Mapping the chemistry of resinite, funginite and associated vitrinite in coal with micro-FTIR

Authors


Yanyan Chen, Department of Geological Sciences, Indiana University, Bloomington, IN 47405-1405, USA; e-mail: mmastale@indiana.edu

Abstract

Micro-FTIR mapping is a powerful tool for nondestructive, in situ chemical characterization of coal macerals at high resolution. In this study, the chemistry of resinite, funginite and associated vitrinite is characterized via reflectance micro-FTIR for Cenozoic high volatile C bituminous coals from Colombia. In comparison with the micro-FTIR spectra of vitrinite and inertinite, the corresponding spectra of liptinite macerals in the same coals are characterized by stronger aliphatic CHx absorbance at 3000–2800 and 1460–1450 cm−1, but less intense aromatic C=C ring stretching vibration and aromatic CHx out-of-plane deformation at 700–900 cm−1. The aliphatic components in resinite have the longest carbon chains and are least branched, bestowing the highest hydrocarbon generation potential on resinite among the three macerals studied. In contrast, funginite exhibits the strongest aromatic character, the highest aromaticity, the lowest ‘A’ factor values and the lowest C=O/C=C ratios among the three maceral groups. Vitrinite generally displays intermediate chemical characteristics. Reflectance micro-FTIR mapping of coal samples further confirms the aliphatic character of resinite and the aromatic nature of funginite. In addition, chemical mapping of resinite and adjacent vitrinite shows that vitrinite immediately adjacent to resinite displays higher aliphatic CHx stretching intensity than more distant vitrinite, suggesting that chemical components from resinite can diffuse over short distances into adjacent vitrinite, specifically causing hydrogen enrichment. It needs to be pointed out, however, that the region of influence is localized and limited to a narrow zone, whose extent likely depends on resinite's properties, such as its size and aliphatic material content. This way, the chemical map of resinite and associated vitrinite provides direct evidence of the intermaceral effects occurring during the peat forming stage or during later coalification. No influence of funginite (primarily fungal spores and sclerotia) on the chemistry of adjacent vitrinite has been demonstrated, which is likely due to the highly aromatic structure of this type of funginite.

Introduction

Determination of the variations in chemical properties of coal macerals (i.e. microscopically distinct organic constituents of coal, ICCP, 1963) is one of the most difficult tasks in coal geochemistry. Although general chemical variations among maceral groups have been extensively investigated, most of these studies are primarily based on the analyses of mechanically separated fractions, for example following density fractionation. Unfortunately, pure separates of macerals are practically impossible to obtain, especially for the macerals that are of small size or occur in small quantities in coals (Mastalerz & Bustin, 1993; Mastalerz et al., 1993a). Resinite and funginite are examples of small and relatively rare macerals for which it is difficult to determine chemical and spectroscopic properties.

Resinite is a maceral of the liptinite maceral group that occurs in many coals as well as source rocks in the form of small spherical, oval or spindle-shaped bodies (Stach, 1982) and displays quite variable chemical properties (Lin & Ritz, 1993). It is important to investigate resinite's chemistry and hydrocarbon generation potential because resinite is considered to be a preeminent source of gas/liquid hydrocarbons in many sedimentary basins (Powell & Snowdon, 1983; Goodarzi & McFarlane, 1991; McFarlane et al., 1993; Guo & Bustin, 1998; Hower et al., 2010). Funginite and secretinite are both macerals of the inertinite group and were previously jointly classified as the maceral sclerotinite (ICCP, 2001). The confusion between these two coal constituents necessitates a close examination of the properties of distinct funginite (Hower et al., 2010). Recently, the associations of funginite with resinite, cutinite, suberinite and huminite were scrutinized (Hower et al., 2010, 2011b; O’Keefe & Hower, 2011), and the fungal role in the formation of inertinite macerals has been recognized and established (Hower et al., 2011a). However, there has been a dearth of information about the distinct chemistries of resinite and funginite so far.

