Thermal and maturation history for Carboniferous source rocks in the Junggar Basin, Northwest China: implications for hydrocarbon exploration

The reconstruction of thermal history is an important component of basin evolution and hydrocarbon exploration. Based on vitrinite reflectance data, we integrate the paleo-temperature gradient and paleo-heat flow methods to reconstruct the thermal history of Junggar Basin. Compared with present thermal state, the Junggar Basin experienced much a higher heat flow of ca. 80–120 mW/m2 during the Carboniferous. This feature can be attributed to large-scale volcanic events and related thermal effects. The hydrocarbon maturation history of Carboniferous source rocks indicates that the temperature rapidly reached the threshold of hydrocarbon generation during the Late Carboniferous and has never achieved such a high level since then. This characteristic resulted in the early maturation of hydrocarbons in Carboniferous source rocks. Meanwhile, the results reveal that hydrocarbon maturities are different among various tectonic units in Junggar Basin. The kerogen either rapidly broke through the dry gas period so that cracking of gas occurred or remained in the oil maturation window forming oil reservoirs, which depended on the tectonic background and depositional environment. In this study, we present the thermal and hydrocarbon maturation history since the Carboniferous, which has important implications for further hydrocarbon exploration in Junggar Basin.


Introduction
With the growing demand for hydrocarbon resources and the deepening of exploration activities, the target stratigraphy for present oil and gas exploration has been shifted to the deep (> 4500 m) and ultra-deep zones (Sun et al. 2013;Zhao et al. 2013), which are currently important strategic areas for hydrocarbon prospecting and exploitation . The Junggar Basin is one of the largest oil and gas basins in western China and is characterized by a thickened basement crust, low geothermal gradient, and widespread deep-seated strata (Pan et al. 1997;Qiu et al. 2008;Rao et al. 2013;Wang et al. 2001;Zhang et al. 2007). During recent decades, several deep Carboniferous oil and gas fields such as the Wucaiwan, Shixi, and Karameli gas fields have been discovered and exploited, which bodes well for the deep zone of the Junggar Basin (He et al. 2010;Li et al. 2009). The thermal evolution of basin is closely linked to the generation, migration, accumulation, and preservation of hydrocarbons in the traps combined with the burial history (Mashhadi

Edited by Jie Hao
Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s1218 2-019-00392 -2) contains supplementary material, which is available to authorized users. et al. 2015). Hence, thermal evolution of the source rock is related to evaluate the petroleum prospectivity in the basin.
The Junggar Basin is a multi-cycle superimposed basin and has experienced multi-stage tectono-thermal evolution since the Late Carboniferous. Previous researchers have conducted many studies on the thermal regime of the Junggar Basin and the evolution of the Carboniferous source rocks. The results could be summarized as follows: (1) the high geothermal gradient gradually decreased from the Carboniferous to the Cenozoic (Pan et al. 1997;Qiu et al. 2005;Wang et al. 2001); (2) the source rocks evolved early due to the high geothermal effect during the Late Paleozoic (Cao et al. 2005;Qiu et al. 2005); and (3) Carboniferous pyroclastic rocks formed by large-scale volcanic activity are not only effective source rocks but also provide favorable reservoir conditions for later oil and gas accumulation (He et al. 2010). However, previous studies have mainly focused on the thermal history since the Permian, and only an approximate estimate of the thermal state has been provided during the Carboniferous. Meanwhile, the main controlling factors of thermal evolution and their relations with regional tectonic events have not been properly evaluated. There is also a little understanding of the hydrocarbon maturation period of Carboniferous source rocks in Junggar Basin.
In this study, the 1D reconstruction of thermal history recorded by considerable vitrinite reflectance (R o ) data from wells LN-1, MS-1, CS-2 and Ca-6, utilizing the paleo-geothermal gradient and paleo-heat flow methods. Moreover, we precisely simulate the thermal history and hydrocarbon maturation history of the Junggar Basin since the Late Carboniferous by the basin modeling, and the thermal maturity stage of the Carboniferous source rock is established under the constraints of the regional geodynamic background. We also discussed the genesis of locally high heat flow in the Carboniferous and the potential of the Carboniferous hydrocarbon system. The thermal and maturation history will help to understand the tectonic evolution during Carboniferous and have important implications for further hydrocarbon exploration in Junggar Basin.

