Journal of Earth Science  2017, Vol. 28 Issue (6): 996-1005 PDF     0
Shale Oil Resource Potential of Es3L Sub-Member of Bonan Sag, Bohai Bay Basin, Eastern China
Shuangfang Lu1, Wei Liu1, Min Wang1, Linye Zhang2, Zhentao Wang3, Guohui Chen1, Dianshi Xiao1, Zhandong Li3, Huiting Hu4
1. Research Institute of Unconventional Petroleum and Renewable Energy (RIUP & RE), China University of Petroleum, Qingdao 257015, China;
2. Geology Scientific Research Institute of Shengli Oilfield Co. Ltd., Dongying 257015, China;
3. Shandong Geological Environmental Monitoring Station, Jinan 250014, China;
4. College of Petroleum Engineering, Northeast Petroleum University, Daqing 163318, China
Abstract: Following shale gas, shale oil has become another highlight in unconventional hydrocarbon exploration and development. A large amount of shale oil has been produced from a host of marine shale in North America in recent years. In China, lacustrine shale, as the main source rock of conventional oil and gas, should also have abundant oil retained in place. In this study, geochemical and geologic characteristics of lacustrine shale from Es3L sub-member in Bonan sag were characterized by using total organic carbon (TOC), Rock-Eval pyrolysis, X-ray diffraction, and ∆log R method. The results show that the Es3L sub-member shale have TOC contents ranging from 0.5 wt.% to 9.3 wt.%, with an average of 2.9 wt.%. The organic matter is predominantly Type Ⅰ kerogen, with minor amounts of Type Ⅱ1 kerogen. The temperature of maximum yield of pyrolysate (Tmax) values ranges from 424 to 447 ℃, with an average of 440 ℃, and vitrinite reflectance (Ro%) ranges from 0.7% to 0.9%, indicating most of shales are thermally mature. The dominant minerals of Es3L shale in Bonan sag are carbonates (including calcite and dolomite), averaging 51.82 wt.%, and the second minerals are clay (mostly are montmorillonite-illite-mixed layer and illite) and quartz, averaging about 18 wt.%. Finally, its shale oil resources were evaluated by using the volumetric method, and the evaluation result shows that the shale oil resource is up to 5.94 billion tons, and mostly Class Ⅰ resource. Therefore, the exploration of the lacustrine shale oil of Es3L in Bonan sag should be strengthened.
Keywords: shale oil    resource potential    Es3L sub-member    Bonan sag    Bohai Bay Basin
0 INTRODUCTION

The growing energy demand and continuous consumption of conventional oil and gas resources cause a sharp contradiction between oil and gas supply and demand. Therefore, unconventional energy is drawing more and more attention (Clarkson et al., 2013; Wang and Carr, 2013; Slatt and Rodriguez, 2012; Zhu et al., 2012; Ross and Bustin, 2008; Bowker, 2007; Jarvie et al., 2007). The great success of shale gas exploration and development in North America triggered a worldwide upsurge in shale gas exploration and development (Pan et al., 2015; Badics and Vetö, 2012; Horsfield and Schulz, 2012; Zou et al., 2010). Inspired by the development of shale gas and stimulated by the decrease in natural gas price, the investors turned around shale oil exploration and development (Kirschbaum and Mercier, 2013; Jarvie, 2012; Kinley et al., 2008). The shale oil and gas resources potential is huge in China, including 15×1012-30×1012m3 of shale gas resources (Zhang et al., 2008), comparable to that in the USA. Continental facies shale oil of China is usually accumulated continuously in lake basin center, and it has a preliminarily estimated recoverable resources of about 30×108-60×108 t (Zou et al., 2013a, b). Many fractured shale reservoirs or oil flows have been discovered during the past conventional oil and gas exploration (Lin et al., 2013; Ning, 2008; Chen, 2006), such as basins of Bohai Bay, Songliao, Jianghan, Nanxiang and Subei in eastern China, basins of Qaidam, Tuha, Jiuquan, Junger and Tarim in western China, and Sichuan Basin in southwestern China. A number of shale oil flows have been discovered in the eastern garben basins of China, such as Well Shugu-165 in Liaohe depression (24 m3/d), Well Pushen-18-1 in Puyang depression (420 m3/d), Well Anshen-1 (4.68 m3/d) and Well HF1 (23.6 m3/d) in Biyang depression (Zhang J C et al., 2012a). The cumulative oil production in some wells can be up to tens of thousands tons, such as Well Luo-42 and Well Xinyishen-9 in Shengli oilfield, Well Xin-197 in Jilin oilfield, the cumulative oil production of each is 1.36×104, 1.13×104 and 3.03×104 t, respectively. Since the development of shale oil is much more difficult than conventional oil-gas and shale gas, the worldwide development of shale oil is behind the shale gas development.

