity show a potential for gas condensate generation. Bulk kinetic parameters of the immature Yanchang shale reveal a relatively broad distribution of activation energies and indicate lower stabilities than marine Cambrian type II shale in south China. Hydrocarbon generation could be characterized by a frequency factor A = 2.20 × 1012 S−1 and a main activation energy at 50 kcal/mol. Extrapolation to the geological heating rate of 1.0 °C/Ma in SOB, the onset (transformation ratio = 10%) and peak generation temperatures were 115 and 124 °C, respectively. Compositional kinetic modeling predicts that the generated gas fraction mainly consists of C1, C2, and C3, while the liquid phase is predominated by compound groups of C7−15 and C16−25. Furthermore, the gas/oil ratio (GOR) varies between 83.6 Sm3/Sm3 (97.1 m3/t) and 168.2 Sm3/Sm3 (195.3 m3/t). The saturation pressure (Psat) and formation volume factor (Bo) display a linear correlation as a function of the transformation ratio (TR). The property of the generated hydrocarbons is in agreement with naturally occurring petroleum fluids. Using the pressure−temperature (P−T) envelope defined from these experiments, only a single liquid phase (black oil) is predicted at different TRs (10−70%). This research provides the first case study with respect to phase kinetics description of Yanchang shale oil and shale gas in the study area.

1. INTRODUCTION

Predicting petroleum composition and phase behavior is of crucial importance in both conventional and unconventional systems.10−12 This is a prerequisite for economic evaluations at both regional and prospect levels. In conventional systems, predicting the gas/oil ratio and charge volumes ahead of drilling is routinely undertaken. Published case-specific compositional kinetic models have made predictions that are in close accordance with field calibrations.13,14 Kinetic parameters for oil to gas cracking12 and late gas generation15 also assist in evaluating liquid and gas potentials. In shale oil, the liquid−gas cutoff must be known precisely and production from the liquidprone zone must be optimized; however, the application of compositional predictions is still in its infancy.

In 1907, China’s first petroleum discovery was achieved in the Ordos Basin.1 After several decades of modern industrial exploration and exploitation, many large conventional oil and gas fields (e.g., the Sulige Gasfield) have been discovered in this basin.2,3 More recently, coalbed methane (CBM) production has also been developed.4 However, despite the long period of conventional petroleum and CBM research, strategic surveys and investigations related to shale oil and shale gas only began a few years ago in the Southeast Ordos Basin (SOB).5,6 The upper Triassic Yanchang shale has been identified as an important source rock for conventional petroleum fields7 and has also been recognized recently as a prolific shale oil and shale gas resource.8,9 In fact, some vertical wells have been drilled in the SOB and have produced commercial oil and gas from fracturing.5,6,8 © 2014 American Chemical Society

Received: May 11, 2014 Revised: August 7, 2014 Published: August 19, 2014 5632

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Figure 1. Location, sedimentary facies, and shale thickness of the upper Triassic Yanchang formation in SOB (the facies map is modified with permission from ref 18).

Table 1. TOC and Rock-Eval Data sample ID

well

depth (m)

formation

TOC (%)

S1 (mg/g)

S2 (mg/g)

S3 (mg/g)

Tmax (°C)

HI (mg of HC/g of TOC)

OI (mg of CO2/g of TOC)

G012139 G012140 G012142 G012143 G012144 G012145 G012146 G012147 G012148 G012149 G012150 G012151 G012152 G012153 G012154 G012155 G012156 G012157

Y127 Y171 Y175 Y178 Y009 Y005 Y12 Y39 Y51 Y016 Y061 Y109 Y36 Y8 Y16 Y6 Y6 Y6

1669.73 1780 1755.8 1608.3 798.8 694.57 902.09 1121.41 1437 870.9 526.61 847.27 1378.2 1528.5 1401.77 1471.73 1644.17 1474.53

Chang 9 Chang 7 Chang 7 Chang 9 Chang 9 Chang 7 Chang 9 Chang 7 Chang 7 Chang 9 Chang 7 Chang 7 Chang 7 Chang 7 Chang 7 Chang 7 Chang 9 Chang 7