The heterogeneity in chemical properties within individual maceral groups has been well established. Several studies suggested that the associated liptinite (i.e. resinite, alginite) and inertinite macerals are likely to influence the chemical properties of vitrinite (Hutton & Cook, 1980; Mastalerz et al., 1993a, b; Suárez-Ruiz et al., 1994; Stankiewicz et al., 1996; Iglesias et al., 2006). However, there is still no consensus on the reasons leading to the variation of vitrinite's chemistry. Mastalerz et al. (1993a, b) and Suárez-Ruiz et al. (1994) attributed the cause for vitrinite's chemistry variations to botanical precursors and depositional environments. Stankiewicz et al. (1996) pointed out that a microbial contribution to the lignin matrix could cause hydrogen enrichment in vitrinite, which would subsequently alter the chemical properties of vitrinite. Hutton and Cook (1980) concluded that vitrinite could be modified by the leakage of material from alginites into vitrinite. However, no direct evidence about leakage of hydrocarbons from liptinite into vitrinite has been observed in their study.

Micro-Fourier transform infrared (FTIR) spectrometry is uniquely qualified to investigate the in situ chemical properties of small-size macerals, such as resinite and funginite. In situ techniques are based on high-resolution point analyses on polished surfaces of coal and therefore (1) avoid the difficulty of mechanically isolating individual maceral fractions; (2) yield far more detailed information about macerals’ origins, heterogeneity and history than bulk measurements and (3) can document intra- and intermaceral zonal heterogeneities at a microscopic level. Micro-FTIR can be performed under reflectance or transmission modes. Although reflectance micro-FTIR yields spectra of poorer quality than transmission micro-FTIR, the straightforward sample preparation and the ability to investigate the exact same maceral area as in standard reflected light microscopy make this technique highly valuable (Mastalerz & Bustin, 1995). The particular strength of reflectance micro-FTIR on the quantification of the abundance of chemical functional groups in coal's structure has been confirmed by several studies (Mastalerz et al., 1993a, b; Iglesias et al., 1995, Mastalerz & Bustin, 1995). In addition, the combination of reflectance micro-FTIR and visible light microscopy further opens the possibility of mapping the abundance of functional groups across a region, which has been recently used in a variety of fields, such as conservation sciences (Mazzeo & Joseph, 2007), medical applications (Wentrup-Byrne et al., 1995; Tesch et al., 2001) and polymer industries (Skrtic et al., 2004). The applicability of reflectance micro-FTIR mapping to the characterization of commonly occurring coal macerals, such as vitrinite and sporinite, has been demonstrated in our previous work (Chen et al., 2012).

The purpose of this study is to (1) document the chemistry of resinite, funginite and associated vitrinite maceral groups in intact coals using in situ reflectance micro-FTIR and (2) investigate resinite-vitrinite and funginite-vitrinite chemical interactions via micro-FTIR mapping.

Experimental section

Sample material

For this study, high volatile C bituminous coals were collected from the Amaga Formation in the Sinifana Basin in Colombia. These coals were selected because they contain abundant and distinct resinite and funginite occurrences. The coal samples were collected from an active coal mine in Colombia, and prepared into pellets shortly after collection. The Amaga Formation is of Cenozoic age and may have been deposited between the Upper Oligocene and the Lower Miocene (Blandón Montes, 2007). The four selected coals for this study cover a narrow range of rank corresponding to vitrinite reflectance Ro 0.54–0.55% (Table 1).

Table 1.  Proximate analysis and vitrinite reflectance Ro of analysed coal samples.
SampleTotal moisture, ar (%)Ash,d (%)Volatile matter, daf (%)Fixed carbon, daf (%)Sulfur, ar (%)Heating value, daf (kJ/kg) Ro (%)
  1. ar, as received; d, dry basis; daf, dry ash free basis.

#12022413.13.446.153.90.41303450.55
#12022813.23.248.351.70.40308520.54
#12022912.74.649.450.60.44307000.55
#12023112.23.648.651.40.38306240.54

Sample preparation

Four coal samples were characterized and chemically mapped by micro-FTIR (Table 1). Samples for vitrinite reflectance and micro-FTIR analyses were prepared as polished blocks, following standard coal petrographic procedures (ICCP, 1963).