Fractured shale oil had been produced in Jiyang depression of Bohai Bay Basin as early as 1973, represented by Well Tanhe-54 in the central uplifted zone, and the shale oil pay zone is Es3Lsub-member. When tested with 5 mm nozzle, the well had daily open flow of 91.4 t of oil and 2 740 m3 of gas, cumulative oil production of 754 t, cumulative gas production of 21 320 m3 during the test, and cumulative oil production of 27 896 t after put into production (Zhang L Y et al., 2012, 2008; Dong et al., 1993). There was also an important discovery in Well Luo-42 and Well Luo-19 in Luojia region of Bonan sag in 1990: when drilling through the taupe shale in Es3L, Well Luo-42 blew out, tested flow with 6 mm nozzle, the daily oil production was 79.7 t, daily gas production 7 746 m3, and the cumulative oil production of the well has reached 13 605 t after put into production (Zhang S W et al., 2012). According to preliminary statistics, by the end of 2010, shale oil and gas shows had been discovered in more than 320 exploration wells in Jiyang depression, and commercial oil and gas flow had been tapped in more than 30 wells; the shale oil and gas shows are widely distributed in Dongying, Zhanhua (mainly in Bonan sag) and Chezhen sags in Jiyang depression, and the zones are mainly in shale oil resource systems of Es4, Es3 and Es1 of Paleogene.

In this study, geochemical and geologic characteristics of lacustrine shale, Es3L sub-member in Bonan sag of Jiyang depression, Bohai Bay Basin, were characterized by using total organic carbon (TOC), Rock Eval pyrolysis, X-ray diffraction, and ∆log R method. The major objectives of this study are to: (1) investigate the geochemical and mineralogical characteristics of Es3L lacustrine shale in the Bonan sag; and (2) characterize the heterogeneity of TOC and S1 (free oil) of lacustrine shale; and (3) establish grading evaluation criteria of shale oil resource; and (4) give a preliminary estimation of lacustrine shale oil resource potential of Bonan sag.

These studies can provide useful data for the understanding and predicting of lacustrine shale oil favorable targets in the Bonan sag, and it will enlighten other relevant research of lacustrine shale oil in basins of eastern China.

1 GEOLOGICAL BACKGROUNDS

Bonan sag, located in the mid-west of Zhanhua sag of Jiyang depression, is the largest secondary negative structural unit in Zhanhua sag, a half graben-like depression, steep in northwest and gentle in south-east in east-north trending (Jiu et al., 2013; Song, 2011; Shi et al., 2005). To the north, it is connected with Chengdong bulge by the Chengnan-Chengdong fault, and to the east, connected with Gubei sag and Gudao buried mountain structural belt by the Guxi fault; to the south, the gentle slope transitions to Chenjiazhuang bulge; in the west, the fracture belt is connected with Yihezhuang bulge (Fig. 1).

 Download: larger image Figure 1. Location and structural map of Bonan sag.

By 2011, an area of 600 km2 has been covered by three-dimensional seismic survey in the region of interest and 377 wells have been drilled with the finished drilling footage of 1 180 000 m, and the exploration well density is 0.628 well per square kilometers. The total oil resources in the zone is about 8.52×108t, and the cumulative total proven OOIP are 1.84×108t, the controlled OOIP are 0.82×108t, and the predicted OOIP are 1.68×108t. The formations of Kongdian, Shahejie, Dongying, Guantao and Minghuazhen developed in the Tertiary system from bottom-up, and Shahejie Formation is the major oil-bearing system in which six oil fields have been discovered, namely Bonan, Chenjiazhuang, Yidong, Kenxi, Shaojia, and Luojia oil field (Song, 2011).