Geological background
The Junggar Terrane, which is near the northwestern margin of China and is tectonically a part of the Central Asian Orogenic Belt, consists of the West Junggar Terrane, the East Junggar Terrane and the Junggar Basin ( Fig. 1a) (Han et al. 2010;Li et al. 2016;Şengör et al. 1993;Xiao et al. 2008). The Junggar Basin is bounded by the Tianshan Mountains on the south and the Altai Mountains on the north (Fig. 1b), with an area of approximately 13 × 10 4 km 2 (Ma et al. 2018). The West Junggar Terrane and East Junggar Terrane are mainly fold-thrust orogenic belts composed of Paleozoic accretionary complexes, volcanic arcs and high-grade metamorphic ophiolite zones (Xiao et al. 2008(Xiao et al. , 2011Zhang et al. 2009).The NE West Junggar folded orogenic belt is a suture zone where the Junggar Terrane collided with the Kazakhstan plate. The East Junggar Terrane was formed during the northward subduction of the southern Paleo-Asian Ocean, and the North Tianshan fold orogenic belt might be a collision product between the Junggar plate and the Tarim Block (Li et al. 2015(Li et al. , 2016Xiao et al. 2011).

Basin evolution
The Junggar Basin is a superimposed basin that has developed since the Late Carboniferous; Xiao et al. (2008) concluded that the basement of the Junggar Basin is mainly composed of volcanic arcs, accretionary complexes and trapped residual oceanic crust from the Paleozoic. He et al. (2013) determined that the SHRIMP U-Pb age of a Carboniferous andesitic tuff in the MS-1 well is 331.7 ± 3.8 Ma; thus, the Early Carboniferous is a crucial period from the closing of the Junggar Ocean to the rapid growth of new continental crust based on Hf isotope analysis . A number of unconformities and tectonic layers were formed after multiple periods of tectonic movement corresponding to the rifting stage from Late Carboniferous to Permian (Carroll et al. 2010), the Triassic-Paleogene intracontinental depression stage (Jolivet et al. 2010) and the rejuvenated foreland basin stage since the Neogene (Fig. 2). The Junggar Basin is generally subdivided into six first-order tectonic units according to the basic structural characteristics ( Fig. 1c; I-VI, respectively): Wulungu Depression, Luliang Uplift, Western Uplift, Central Depression, Eastern Uplift and Southern Depression.

Sedimentary strata
The sedimentary cover of the Junggar Basin is well developed, and the depositional thickness can be up to 15,000 m locally (Fig. 3). The Carboniferous, Permian, Triassic, Jurassic, Cretaceous and Cenozoic systems from bottom to top are overlapped on top of pre-Carboniferous volcanic and metamorphic rocks (Cai 2009;Yang et al. 2012b). A transitional facies sedimentary system accompanied by intermittent volcanic eruptions is recorded by Late Paleozoic sediments, whereas a set of clastic sediments of Mesozoic and Cenozoic ages reveals continental facies.
The Carboniferous is a critical period for the cratonization of the Junggar terrane, when two continental passive margins were developed on the southern and northern margins, and thick volcaniclastic rocks (Fig. 3) filled in the rifts (He et al. 2010). The Carboniferous strata in the Junggar Basin mainly deposited as marine-terrestrial alternating facies (Li et al. 2009). Only low-grade metamorphic sandstone, slate, siliceous slate and intermediate acid volcanics of the Lower Carboniferous (C 1 ) occur in the Altai region, whereas abundant sandstone, siltstone, tuff and volcanic rocks of the Lower to Upper Carboniferous (C 1 -C 2 ) are widespread in other regions (Han et al. 2010;He et al. 2013;Jin et al. 2008;Yang et al. 2012b). Meanwhile, many recent chronological studies on Carboniferous strata yield U-Pb ages of zircons from 350.0 ± 6.3 to 299.8 ± 5.2 Ma based on different lithologies, including rhyolite, basalt, tuff and andesite (Tan et al. 2009;Yang et al. 2012b).