1.33 0.55 0.40 2.93 1.70 3.03 0.54 5.54 0.67 0.53 0.47 0.43 0.59 5.96 1.46 2.72 6.08 5.64

0.41 0.07 0.07 2.31 0.54 0.57 0.09 1.10 0.19 0.09 0.07 0.03 0.06 4.83 0.72 4.66 3.78 4.32

1.67 0.23 0.18 5.67 3.37 6.79 0.33 9.15 0.54 0.37 0.19 0.13 0.23 13.71 1.94 6.76 12.85 14.13

0.17 0.17 0.26 0.28 0.25 0.21 0.12 0.22 0.06 0.14 0.12 0.11 0.09 1.35 0.27 0.35 0.27 0.37

460 475 470 448 448 447 460 454 455 460 456 460 457 431 449 441 452 447

126 42 45 194 198 224 62 165 80 70 40 30 39 230 133 249 211 251

13 31 65 10 15 7 22 4 9 26 25 26 15 23 18 13 4 7

ature conditions. A further refinement of prospectivity studies is conducted by applying the experimental data and modeling results into the regional context. In addition, this paper provides an emblematical case study regarding modeled data, with experimental results obtained on a multitude of rock samples.

Thus, kinetics analyses regarding aspects of shale oil and shale gas for the Yanchang formation are scarce or non-existent. The applied focus of this paper is on solving kinetics and phase behavior issues. To do this, we built an understanding of the dynamics of the Yanchang system with respect to hydrocarbon generation and migration in SOB. Taking the thermal stability of kerogen into account, in this study, we describe the petroleum potential, source rock kinetics, and physical properties of the Yanchang petroleum fluids under varying pressure and temper-

2. GEOLOGY OF THE STUDY AREA The Ordos Basin is a large sedimentary basin (3.7 × 105 km2) located in the North China Block (Figure 1). The basin 5633

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framework is a huge asymmetric syncline floored by an Archean and Proterozoic crystalline basement. Tectonically, it can be subdivided into six structural units: the Yimeng Uplift, the Jinxi Fault Belt, the Weibei Uplift, the Tianhuan Depression, the Western Edge Fault Belt, and the Yishan Slope. There are four major evolutional stages: early Paleozoic shallow marine platform, late Paleozoic marine and terrestrial transition, Mesozoic foreland basin, and Cenozoic margin faulting and subsidence.16 The sedimentary strata in this basin comprise Paleozoic−Cenozoic sediments with the maximum thickness up to 10 km.17 There are no Silurian or Devonian deposits because of a regional unconformity. According to previous analyses, three potential series of source rocks have been indentified: Lower Paleozoic marine carbonate, Upper Paleozoic Carboniferous− Permian coal, and Mesozoic Triassic lacustrine shale.18−20 Intense tectonic activity during the Late Triassic caused rapid expansion of this lake basin, which resulted in the formation of lacustrine sediments.21 The study area (SOB) is located in lacustrine facies and merged with delta-plain facies. The sedimentary strata consist mainly of shale, mudstone, and siltstone. This kind of depositional environment provides the basic geological condition for the development of high-quality source rocks. The upper Triassic Yanchang formation is in excess of 5 × 104 km2, with a broad northwest to southeast distribution over the whole basin (Figure 1). Its thickness is generally between 60 and 120 m. Meanwhile, the black shales show good gas concentration and flow characteristics in SOB.