Many sets of strata developed in Bonan sag, including (from bottom to top) Paleozoic, Mesozoic, Paleogene Kongdian Formation, Shahejie Formation and Dongying Formation, Neogene Guantao and Minghuazhen formations, and Quaternary Pingyuan Formation. Among them, Paleogene Shahejie Formation is the main oil generation strata, with large thickness and wide distribution. Therefore, it is the main hydrocarbon source rock in Bonan sag. Bonan sag is one of the best oil and gas generation sags in Jiyang depression, where oil and gas shows have been discovered in shale of several wells (Wang et al., 2015; Song, 2011).

2 GEOCHEMICAL AND GEOLOGICAL FEATURES OF SHALE 2.1 Material and Methods

More than 200 shale core samples of Es3L were selected to characterize variation in TOC. Shale core samples were powdered to 100 mesh after surface cleaning, and a Rock-Eval-Ⅵ instrument was used for carrying out pyrolysis and TOC analysis of the crushed un-extracted powder. Parameters measured include total organic carbon (TOC), free oil or volatile hydrocarbon content, mg HC/g rock (S1), remaining hydrocarbon generation potential, mg HC/g rock (S2) and temperature of maximum pyrolysis yield (Tmax). The shale was studied for maceral analysis using Leica DMRXP, and vitrinite reflectance was analyzed by using the microscope photometer (UMSP-50).

A total of 421 core samples were analyzed for whole-rock (bulk) and clay fraction ( < 2 μm) mineralogy. Crushed samples (80-100 mesh) were mixed with ethanol, hand ground in a mortar and pestle, and then smear mounted on glass slides for X-ray diffraction (XRD) analysis. The XRD data collection was performed using a Panalytical X'Pert PRO Diffractometer with Cu Karadiation (40 kV, 30 mA) and scanning speed of 2° per minute with the accuracy of the measurement better than 2θ.

2.2 Organic Matter Abundance and Kerogen Type

Geochemical data show that Es3L shale have a TOC content range of 0.5 wt.% to 9.3 wt.%, 2.9 wt.% on average, and TOC content of over 70% samples is more than 2.0 wt.%, indicating the very good hydrocarbon source rock (Fig. 2a), with high hydrocarbon generation potential. The content of chloroform bitumen "A" is between 0.07% and 2.14%, with an average of 0.8%. And the main values distribute in the range of the "very good" interval (Fig. 2c), suggesting that hydrocarbon generation from Es3L shale is fairly good, and a lot of residual hydrocarbons are retained in the shale. Hydrocarbon generation potential (S1+S2) shows that 80% of samples are in "good" and "very good" ranges based on the evaluation indexes (Fig. 2b). The organic matters in Es3L sub-member are mainly Type Ⅰ and Type Ⅱ1 (Fig. 2d).

 Download: larger image Figure 2. Geochemical characteristics of Es3L shale in Bonan sag. (a) TOC distributions showing the shale quality; (b) hydrocarbon potential (S1+S2) distributions; (c) chloroform bitumen "A" distributions; (d) plot of the hydrogen index (HI) versus the pyrolysis Tmax for the Es3L shale, showing the kerogen quality and the thermal maturity stages.
2.3 Thermal Maturity

Exploration practice proved that the distribution of hydrocarbon reservoirs in a basin is controlled by source rocks (Zhao et al., 2003), and only in an area with mature source rocks has a higher success rate for oil exploration. The thermal evolution of source rocks in the basin directly affects the oil and gas exploration prospects, and also determines whether it is rich of oil or gas in the sag. The vitrinite reflectance (Ro) is considered to be one of the best parameters to study the kerogen thermal evolution and maturity (Durand et al., 1986). Vitrinite reflectance (Ro) will change regularly with the deepening of thermal evolution of organic matter. In general, vitrinite reflectance (Ro) increases with the buried depth of the source rocks. From the relationship between Ro and depth, Ro values in Es3L of Bonan sag are mainly between 0.7% and 0.9%, in a fairly concentrated manner (Fig. 3). It is worthy to note that the previous studies have shown that the organic maturity (Ro) in this area is suppressed, represented as lower value, about 0.2% lower than the normal (Li et al., 2008). The reason of suppression may be due to invasion of hydrocarbon (mainly oil) to the organic matter. Most of the samples are in oil generating window, and the location for sampling is in shale oil production region (Fig. 3).