Carboniferous source rocks
Multiple depocenters were formed during the Carboniferous in the Junggar Basin, and the volcaniclastic rocks can be up to 2.0-5.0 km in thickness (He et al. 2010). The Dishuiquan Formation (C 1 d) and Batamay Formation (C 2 b) occur within or around the basin, and the thickness of effective source rocks can reach 400-800 m. The Carboniferous source rocks included organic-rich gray mudstone, tuffaceous mudstone, silty mudstone and tuffs, which primarily deposited in marine-terrestrial environment containing type II-III kerogen (Wang et al. 2013). The total organic carbon (TOC) value of Carboniferous source rocks changed dramatically and indicated great oil generation potential. Typically, the mudstones have TOC values with the average of 1.45%; carbonaceous with the average of 15.53%; coals with the average of 43.78%; tuffs with the average of 1.56% (Li et al. 2009  will be described in next chapter. Hence, the Carboniferous source rocks are mainly in high-post mature stage. The oil correlation results of Carboniferous source rocks suggest that the oil from volcanic reservoir rocks within the C 2 b Formation generally correlates with the C 1 d source rocks based on molecular and carbon isotopic data (Yu et al. 2014). However, a small amount of younger source rocks from the Middle Permian and Lower Jurassic also can feed to the Carboniferous reservoirs in some regions (Yu et al. 2014). Overall, these source rocks could provide effective supplies of oil and gas for the Carboniferous pyroclastic and the Permian clastic reservoirs (He et al. 2010;Li et al. 2009;Wang et al. 2010).

Carboniferous reservoir
The discovered Carboniferous hydrocarbons mainly reserved on the volcanic rocks of Batamay Formation (C 2 b) by now, although the sandy conglomerates of Lower Carboniferous could be potential reservoir. The Batamay Formation (C 2 b) contains grayish green basalts, andesites, tuffs with minor gray sandstones, dark gray mudstones, carbonaceous mudstones and coals (Wang et al. 2010). These volcanic rocks could form the weathering crust and the interior interval reservoirs with favorable porosity and permeability condition owing to the long periods of weathering and leaching, the dissolution of organic acid or deep thermal fluids (He et al. 2010). Except for regional mudstone seal from Triassic or Permian, the thick source rocks of Batamay Formation (C 2 b) also could be good seal to form self-generated and self-stored reservoir.

Methodology and data
The thermal stage of a basin plays an important role not only in regional tectonic evolution but also in controlling the generation, migration, accumulation and preservation of oil and gas (Hudson and Hanson 2010). The principles for the reconstruction of the thermal history of a sedimentary basin can be summarized as follows: 1. On the lithospheric scale, the heat flow history of the basin can be determined by a mathematical and geophysical model, depending on the geodynamic mechanism of the basin and the heat transfer pattern from the Earth's interior. Through adjusting the parameters of model, the modeled results are accordance with observed temperature and subsidence characteristics. 2. On the basin scale, the thermal history can be quantitatively recovered through types of indicators recording paleo-temperature information (Chang et al. 2016(Chang et al. , 2018Sweeney and Burnham 1990). The modeling methods fall into three categories: the stochastic inversion of a single sample, the paleo-geothermal gradient method based on a vertical profile and the paleo-heat flow method based on a suite of downhole samples (Lerche et al. 1984;Tang et al. 2014).

Inversion method
Considering the complex geological background of the Junggar Basin, the paleo-geothermal gradient and the paleo-heat flow methods (Lerche et al. 1984;Wang et al. 2001) are    Thermal conductivity values (K) are from Rao et al. 2013, which are used for later thermal modeling integrated to reconstruct the thermal history. We use the paleo-geothermal gradient method to define the thermal history before reaching the maximum paleo-temperature and invert the uncertain thermal history through various heat flow paths coupled with the burial history. The best-fit path coinciding with thermal indicator data is selected as the final result. Inversion modeling is performed by the software Thermodel (Hu et al. 2001) constrained by a heat conduction and compaction model. Present geothermal gradients, present heat flow, thermal conductivity and heat production data are from Rao et al. (2013), while other related parameters such as the compacting factor and porosity are adopted according to the software default values.