(FID) was connected to the HP-Ultra 1 column with helium as the carrier gas. The GC unit temperature was programmed from 30 to 320 °C at 5 °C min−1. 3.2.3. Bulk Kinetics. Rock-Eval using an open pyrolysis system at four different heating rates (0.7, 2.0, 5.0, and 15 °C min−1) was employed to characterize the kinetic parameters of organic matter to hydrocarbon conversion using a source rock analyzer (SRA, Humble). The varying heating rates ensured the mathematical model and the parameter calculation based on equation iteration. The temperature was programmed from 250 to 600 °C. Four ground material aliquots (ranging from 10 to 200 mg according to the above-mentioned heating rates and organic matter richness) of each sample were weighed into small vessels and subsequently pyrolyzed. Bulk generated products were carried to the FID by helium gas flow at the constant rate of 50 mL min−1. The peak generation temperature shifted via ranging heating rates. To avoid the temperature problem related to the fast heating rate, slow heating rates (0.7, 2.0, and 5.0 °C min−1) were employed to calculate and model the evolution curves using the KINETICS05 and KMOD programs.24 3.2.4. Microscale Sealed Vessel (MSSV) Py−GC. Non-isothermal closed system pyrolysis with a MSSV22 was conducted for the artificial maturation analysis. This approach was applied to provide compositional information on hydrocarbon generation. A milligram aliquot (ca. 1−20 mg) of finely ground sample was accurately weighed into a glass capillary tube. Precleaned quartz powder sand was used to fill the remaining internal volume of the sample tube, which was later sealed by a H2 flame. Off-line pyrolysis was performed in a special heating oven with a homogeneous temperature control. The sample capillaries were heated at a rate of 0.7 °C min−1 to desired end temperatures corresponding to 10, 30, 50, 70, and 90% transformation ratios (TRs) as derived from the bulk kinetics data. Afterward, the tubes were removed and placed in a Quantum MSSV-2 thermal analyzer interfaced with an Agilent GC6890A. After remobilization at 300 °C for 5 min, the samples were then cracked open by a piston device. The generated products were measured as described above for open Py−GC. 3.2.5. Phase Kinetic Modeling. Linking source rock characteristics to petroleum-type organofacies is critical for exploration and exploitation. Bulk and compositional kinetic information could be determined by open and closed system pyrolysis techniques. Further, the phase kinetics approach13 allows for the application of these data to predict hydrocarbon physical properties, i.e., gas/oil ratio (GOR), saturation pressure (Psat), and formation volume factor (Bo). A total of 14 compounds determined from MSSV analysis were assigned to the activation energy distribution. The gas compositions (C1, C2, C3, i-C4, nC4, i-C5, and n-C5) were corrected according to ref 13. The liquid hydrocarbons comprise pseudo-C6, C7−15, C16−25, C26−35, C36−45, C46−55, and C56−80. Under changing subsurface conditions, the petroleum phase behavior prediction (phase kinetics approach) can be conducted in the following four steps: (1) definition of petroleum-type organofacies, (2) determination of thermal response, (3) combination of compositional evolution with thermal response, and (4) input of compositional evolution into PVTsim modeling.

3. MATERIALS AND METHODS 3.1. Sample Set. The lacustrine Yanchang formation can be subdivided into 10 member units, that is, the Chang 1−Chang 10 intervals from top to bottom. Chang 7 and Chang 9 were identified as potential shale gas and shale oil exploration targets.5,6,8 A total of 18 samples from the two intervals from 16 drilling wells were selected for analysis in this research (Table 1). Most of these obtained samples are black carbonaceous or mud shales. 3.2. Experimental Approach. 3.2.1. Total Organic Carbon (TOC) and Rock-Eval. The TOC content was measured using a Leco SC-632 device. After the carbonate is removed, the amount of carbon is measured as carbon dioxide. Rock-Eval pyrolysis was performed using a Rock-Eval 6 instrument to characterize Tmax and bulk thermal parameters S1, S2, and S3. The pyrolysis temperature was programmed from 300 °C (3 min) to 650 °C (0 min) at 25 °C min−1. These measurements were conducted by Applied Petroleum Technology AS, Norway. All experimental procedures follow the Norwegian Industry Guide to Organic Geochemical Analyses (NIGOGA), 4th edition (http://www.npd.no/engelsk/nigoga/). 3.2.2. Open Pyrolysis Gas Chromatography (Py−GC). Nonisothermal open system pyrolysis was performed to reveal the kerogen properties of organic matter in the lacustrine Yanchang shale formation. Dependent upon the organic matter richness, 5−20 mg of crushed shale rock sample was analyzed using a Quantum MSSV-2 thermal analyzer (pyrolysis oven unit) interfaced with an Agilent GC6890A.22 The sample material was positioned in the central part of a ∼25 mm long glass capillary tube with an inner sleeve diameter of ∼2 mm. The remaining internal space was filled with quartz glass wool, which was thermally precleaned at 600−650 °C in air for 30−60 min. To release volatile products, the sample tube was flushed in heating helium flow up to 300 °C for 5 min. Afterward, the glass tube was pyrolyzed from 300 to 600 °C at the rate of 50 °C min−1. The end temperature (600 °C) was isothermally maintained for 2 min.23 Pyrolysis products were collected in a cryogenic trap (liquid nitrogen cooled) and later liberated by ballistic heating (300 °C) into an Agilent GC6890A equipped with a dimethylpolysiloxane HP-Ultra 1 capillary column (50 m length, 0.32 mm inner diameter, and 0.52 μm thickness). A flame ionization detector