 Download: larger image Figure 3. Depth profile of organic maturity (Ro vs. depth) of Es3L sub-member of Bonan sag.
2.4 Mineral Composition

The XRD data of 421 shale samples from Es3L member show that the mineralogy is dominated by calcite (0 wt.%-88 wt.%, ave. 51.82 wt.%), followed by clay minerals (1 wt.%-48 wt.%, ave. 18.86 wt.%) and quartz (3 wt.%-48 wt.%, ave. 18.05 wt.%) (Fig. 4a). Although calcite is the predominant carbonate, small amounts of dolomite (ave. 5.83 wt.%) and siderite are present as well. There is also some pyrite ranging from 0 to 16 wt.%, ave. 3.84 wt.%. The content of feldspar (including K-feldspar and plagioclase) ranges from 0 to 12 wt.%, with an average of 1.37 wt.%. Of the clay mineral component, the main species are I/S (illite/smectite) mixed-layer (ave. 61.48%), illite (ave. 29.79%), and kaolinite (ave. 6.18%), together with a little chlorite ( < 3%) (Fig. 4b).

 Download: larger image Figure 4. Mineral composition characteristics of shale in Es3L sub-member of Bonan sag. (a) Whole-rock mineral composition; (b) clay mineral composition.
2.5 Development Characteristics of Shale in Es3L Sub-Member

The sedimentary facies of Paleogene strata in study area are mainly deep lake to semi-deep lake, and there are small scale braided river delta deposits in west edge Yihezhuang salient and the southeast of the sag (Wang et al., 2010; Shi et al., 2005). Three mudstone series developed in Paleogene Shahejie Formation, i.e., Es4U, Es3L, and Es1L sub-members, among which the best one is Es3L sub-member. The lithology of Es3L sub-member are dark-gray mudstone and brown oil shale, interbedded with small amount of thin gray limestone, sandstone, dolomite and turbidite sandstone (Wang et al., 2015). The rock is brittle and developed in micro-fractures. The discovered commercial shale oil flow wells are mainly concentrated in Es3L, followed by Es1L, and a few in the Es4U (Wang et al., 2015; Zhang et al., 2014). As shown in Fig. 5, shale in Es3L sub-member of Bonan sag is widespread, mainly in the middle part of the study area, with a thickness of 100-700 m.

 Download: larger image Figure 5. Shale thickness contour of Es3L sub-member of Bonan sag.
3 SHALE OIL RESOURCE EVALUATION 3.1 Grading Evaluation Criterion

There is a large amount of oil retained in shale, however, because of its lower porosity and permeability, only oil in high abundance can be economically feasible for production. The shale oil is divided into three classes based on the enrichment of oil content (Lu et al., 2012). Based on statistical analysis of geochemical data, chloroform bitumen "A" and S1 (free oil) increase with the increasing of TOC (Fig. 6). There are two inflection points in the trend line, which can divide the trend line into three sections (Fig. 6). And according to these points, shale oil resources can be divided into three classes. Class Ⅲ: in the stable low value section, the shale unsaturated with hydrocarbon, has a lower TOC, lower oil content and strong adsorption, therefore, this kind of shale oil is difficult to be extracted, i.e., invalid resource. Class Ⅱ: in the middle rising section, the shale with proper oil content, and the oil can only be exploited when further technical progress was made, therefore this kind of shale oil can be regarded as inefficient resource. Class Ⅰ: in the stable high-value section, the shale is saturated with oil, and it is the first choice for evaluation and exploration, regarding as rich resource. The classification criterion is given in Table 1.

 Download: larger image Figure 6. Relationship of oil content and TOC. (a) Chloroform bitumen "A" vs. TOC; (b) S1vs. TOC.
Table 1 Grading criterion of lacustrine shale of Es3L sub-member, Bonan sag
3.2 Evaluation Methods

The evaluation methods of shale oil and gas resources include analogy method, statistical method, genetic method and volumetric method, however, the application range of different methods is different. Analogy method is a commonly used way in the evaluation of a region with low exploration degree; the parameters in calculation of statistical method are too simple, and lack of clear links with geological analysis; while the credibility of genetic method depends on the credibility of many parameters in the model of oil generation and expulsion. Under the current data available, volumetric method is the most appropriate method for the evaluation of shale oil resources.

The principle to evaluate the amount of shale oil resource with volumetric method is as follows

 $Q{\rm{ = }}S \times H \times \rho \times A ({\rm{or}}\;{S_1})$ (1)

where Q—shale oil resource amount, kg; S—source rock area, m2; H—thickness, m; ρ—density of source rock, t/m3; A—chloroform bitumen "A" content after correction, %; S1—free oil content after correction, kg HC/t rock.