Analytical methods

Vitrinite reflectance measurements and maceral analyses of coal samples were performed via a Zeiss RS-III microscope. The maceral composition of coal samples was determined using a standard point counting procedure (500 points counted per pellet). Micro-FTIR measurements were carried out using a Nicolet 6700 spectrometer connected to a Nicolet Continuum microscope operated in reflectance mode. The microscope was equipped with a video camera, a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector (Nicolet Instrumentations Inc., Madison, WI, USA), and a motorized mapping stage. The OMNIC program was used for spectral curve-fitting, deconvolution and peak-area integration. The microscope software Atlus in OMNIC controlled micro-FTIR mapping data collection and data processing. Reflectance micro-FTIR spectra were obtained at a resolution of 4 cm−1 with a gold plate as background. The obtained spectra were subjected to Kramers-Kronig transformation. Peak assignments of spectra were based on Painter et al. (1985) and Wang and Griffiths (1985). The micro-FTIR used in this study is able to characterize areas as small as 25 by 25 μm. We note that this area is relatively large compared to the size of many fungal remains, especially in case of fungal hyphae which are most likely to influence the chemistry of surrounding macerals (Hower et al., 2010, 2011b; O’Keefe & Hower, 2011). However, a smaller aperture size would result in spectra of poor quality due to deteriorating signal-to-noise ratios. Therefore, only large spores and sclerotia were examined in this study.

Calculations of semi-quantitative ratios

Numerous semi-quantitative ratios have been used to characterize chemical properties of macerals (Guo & Bustin, 1998; Walker & Mastalerz, 2004). The semi-quantitative ratios used in this study include (1) CHar at 700–900 cm−1 versus CHal (AR) ratio, i.e. aromaticity (Guo & Bustin, 1998; Walker & Mastalerz, 2004); (2) CH2/CH3 deconvoluted from the 2900–3000 cm−1 region, which is a widely accepted proxy for chain length (Lin & Ritz, 1993); (3) absorbance at 870 cm−1 versus absorbance at 815 cm−1 which is an index for the degree of substitution of aromatic rings with alkyl functional groups (DOS); (4) hydrocarbon-generating potential and maturation level, reflected by ratios of CHal at 3000–2800 cm−1 versus [CHal at 3000–2800 cm−1+ C = C at ∼1600 cm−1] (i.e. ‘A’ factor) and of C = O at 1710 cm−1 versus [C = O at ∼1710 cm−1+ C = C at ∼1600 cm−1] (i.e. ‘C’ factor) (Ganz & Kalkreuth, 1987; Lin & Ritz, 1993; Iglesias et al., 1995). Due to the negligible CHx stretching peak at 3000–3100 cm−1, the semi-quantitative ratios involved this integrated peak area are not discussed in this paper. Curve-fitting of peaks in the ∼1560–1800 cm−1 region provided integrated areas of oxygenated and aromatic carbon groups.

Experimental results

Petrographic characterization

Proximate analysis results and vitrinite reflectance measurements are presented in Table 1. All samples have a vitrinite reflectance of ∼ 0.55%, which classifies them as high volatile C bituminous coals. The coals’ heating values and volatile matter (on dry ash free basis) compositions fall in the range of 30 345–30 852 kJ/kg and 46–49%, respectively. The sulphur contents are all below 1.0 wt.%.

Huminite/vitrinite is the major maceral group in the samples and accounts for 62–71% by volume (Table 2). Liptinite accounts for 22–26 vol.% and is dominated by liptodetrinite (8–11 vol.%), resinite (2–4 vol.%) and exsudatinite (2–4 vol.%). Inertinite macerals are represented mainly by inertodetrinite (2–4 vol.%) and funginite (∼2 vol.%). Most of the fungal remains found in the coal samples are fungal spores and sclerotia.