4. RESULTS AND DISCUSSION 4.1. Bulk Geochemical Characterization. The TOC content and Rock-Eval are widely used techniques to rapidly analyze generative potential and maturity of source rocks. The TOC content characterizes the source rock quality and indirectly indicates possible petroleum potential. Meanwhile, hydrocarbon generation also depends upon the degree of evolution as well as the initial chemical structure of the biomacromolecules (kerogen). The maturity of a screened source rock sample can be rapidly assessed from its Tmax, which is the temperature required to break most kerogen bonds associated with S2 pyrolysate generation. In general, a Tmax below or around 435 °C is usually interpreted as an immature signature.25 The TOC content of the investigated Yanchang shales varies from 0.40 to 6.08%, with remaining potential hydrocarbon 5634

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Figure 2. Plot of organic matter richness (TOC) versus remaining hydrocarbon generation potential (S2).

Figure 5. Predicted petroleum type from Horsfield.26

index (HI) ranges from 30 to 251 mg of HC g−1 of TOC. The kerogen type and maturity level of the studied shale samples can be estimated from the HI versus Tmax diagram (Figure 3). The organic matter in Yanchang shales is predominantly type II2 kerogen. Most rock samples are in the oil to wet gas maturity range, which is consistent with the Tmax values. 4.2. Petroleum-Type Organofacies. For further characterization of detailed kerogen composition and petroleum-type organofacies, Horsfield26 established a ternary diagram in terms of the alkyl chain length distribution. This approach defined five petroleum-type organofacies fields: paraffinic low wax oil, paraffinic high wax oil, paraffinic−naphthenic−aromatic (PNA) low wax oil, PNA high wax oil, and gas condensate. Pyrolysis products of the investigated shale samples display a domination of light hydrocarbon but poor sulfur-containing compounds (e.g., alkylthiophenes) (Figure 4). The Yanchang shales plot in the PNA low wax oil field, with a gradual transition to the gas and condensate area (Figure 5). The restricted basin environment and continental deltaic depositional setting can be cited for the PNA low wax oil and gas condensate generating facies, respectively. Thus, according to the sedimentary geology of the

Figure 3. Tmax versus HI diagram.

generation (S2) ranging from 0.13 to 14.13 mg g−1 of rock displaying a positive correlation (Figure 2). Rock-Eval hydrogen

Figure 4. Py−GC chromatogram of Yanchang shale. 5635

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Figure 6. Activation energy distributions of Yanchang shales.

Figure 7. Transformation ratio versus temperature curves of Yanchang shales.