Thermal indicators
Considering the lack of fission track data in the Junggar Basin (Qiu et al. 2005;Wang et al. 2001), we mainly utilize vitrinite reflectance as the thermal indicator. Vitrinite reflectance (R o ) is one of the most popular thermal maturity indices to measure experienced maximum temperature of organic matter. Each R o value can be translated to a maximum paleo-temperature value using the EASY% R o chemical kinetics model (Sweeney and Burnham 1990). For this paper, 633 R o data were systematically collected from 30 drillings, mainly collected from the PetroChina Xinjiang Oilfield Company. The histogram distributions of R o data are drawn for the Jurassic, Triassic, Permian and Carboniferous (Fig. 4), and the detailed results of R o are shown in the supplementary material.
The relatively low average R o values of Jurassic and Triassic Formations indicate their low-mature organic matter and are consistent with the low thermal background since the Mesozoic (Cao et al. 2005;Qiu et al. 2008). However, Carboniferous R o values show a wider distribution and relatively high thermal maturity. Meantime, the differences in frequency distributions of R o values between Late Paleozoic and Meso-Cenozoic rocks indicate completely different thermal regimes. The difference of current maturity in the Carboniferous source rocks is clear and may show a change from low maturity to high maturity. For example, the R o values of the Dishuiquan group (C 1 d) range from 0.56% to 0.63% in well Cai-28, 1.69%-1.76% in well LN-1 and 1.13%-4.15% in well MS-1 (Fig. 1c). The Batamay group samples (C 2 b) in the northwestern basin have moderate maturity with R o between 0.86% and 1.18%, mainly in oil maturation window, whereas it reaches the overmature phase in other regions (He et al. 2010). Four representative vitrinite profiles show that there is a R o "jump" or "break" phenomenon at the disconformity due to uplift and erosion of underlying strata or changing of the geothermal gradient, which establishes the foundation for quantitative reconstruction of the thermal history of the region.

Thermal history of well LN-1
The simulated results of well LN-1 located in the Luliang uplift are presented (Fig. 1c, II) to introduce the specific reconstruction procedure. The Formations and lithology in well LN-1 are shown in Table 1. The well was drilled from the Oligocene unit to the Carboniferous unit with a total depth of 4905.5 m.
Firstly, the paleo-geothermal gradient method is utilized to acquire the maximum paleo-geothermal gradient and erosion thickness in mainly unconformities. There are several unconformities or disconformities observed in well LN-1, but two are major: the absence of the Upper Carboniferous to Middle Permian and of the Neogene to present units. Thus, the sedimentary strata (Table 1) can be approximately divided into two subsections: Paleogene to Permian (subsection I) and Carboniferous (subsection II). The paleo-temperature profiles are measured by the R o data of different layers (Fig. 5a). The paleo-temperature gradient (dT/dz) for each subsection can be obtained using a linear least-squares fit according to the paleo-temperature profile (Fig. 5b). The eroded thickness is calculated by dividing the difference between the surface temperature (T s ) and the intercept of the paleo-temperature profiles (T i ) at the top unconformity by the estimated paleotemperature gradient using the equation: E i = (T i − T s )/(dT/ dz) i (where i represents the code of each subsection). The results indicate that the paleo-temperature gradient and erosion thickness of subsection II reach 44.9 °C/km and 2550 m, respectively, and 26.5 °C/km and 300 m, respectively, for subsection I (Fig. 5c, d). Meanwhile, the paleoheat flow of well LN-1 decreases from 95.8 mW/m 2 in the Late Carboniferous to 54.2 mW/m 2 at the end of the Late Paleogene (Fig. 5d).
Further, the reasonable paleo-heat flow results and eroded thickness defined by the paleo-geothermal gradient method are imported into the software Thermodel. Then, we can screen the best-fit eroded thickness (He) and paleo-heat flow (Q) through multiple iterations. The detailed burial and thermal history (Fig. 6a, c) show that both a high deposition rate (80 m/Ma) and high heat flow (− 100 mW/m 2 ) occurred in the Early Carboniferous. The heat flow gradually decreased to approximately 80 mW/m 2 in the Early Triassic along with a rapid erosion event, followed by a relatively stable depositional period since the 1 3 Late Triassic. The heat flow was approximately 50 mW/m 2 in the Late Cretaceous, which is close to the present level. Figure 6b verifies the reliability of the inversion results through comparing the modeled R o values with measured R o values.