Table 2.  Maceral composition (vol %) of coals used in this study.
SampleVitriniteLiptiniteInertiniteMM
V tot CtCd L tot SpCtReSuALdEx I tot FuId
  1. Vtot: total vitrinite identified in the sample, Ct: Collotelinite, Cd: Collodetrinite, Ltot: total liptinite identified in the sample, Sp: Sporinite, Ct: Cutinite, Re: Resinite, Su: Suberinite, A: Alginite, Ld: Liptodetrinite, Ex: Exsudatinite, Itot: total Inertinite identified in the sample, Fu: Funginite, Id: Inertodetrinite. , MM–mineral matter

#120224 66.0 51.015.0 26.2 4.61.84.02.21.68.04.0 6.6 1.84.81.2
#120228 68.2 53.814.4 23.6 3.81.63.81.01.010.02.4 7.8 2.05.80.4
#120229 62.0 45.416.6 26.4 1.82.04.82.21.211.03.4 11.0 2.68.40.6
#120231 71.4 49.621.8 22.4 1.80.82.42.24.47.83.0 5.0 2.62.41.2

Resinites of various size, shape and fluorescence intensity are present in the coal samples (Fig. 1a and b). In reflected light, resinite can occur as dark and light varieties, which exhibit different reflectance values. Dark resinite presents a strong yellowish green fluorescence and has Ro values below 0.20% (Table 3). Light resinite shows Ro values as high as 0.29%, and usually is admixed with clay minerals or carbonates. Cutinite and suberinite displaying a weak yellowish fluorescence are also commonly observed in the samples (Fig. 1c and d). The fluorescence of these macerals is consistent with a highly aliphatic character. Funginite shows fungal forms typical of Cenozoic coal (Stach, 1982; Fig. 1e and g). This maceral is nonfluorescent and presents the highest reflectance values of all macerals in these coals (Table 3). Cavities in funginite are frequently impregnated with exsudatinite (Fig. 1e–h).

Figure 1.

Photomicrographs of macerals in coal sample #120228, oil immersion. (a) Resinites of various sizes and shapes in reflected light; (b) resinites of various sizes and shapes in fluorescent light; (c) and (d) cutinite under reflected and fluorescent light; (e) and (f) funginite, single- and multicelled teleutospores under reflected and fluorescent light; (g) and (h) funginite (morphologically similar to Sclerotities sp.) under reflected and fluorescent light.

Table 3.  Reflectance values (%) of resinite and funginite bodies in each of the four coal samples.
Sample number#120224#120228#120229#120231
ResiniteFunginiteResiniteFunginiteResiniteFunginiteResiniteFunginite
  1. Avg, average; Std, standard deviation.

10.140.700.150.770.150.770.210.73
20.210.750.130.770.130.740.290.82
30.170.840.120.790.120.720.200.82
40.230.820.160.870.160.810.150.79
50.240.810.140.820.140.700.180.76
60.200.830.150.780.150.730.230.68
70.200.800.140.710.140.810.200.74
80.240.850.130.760.130.770.170.71
90.210.850.170.730.170.770.200.75
100.220.830.190.760.190.720.200.70
Avg 0.21 0.81 0.15 0.78 0.15 0.75 0.20 0.75
Std 0.03 0.05 0.02 0.04 0.02 0.04 0.04 0.05

Micro-FTIR spectra

Several vitrinite, resinite and funginite macerals from each coal sample were selected for micro-FTIR analysis. Due to the small size of resinite and funginite, the FTIR aperture occasionally had to be reduced from the regular 50 × 50 μm size to only 25 × 25 μm to obtain spectra from pure funginite and resinite. The small aperture enabled us to obtain several pure funginite spectra from samples #120224 and #120228, and one pure funginite spectrum from each of the other two samples.

Resinite shows the strongest intensities of aliphatic CHx stretching at 2800–3000 cm−1, aliphatic CHx deformation at ∼1450 cm−1, and the strongest signal from oxygenated groups at 1650–1800 cm−1, but less intense aromatic C=C ring stretching at ∼1600 cm−1 and CHx out-of-plane deformation at 700–900 cm−1 compared with the spectra of vitrinite and funginite (Fig. 2). The spectra of pure funginite display the most intense C = C ring stretching and out-of-plane deformation vibration peaks, suggesting a strongly aromatic character. All examined vitrinite particles display similar spectra and thus testify to vitrinite's highest degree of structural homogeneity among the three coal macerals. Vitrinite displays intermediate aliphatic CHx stretching at 2800–3000 cm−1, C=C stretching at ∼1600 cm−1 and aromatic CHx out-of-plane deformation. Interestingly, vitrinite shows the strongest OH stretching at ∼3300 cm−1, which is probably related to its phenol-rich biochemical precursor material, for example from lignin.