Ordos Basin,17,18 the organic matter of upper Triassic Yanchang shale is derived from algae and bacteria material influenced by terrigenous debris input. 4.3. Kinetics of Petroleum Generation. Bulk kinetic parameters (activation energy Ea and frequency factor A) for kerogen to hydrocarbon conversion information are calculated on the basis of the mathematical routine.27 Assuming parallel first-order reactions with a single frequency factor and activation energies, different heating rates are used to achieve optimal values. Optimization results in a best fit for calculated curves and measured curves. Four Yanchang shale samples with different maturities were chosen for analysis. The experimental and measured data are

displayed in Figure 6. In general, broad activation energy distributions were observed by single frequency factors. For the immature G012153 Yanchang shale, the discrete activation energy distributions range from 39 to 62 kcal mol−1 with a frequency factor A = 2.20 × 1012 S−1. The dominant activation energies (48−51 kcal mol−1) account for 71% of total kerogen conversion. This is attributed to the limited range of stable chemical bonds, which crack at later thermal stress stages.24 In comparison to marine type II shale (A = 8.43 × 1014 S−1) in south China (54−58 kcal mol−1),28 organic matter within the young upper Triassic Yanchang shale shows lower stability than older Cambrian shale. 5636

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Figure 8. Compositional kinetics data of immature Yanchang shale.

versus temperature curves of generation kinetics are modeled in Figure 7 for a better numerical and graphical comparison. The immature G012153 sample is representative for revealing kinetics parameters. The temperatures for onset (10%), maximum (geological peak temperature, Tmax), and end (90%) transformations of hydrocarbon generation can be determined as 115, 124, and 180 °C, respectively. The prediction confirms that Yanchang organic matter generates hydrocarbons over a very broad temperature range (∼65 °C), which is comparable to the published data for similar types of organic matter.10,11,29 This immature G012153 Yanchang shale rock sample was also selected for closed-system Py−GC. MSSV pyrolysis was applied to different kerogen transformations (10, 30, 50, 70, and 90%) to predict the compositions of the generated hydrocarbons at differing maturation stages. The end heating temperatures for each transformation were based on open system bulk pyrolysis (SRA). The gas fraction generated from upper Triassic Yanchang shale mainly consists of C1, C2, and C3, while the liquid phase is dominated by compounds of groups C7−15 and C16−25 (Figure 8). 4.4. Prediction of Phase Behavior. Physical properties of the petroleum sourced by Yanchang shale can be predicted through integration and modeling of MSSV pyrolysis data sets. For phase behavior prediction, MSSV data provided compositions of 14 compounds (mentioned above), which were used to populate the bulk kinetic potentials. Afterward, the physical properties of fluids were calculated using the compositional information. Following the phase kinetics approach,13 the evolution of the GOR, saturation pressure (Psat), and formation volume factor (Bo) were defined as functions of maturity. Finally, this model was applied in the regional context of the SOB. The burial and thermal history was employed according to the Shancan 1 well in the Ordos Basin.30 For Yanchang, the organic matter conversion ranges from 10 to 70%, GOR ranges from 83.6 Sm3/Sm3 (97.1 m3/t) to 168.2 Sm3/Sm3 (195.3 m3/t), Psat ranges from 93.5 to 126.3 bar, and Bo ranges from 1.36 to 1.70 m3/Sm3 (Figure 9). The calculated GOR values are rather higher than more aliphatic marine or lacustrine source rocks, with GORs below 100 Sm3/Sm3 at comparable maturity stages reported in the literature.13 Thus, black oils (GOR < 200 Sm3/Sm3) are generated. The predicted

Figure 9. Physical properties of generated fluids.

Figure 10. Hydrocarbon-generating history of upper Triassic Yanchang shale (burial evolution is modified with permission from ref 30).

Extrapolating to a geological heating rate of 1.0 °C Ma−1, which corresponds to the thermal evolution of burial history in the Ordos Basin (lower than the average geological heating rate of 3.0 °C Ma−1 in sedimentary basins11), transformation ratio 5637

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Figure 11. Phase envelope curves of generated hydrocarbons.