Thermal characteristics of the Junggar Basin
For superimposed basins, the thermal history recorded by various wells should be jointly analyzed to form an integrated complete thermal sequence. Therefore, we reconstruct the thermal history (Fig. 7) of 3 other wells in the Junggar Basin, including MS-1 (III, Central Depression), CS-2 (VI, Eastern Uplift) and Ca-6 (V, Southern Depression) (Fig. 1c). Well MS-1 is in the Central Depression and has a drilling depth of 7500 m, which is one of the deepest wells in the Junggar Basin. The reconstructed thermal history of the Central Depression is characterized by high paleo-heat flow (> 120 mW/m 2 ) in the Carboniferous, followed by rapid uplift and erosion from 290 to 260 Ma (Fig. 6a). The eroded thickness is close to 4.0 km, and the heat flow decreases to approximately 85 mW/m 2 during this stage. A similar In general, the Junggar Basin has experienced constantly decreasing heat flow since the Late Carboniferous. The reconstructed thermal histories of all modeled wells indicate paleo-heat flows of 80-100 mW/m 2 from the Late Carboniferous to Early Permian, decreasing to the present level of 40-50 mW/m 2 . The heat flow evolution of the tectonic units in the basin shows lateral heterogeneity. During the Carboniferous, the highest heat flow of the basin was in the Eastern Uplift, but during the Permian, the Central Depression showed the highest heat flow value, while the heat flow of the southern part of the basin always maintained a relatively low level. The heat flow distribution of the entire basin has been consistent with the present since the Triassic.
Previous researchers mainly focused on thermal history and eroded events after Permian (Wang et al. 2013). Li et al. (2009) and Qiu et al. (2005) consider the eroded thickness at about 0.5-1.5 km between late Carboniferous to early Permian in Central Depression, and these results are reasonable in the Wucaiwan and Dongdaohaizi sag. Our burial history indicates that the eroded thickness could be reach to 2.0-4.0 km since late Carboniferous because these drillings are located in uplift region (Fig. 1c). Well Ca-6 even reveals the unconformity between late Carboniferous and Jurassic in the North Tianshan Thrust Belt.

Thermal maturity history
The thermal evolution of source rocks refers to the maturity state of source rocks in different geological periods and is an important part to evaluating hydrocarbon generation. Based on the reconstruction of the burial and heat flow history (Fig. 7), we can acquire the temperatures of hydrocarbon source rocks at various stages. Furthermore, the R o vs time curves can be estimated to infer the maturity of source rocks according to the thermal paths and the EASY% R o chemical kinetics model (Sweeney and Burnham 1990).
The geothermal and maturity histories of the CS-2 well (Fig. 8) show that the burial depth of the Lower Carboniferous (C 1 ) was only approximately 1200 m, but the temperature had already reached more than 90 °C by 320 Ma, which broke through the oil maturation threshold and quickly reached the oil window peak. The C 1 strata reached their highest temperature of approximately 150 °C in the Late Carboniferous and entered the late oil window period. Since then, the temperature of the C 1 has never been more than 150 °C due to later erosion and decreasing thermal background. Hence, the Lower Carboniferous source rocks of well CS-2 matured rapidly during the Late Carboniferous-Early Permian and have remained in the late oil window period since ca. 250 Ma (Fig. 8). Meanwhile, the results indicate that the hydrocarbon maturation history is very different among various tectonic units in the Junggar Basin (Fig. 9). The R o of Carboniferous strata in well MS-1 has been close to 4.0% during geological time, corresponding to the overmature