Figure 2.

Micro-FTIR spectra of resinite, vitrinite and funginite (with exsudatinite impregnation). Resinite shows the strongest aliphatic CHx stretching signal and the most intense oxygenated group stretching at ∼1710 cm−1, but the lowest intensities for C=C ring stretching at ∼1600 cm−1, aromatic CHx stretching, and out-of-plane deformation. Funginite plus its impregnation by exsudatinite exhibits relatively high intensities of aliphatic CHx stretching and aromatic CHx out-of-plane deformation. Vitrinite displays the weakest aliphatic CHx and oxygenated groups stretching.

The highest CH2/CH3 ratios for resinite suggest the presence of the longest and least branched aliphatic side chains of all measured macerals (Fig. 3). The highest ‘C’ factor of resinite indicates the highest C=O/C=C ratio. In addition, resinite exhibits the highest ‘A’ factor, i.e. the highest hydrocarbon-generation potential among the three macerals. In contrast, funginite displays the highest aromaticity, lowest ‘A’ factor and lowest C=O/C=C ratio or ‘C’ factor, indicating its most aromatic character among the three macerals. The lowest 870 cm−1/815 cm−1 ratio of funginite witnesses the lowest degree of substitution of aromatic sites by alkyl groups. Vitrinite shows intermediate values of aromaticity, ‘A’ factor, C=O/C=C ratio and alkyl substitution, all of which are below the respective values for resinite. The chemical character of vitrinite is therefore between those of resinite and funginite.

Figure 3.

Variations in macerals’ chemistries. Error bars indicate standard deviations. Resinite (left-slash columns) exhibits the lowest aromaticity and the highest CH2/CH3 ratios and ‘A’ and ‘C’ factors. Funginite (right-slash columns) displays highest aromaticity, low CH2/CH3 ratios, a low degree of alkyl substitution of aromatic rings (DOS), and low ‘A’ and ‘C’ factors. Vitrinite generally shows intermediate values for these ratios. Although there is a difference in the ratios between individual macerals within each sample, ratios for the same type of macerals are similar in all samples.

Reflectance micro-FTIR mapping

The overall similarity of the four samples with regard to maceral composition, vitrinite reflectance and functional group distribution makes the observations from one sample representative for all four samples. Therefore, the micro-FTIR mapping results of representative sample #120228 (Ro= 0.55%) are solely displayed to demonstrated the applicability of this technique to the characterization of small-sized coal macerals.

Resinite and its influence on adjacent vitrinite

A line map of region A with two resinite bodies was initially generated from sample #120228 to characterize the chemical properties of resinite and vitrinite, as well as the influence of resinite on adjacent vitrinite's chemical properties (Fig. 4a). The size of the FTIR aperture or the sampling area was 50 × 50 μm. The sampling area at point A1 contained pure resinite, points A2 and A3 represented pure vitrinite and sampling area A4 covered half of vitrinite and half of resinite. The intensity of aliphatic CHx stretching was highest at point A1 (resinite), decreased around points A2 and A3 (vitrinite) and rose again towards the other resinite at point A4 (Fig. 4b and c). The reverse pattern holds true for aromatic C=C stretching absorbance whose signal strength increased in the approach to vitrinite and decreased toward resinite (Fig. 4d).

Figure 4.

Chemical mapping along a linear transect in region A with two resinite grains located at both ends. Ten sampling points are evenly distributed along a transect. The sampling area at point A1 contains pure resinite, and points A2 and A3 represent pure vitrinite. The sampling area at point A4 covers half of resinite and half of vitrinite. (a) Micro-FTIR microscopic image observed through a dry 15× objective; the mapping sequence is indicated by yellow dots; (b) micro-FTIR spectra of all sampling points region A; (c) the integrated area of aliphatic CHx stretching at 2800–3000 cm−1 as a function of distance from point A1. The aliphatic character is strongest in resinite (point A1), decreases towards vitrinite (points A2 and A3) and increases in the approach to point A4 that receives a mixed signal from both vitrinite and resinite; (d) the integrated area of aromatic C=C stretching at 1550–1680 cm−1 as a function of distance from point A1. Units in panels b–d are arbitrary absorbance units (AU).