Most of light hydrocarbon components (C14-) in chloroform bitumen "A" will be lost during the extraction analyzing process, therefore, it requires to compensate for the loss of light hydrocarbon components when calculating shale oil resources with the volumetric method by using parameters of chloroform bitumen "A". Affected by the core storage conditions, experimental testing analysis and the adsorption of kerogen, the measured S1 value is usually far below the actual one underground. Therefore, the mount of heavy hydrocarbon handling and light hydrocarbon losing should be taken into consideration (Wang et al., 2014). The correction factors of S1 and chloroform bitumen "A" (Table 2) were worked out with the compositional kinetic method based on Rock-Eval, PY-GC experiments.

Table 2 Correction factors of S1 and chloroform bitumen "A" for shale with different maturities
3.3 Acquisition and Correction of Key Parameters 3.3.1 Logging evaluation of chloroform bitumen "A" and S1

Shale oil is residual liquid HCs in the source rock, therefore, chloroform bitumen "A" and S1 (free oil) (which are the indicators to describe residual hydrocarbon in the source rocks) can be used to evaluate shale oil content. The organic matter abundance TOC and free oil (S1) content and chloroform bitumen "A" of shale are important parameters for shale oil resource evaluation. The development and distribution of organic matter are strongly heterogeneous, due to sedimentary environment changes caused by tectonic events, climate change, and depositional filling. Restrained by analysis period, cost, and number of the samples, the measured data are always difficult to meet the needs of fine description of shale organic matter heterogeneity. However, since the development characteristics of organic matter have a significant response to a number of well logging (such as acoustic wave, resistance, neutron, density, gamma, etc.), it is possible to evaluate organic matter heterogeneity with logging data (Passey et al., 1990). In recent years, the logging geochemical principles and techniques represented by ∆log R model have been studied and applied widely and successfully in TOC heterogeneity evaluation of source rocks in conventional and unconventional oil and gas exploration (Witkowsky et al., 2012; Passey et al., 2010; Coope et al., 2009). However, TOC is obviously not as good as S1 in reflecting shale oil content objectively. And in principle, S1 content changes will also be reflected on log responses, which makes it possible to evaluate the shale oil potential with logging data.

The modified ∆log R model (Liu et al., 2011), resistivity (R2.5) and SDT (AC) curves were used in modeling organic heterogeneity on organic matter abundance (TOC, S1 and "A") of Well Luo-69 in Es3L sub-member, and comparison results between the measured and calculated data are shown in Fig. 7. The relationship between magnitude difference (the distance from borehole log to the baseline) and organic matter abundance is shown in Fig. 8. It can be seen that the correlation coefficient (R2) of measured value and the calculated is more than 70%. Therefore, the model can be used.

 Download: larger image Figure 7. Results of evaluation on logging organic matter heterogeneity for Well Luo-69 (cal.=calculated; mea.=measured).
 Download: larger image Figure 8. Logging evaluation model for organic matter abundance of Well Luo-69 in Es3Lsub-member of Bonan sag. (a), (b) TOC model; (c), (d) S1 model; (e), (f) "A" model; TOCpd, S1pdand "A"pdare measured values; TOCcal., S1cal. and "A"cal. are calculated data from logging model; TOCR2.5-AC, S1R2.5-AC and "A"R2.5-ACare resistivity and acoustic wave magnitude difference respectively in TOC, S1and "A" logging model.
3.3.2 Distribution characteristics of shale oil

The lacustrine shale oil resources are huge, but due to the characteristics of strong adsorption, low porosity and low permeability of organic-rich shale, it is difficult to recover the liquid hydrocarbons in such shale or the recovery rate is limited under current economic and technical conditions. Relatively, the oil in shale with high porosity and permeability, fractured and organic lean siltstone, carbonate interbed is more worth of exploitation. Luojia oilfield in the region of interest is dominated with fractured shale oil, and the cumulative shale oil production from Well Luo-42 has reached 13 605 t.