correlations of Psat versus Bo and Psat versus GOR are representative of natural fluid properties.31 The phase behavior of petroleum formed during Yanchang shale maturation was determined for TRs from 10 to 70%, corresponding to SOB regional context. The geo-temperatures are displayed in Figure 10, and applied reservoir pressures can be calculated on the basis of burial depths (100 bar km−1). The phase envelope parameter of the bubble point pressure indicates the value for which the gas fraction separates from the liquid phase. Subsequent to subsidence from middle Triassic to Jurassic, the onset of petroleum generation for Yanchang shale occurred in the early Cretaceous (125 Ma) at a burial depth of 2380 m (Figure 10). The phase envelope curve for the generated fluid reveals a bubble point pressure around 94 bar, which is much lower than the reservoir pressure of 238 bar (Figure 11a). This means that the black oils generated at this period existed as a single liquid (oil) phase under reservoir conditions. In the case of petroleum expulsion and migration, the generated hydrocarbons may have partly accumulated in overlying formations. Consequently, an oil−gas phase fluid will appear when the reservoir pressure is lower than the bubble point pressure. On the basis of the kinetic analysis, black oil (GOR < 200 Sm3/Sm3) generation continued into the late Cretaceous (93 Ma), at a maximum depth of 3100 m and a geo-temperature around 146 °C (panels b−d of Figure 11). Although the bubble point pressure increased up to 112 bar, the reservoir pressure was still much higher (310 bar). Hence, the single liquid (oil) phase system was maintained during that period.

rock potential. The detailed kerogen composition shows organofacies generating PNA low wax oil, with some potential for gas condensate. A broad activation energy distribution spanning 23 energies (39−62 kcal mol−1) can be observed for immature Yanchang shale, described by a single frequency factor A = 2.20 × 1012 S−1. The main activation energies are in the range of 48−51 kcal mol−1, accounting for 71% of the bulk reactions. Extrapolating to the regional context heating rate of 1.0 °C Ma−1, a very broad generation temperature range of ∼65 °C can be identified. Black oils [GOR ranging between 83.6 Sm3/Sm3 (97.1 m3/t) and 168.2 Sm3/Sm3 (195.3 m3/t)] are generated throughout the whole kerogen conversion from 10 to 70%. Meanwhile, a potential for gas generation exists at relatively high maturity. The predicted parameters of Psat and Bo suggest a linear correlation. Light hydrocarbons (C1, C2, C3, C7−15, and C16−25) dominate the products generated from upper Triassic Yanchang shale. Reservoir pressures were higher than the corresponding bubble point pressures for the whole evolutional process. Only a single phase fluid occurred in the Yanchang shale system. However, the generated black oils would have evolved into a two-phase fluid if petroleum expulsion and migration occurred.

5. CONCLUSION The organic matter of upper Triassic Yanchang shale in the SOB contains up to 6.08% TOC and comprises terrigenously influenced type II2 kerogen, which is in the oil to wet gas maturity stage. The lacustrine Yanchang shales possess source

ACKNOWLEDGMENTS Our research is supported by the Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources (Grants MRE201201 and 20121108609), the Fundamental Research Funds for the Central

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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(29) Braun, R. L.; Burnham, A. K.; Reynolds, J. G.; Clarkson, J. E. Energy Fuels 1991, 5, 192−204. (30) Shuai, Y.; Zhang, S.; Mi, J.; Gong, S.; Yuan, X.; Yang, Z.; Liu, J.; Cai, D. Org. Geochem. 2013, 64, 38−46. (31) di Primio, R.; Dieckmann, V.; Mills, N. Org. Geochem. 1998, 29, 207−222.

Universities (Grant 2-9-2013-144), the Open Research Program of Tectonics and Petroleum Resources Key Laboratory, Ministry of Education (Grant TPR-2013-05), and the National Natural Science Foundation Research (Grants 41272167 and 41102088). The authors are grateful to Ferdinand Perssen for his technical assistance at the German Research Centre for Geosciences (GFZ). Associate Editor Ryan P. Rodgers and all anonymous reviewers are gratefully acknowledged.

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NOMENCLATURE GOR = gas/oil ratio (Sm3/Sm3 or m3/t) Psat = saturation pressure (bar) Bo = formation volume factor (m3/Sm3) TR = transformation ratio (%)

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REFERENCES

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