Sampling areas at points A2 and A3 are essential for the examination of the influence of resinite on the chemistry of the adjacent vitrinite. Although points A2 and A3 are both located within pure vitrinite, the closer proximity of point A2 to resinite is expected to make chemical interaction more likely for point A2 than for point A3. Therefore, we evaluated the chemical difference of vitrinite between points A2 and A3, particularly the intensity of aliphatic CHx and aromatic C=C stretching bands (Fig. 4b–d). This examination demonstrated that no distinct variation in the absorbance of aliphatic CHx stretching exists between these two vitrinite spots (Fig. 4c), although the intensity of aromatic C=C stretching at point A3 was slightly higher than at point A2 (Fig. 4d). One possible reason for the observed homogeneity in vitrinite between points A2 and A3 could be that the chosen sampling area was too large to capture narrow zonal differences. Subsequently, the aperture size was reduced to 25% (i.e. 25 × 25 μm) for more detailed chemical mapping. Considering that the influence of resinite might not be identical in all directions in vitrinite, we employed two-dimensional area mapping (in region B) rather than linear mapping along a transect to characterize the possible influence of resinite on adjacent vitrinite's properties.

A different microscopic region B in the same block of coal featured multiple resinite macerals adjacent to vitrinite for detailed chemical mapping of the resinite/vitrinite contact area (Fig. 5a and b). Mapping of the integrated areas of aliphatic CHx stretching (2800–3000 cm−1), aromatic C=C stretching (1550–1650 cm−1) and absorption by oxygenated groups (1650–1800 cm−1) yielded a good match with the petrography of the same area, thereby affirming the applicability of micro-FTIR to chemically characterize fine-grained coal macerals (Fig. 5c–h). The signal intensities of aliphatic CHx stretching at 2800–3000 cm−1 and of oxygenated groups were higher in resinite (i.e. red and yellow colours in Fig. 5c and g) than in vitrinite. On the contrary, the absorbance of aromatic C=C was higher in vitrinite (Fig. 5e). These two-dimensional findings are consistent with the initial linear results from micro-FTIR along a transect. Aromatic CHx stretching at 3000–3100 cm−1 and out-of-plane deformation at 700–900 cm−1 were not included, due to the large spectral noise and small peak areas.

Figure 5.

Chemical maps of a microscopic field with multiple resinite bodies in a vitrinite matrix in region B. (a) Micro-FTIR microscopic image observed through a dry 10× objective; (b) viewing the field of study through an infrared objective with 15× magnification; the mapping sequence is indicated by yellow dots; (c) chemical mapping of aliphatic CHx groups (2800–3000 cm−1 region) in the field of study, and (d) the corresponding chemical map superimposed on the microscopic image; (e) chemical mapping of aromatic C=C stretching (1550–1650 cm−1 region) and (f) the corresponding chemical map superimposed on the microscopic image; (g) chemical mapping of oxygenated groups (1650–1800 cm−1) and (h) the corresponding chemical map superimposed on the microscopic image. The scales of panels e and g are the same as that of c, and the scales of d, f, h are the same as that of b. Units in panels c, e and g are arbitrary absorbance units (AU).

In microscopic region B, spots B1B7 were selected to trace the influence of resinite on the adjacent vitrinite (Figs 5a and 6). Point B1 falls in the middle of resinite, thus the spectrum at this point represents pure resinite. Points B2 and B5 are located in vitrinite where chemical properties are expected to be influenced by the adjacent resinite. The sequence of points B3, B4, B6 and B7 traces a suite of vitrinites with increasing distance from resinite. The spectrum at point B7 serves as a pristine vitrinite background, because there is no nearby resinite. Consistent with the earlier findings, resinite expressed the most aliphatic characteristics with the strongest aliphatic CHx stretching and deformation, and the weakest aromatic C=C vibration (Figs 5b and 6, point B1). From the resinite-vitrinite boundary at point B2 to a more distant vitrinite region at point B3, the integrated area of aliphatic CHx decreased by more than three arbitrary absorbance units (AU) or ∼40%, which further declined by 1 AU toward point B4. The aliphatic CHx signal rose by around 2 AU, i.e. ∼50%, at the next resinite/vitrinite boundary near point B5. The integrated area of aliphatic CHx stretching then declined with increasing distance from resinite, and finally reached its lowest value at point B7 in vitrinite. We conclude that the integrated area of aliphatic CHx stretching consistently increases in vitrinite with decreasing distance from resinite, in agreement with the hypothesis that resinite directly affects the chemical properties of neighbouring vitrinite.