Genesis of high heat flow in the Carboniferous
The reconstructed thermal history in the Junggar Basin is closely related to the geodynamic background of this region. Previous research and our modelings (Fig. 7) indicate that the Junggar Basin has high heat flow values with the average greater than 80 mW/m 2 in the Carboniferous (Qiu et al. 2005). The heat flow of some region (Mosuowan swell) even could have reached 100-120 mW/m 2 , which is similar to the heat flow in mid-oceanic ridges and active volcanic areas. These results may be attributed to intense plate tectonic and simultaneous large-scale volcanic activity (Yang et al. 2012a). The Junggar Basin went through an important transition period from subduction and accretion to collision and amalgamation in the Carboniferous (Li et al. 2015;Xiao et al. 2008). Because of the subduction of oceanic crust and the activation of continental crust (Fig. 10b), frequent and 1 3 large-scale subduction-related volcanic rocks were well developed in the basin as the Junggar Ocean disappeared (Tan et al. 2009;Xiao et al. 2011;Zhang et al. 2009;Zheng et al. 2007). Figure 10a gives the distribution of Carboniferous volcanic rock, and our modeling wells are located at these volcanic field (He et al. 2010). The age distribution of igneous rocks in the Junggar Basin during the Carboniferous (Fig. 10c) shows that the main peak of igneous events was from 335 to 300 Ma, which indicates that these magma maybe release plenty of heat to form a locally high geothermal gradient field. It is worth to mention that the Tarim Basin is neighbored to the Junggar Basin (Fig. 1a) and has similar thermal history in Paleozoic (Chang et al. 2014(Chang et al. , 2017. The Tarim Basin experienced an intracratonic rift from the Middle Carboniferous to the Permian, and the early Permian Large Igneous Province has been systemically studied (Xu et al. 2014). The dramatic magmatic activity could affect the thermal regime of the basins, and the thermal gradient could reach to 30-40 °C/km in the Tarim Basin during this period (Qiu et al. 2012). This situation is also similar to the thermal regime related to the eruption of the Emeishan Large Igneous Province at 259 Ma in the Sichuan basin (Xu et al. 2001;Zhu et al. 2010).
Furthermore, the rapid uplift and extensive erosion recorded by the unconformity in the Late Carboniferous are coincident with our modeling results (Fig. 7) and are closely related to the compressional tectonic system of the basin that formed in the process of collision (Li et al. 2015(Li et al. , 2016. Hence, we suggest that the high heat flow of the Junggar Basin in Carboniferous could be a result of large-scale volcanic events and related thermal effects.

Hydrocarbon maturation style of Carboniferous source rocks
According to the thermal maturation history of different drillings (Fig. 9), we deduce the hydrocarbon maturation styles of Carboniferous source rocks in the Junggar Basin (Fig. 11). The general hydrocarbon maturation styles can  Fig. 10a). Then, the kerogens quickly broke through the oil window and the gas window, reaching to the overmature phase with a R o value of more than 3.0%.
(2) The times of hydrocarbon maturation in the Wucaiwan sag (East Uplift) and the Sikeshu sag (Southern Depression) started early and simultaneously, but have lower levels of thermal evolution with moderate R o at 1.2% and 0.9%, respectively, corresponding to the oil maturation window. These maturation styles mean that both oil and gas fields could have formed in Carboniferous source rocks. These different styles in maturity may reflect the combined effects of magmatic activity during the Carboniferous Period and the later sedimentary environment in various tectonic positions. As the lithosphere thickened and the supply of mantle heat was reduced, the heat flow has decreased rapidly since the Mesozoic. Considerable sedimentation can also do nothing to help source rocks undergoing higher temperatures (Fig. 2). This high thermal background results in rapid hydrocarbon maturation for Carboniferous source rocks and no higher maturity due to a continuous decrease in heat flow after this time. Meanwhile, the rapid deposition since the Mesozoic can lead to the development of stratigraphic overpressure, which is helpful for the migration and accumulation of oil and gas (Luo et al. 2007). Hence, the Carboniferous Formations preserve conditions for the development of large oil and gas fields in the Junggar Basin from the perspective of thermal evolution.

Conclusions
The thermal history of the Junggar Basin is quantitatively restored by integrating the paleo-geothermal gradient and paleo-heat flow methods constrained by considerable vitrinite reflectance data. Furthermore, the thermal maturation states of the Carboniferous source rocks for typical wells are identified based on 1D basin modeling. The following two major conclusions can be derived: 1. The paleo-heat flow of Junggar Basin was relatively high at ca. 80-120 mW/m 2 during the Carboniferous and Early Permian, which could be attributed to intense plate subduction and simultaneous large-scale volcanic activity. Meanwhile, considerable erosion is well documented in the Junggar Basin during this time. Later, the heat flow decreased rapidly during the Mesozoic and reached the present level of 40-60 mW/m 2 by the Cenozoic. 2. The source rocks of the Carboniferous started to generate hydrocarbons in the Late Carboniferous and are characterized by rapid hydrocarbon maturation. Not only could the kerogen rapidly break through the dry gas period so that cracking of gas occurred, but it could also remain in the oil maturation window and form oil reservoirs based on the differences between the tectonic units and the depositional environments. Hence, from the perspective of thermal evolution, the Carboniferous source rocks have the potential to develop abundant hydrocarbon resources.  Legend  Fu-2 PC-2 Pen-4 SC-1 Che-202 Tai Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.