Currently, it is a more effective method to identify the possible shale oil producing interval with geochemistry parameters (Jarvie, 2012). The geochemical plane section of Well Luo-69 (Fig. 9) shows that OSI (oil saturated index) in the two intervals of 2 989-3 011 m and 3 095.4-3 130.0 m is greater than 100 mg HC/g TOC. The first interval has TOCpd of 2.4 wt.% to 6.0 wt.%, generally greater than 3 wt.%; Tmax from 440 to 447 ℃, Ro about 0.75%, organic matter being in the peak hydrocarbon generation period, S2 between 11.9 and 42.7 mg HC/g rock, and HI at 309 and 718 mg HC/g TOC. Based on the fact that the S2 and HI values were still quite high though organic matter was in the peak hydrocarbon generating period, it is inferred that large amounts of hydrocarbons were adsorbed by kerogen; carbonate content is between 13 wt.% and 71 wt.%, generally greater than 40 wt.%, 53.4 wt.% on average; the high content of carbonate minerals can increase the brittleness of rock, favorable for fracturing. The second interval has TOCpd of 0.71 wt.% to 2.0 wt.%, relatively lower than that in the upper interval, Tmax between 424 and 441 ℃, Ro between 0.84% to 0.9%; Tmax significantly lower than that in the upper interval, showing abnormality, which may be because that there are a lot of residual oil (∆S2) in kerogen. The organic matter is in the peak of hydrocarbon generation based on the value of vitrinite reflectance; S2 ranges between 1.79 and 7.71 mg HC/g rock, HI between 161 and 450 mg HC/g, OSI 104 mg HC/g TOC on average, this high OSI and relatively low TOC shows that the mobile oil amount is quite large; carbonate content is between 18 wt.% and 89 wt.%, generally greater than 50 wt.%, 69.4 wt.% on average. The above two intervals with fairly high OSI and carbonate content are promising prospect oil shale intervals. The shale oil resource intensity can better reflect the amount of oil resource per unit area. The intensity value of shale oil was calculated by formula (1) with the area S taken as 1 km2, the plane distribution of resources intensity of corresponding shale oil is shown in Fig. 10. It can be seen that: the resource intensity is higher near Well Boshen-5, where the shale is thicker, with the maximum value of up to 3 000×104t/km2, and gradually reduces to the north and south.

 Download: larger image Figure 9. Geochemical profile of shale in Es3Lsub-member of Bonan sag (Well Luo-69). OSI. Oil saturation index=S1×100/TOC, mg HC/g TOC.
 Download: larger image Figure 10. Shale oil amount isolines of Es3L sub-member, Bonan sag. (a) Chloroform bitumen "A"; (b) S1.

Similarly, resource intensity of each class of Es3L sub-member was also calculated by this way. The shale oil resources after grading are listed in Table 3. Shale oil resource is dominated by Class Ⅰ, with relatively small contribution from classes Ⅱ and Ⅲ. As shown in Table 3, shale oil resource potential in Es3L sub-member is large, with a promising resource exploration and development prospects.

Table 3 Shale oil amount (×109 t) of Es3L sub-member in Bonan sag
4 CONCLUSIONS

(1) The organic matter abundance of shale in Es3L sub-member of Bonan sag is fairly high at the average of 2.9 wt.%, and the kerogen is mainly Type Ⅰ, with a maturity of 0.7% to 0.9%, indicating the organic matter is currently in the oil generation stage. The shale is quite thick (700-900 m), and the dominant minerals are carbonates (including calcite and dolomite), with an average value of 51.82 wt.%.

(2) The shale oil resource is graded according to the relationship between oil content and TOC. Since S1 is easy to obtain and can represent hydrocarbon content in reservoirs, it is recommended using oil content S1 as the basis for shale oil resource grading. Shale oil resources with S1 > 3.2 mg HC/g TOC are classified as Class Ⅰ resources, greater than 0.5 mg HC/TOC and less than 3.2 mg HC/g TOC as Class Ⅱ resources, and less than 0.5 5 mg HC/g TOC Class Ⅲ resources. At current economic and technological conditions, Class Ⅰ resources should be the first target of exploration and development.

(3) Shale oil resources are large in the study area, and mainly distribute in the center of the sag, with the total resources of up to 5.94×109t, and mostly Class Ⅰ resources.

ACKNOWLEDGMENTS

We would like to thank anonymous reviewers for constructive comments. This study was supported by the National Natural Science Foundation of China (Nos. 41672116, 41330313), the Fundamental Research Funds for the Central Universities (No. 17CX05012) and National Science and Technology Major Project of China (Nos. 2017ZX05049004, 2016ZX05046-001). The final publication is available at Springer via https://doi.org/10.1007/s12583-016-0945-4.

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