Figure 6.

Micro-FTIR spectra of region B Measurement points B1–B7 were extracted from the reflectance micro-FTIR area map shown in Fig. 6. The values stated are the integrated areas of corresponding vibrations. Point B1 is located in the centre of resinite, thus the spectrum of this point represents pure resinite. Points B2 and B5 are located at the edge of a resinite maceral. Points B3, B4, B6 and B7 trace a series of measurement points in vitrinite with increasing distances from resinite. The spectrum of point B7 can be regarded as a vitrinite background. The spectra of points B2 and B5 exhibit higher intensities of aliphatic CHx stretching than the spectra of points further away from resinite. No distinct difference in oxygenated groups and aromatic C=C ring stretching were found among points B2, B5 and other sampling areas in vitrinite. Units are arbitrary absorbance units (AU).

However, it is important to realize that vitrinite and its biological precursor wood are never homogeneous, even in the absence of neighbouring resinite. Vitrinite inherits its heterogeneous legacy from woody tissues and, thus, exhibits natural chemical variance, for example, in the integrated areas of aromatic C=C stretching and oxygenated groups expressed by micro-FTIR data. No clear trend was observed in vitrinite's signal strengths of aromatic C=C stretching and oxygenated groups either as a function of distance of vitrinite from resinite, or as a reflection of natural variance (Fig. 6). To investigate the extent of internal chemical variations within vitrinite, 15 random vitrinite spots distant from resinite macerals were micro-FTIR investigated in the same maceral assemblage. The natural variations of their absorbance of aliphatic CHx stretching were no more than 0.5 AU, which is much lower than the difference between points B2 and B3 (3.1 AU), or between points B4 and B5 (2.1 AU). Therefore, the natural chemical variation in vitrinite does not account for the observed large difference in aliphatic CHx stretching in vitrinite at close range to resinite.

Funginite and its exsudatinite impregnation

Funginite cells in this study's coal samples were frequently impregnated with exsudatinite, which resulted in strong chemical gradients from interior exsudatinite fillings to funginite rims. The number of FTIR studies on funginite is limited due to the generally small size of funginite between ∼10–30 μm for fungal spores and ∼10–80 μm for sclerotia and other fungal remains (Moore et al., 1996; ICCP, 2001). Mapping of the chemistries of a thick-walled funginite and its interior exsudatinite impregnation (Fig. 7a and b) indicated that the signal strength of aliphatic CHx stretching bands at 2800–3000 cm−1 closely mirrored the petrography. The exsudatinite filling coincided with the highest aliphatic CHx stretching intensity (i.e. red centre in Fig. 7c and d), whereas the funginite rim matched the surrounding blue perimeter expressing the lowest aliphatic CHx stretching intensity. The integrated areas of the absorbance peaks increased from funginite to adjacent vitrinite (switch to green in Fig. 7c and d). The pattern of the integrated area of aromatic C=C ring stretching also mirrored the petrography, but in reverse order from the trend of the aliphatic CHx stretching bands (Fig. 7e and f). The integrated areas of aromatic CHx stretching and out-of-plane deformation did not show clear trends due to their weak intensities and relatively small signal-to-noise ratios.

Figure 7.

Chemical maps of a microscopic field in region C featuring funginite with exsudatinite impregnation. (a) Micro-FTIR microscopic image observed through a dry 10× objective; (b) viewing the field of study through an infrared objective with 15× magnification; a mapping sequence is indicated by yellow dots; (c) chemical mapping of aliphatic CHx groups (2800–3000 cm−1 region) in the field of study and (d) the corresponding chemical map superimposed on the microscopic image; (e) chemical mapping of aromatic C=C stretching (1550–1650 cm−1 region) and (f) the corresponding chemical map superimposed on the microscopic image. Units in c and e are arbitrary absorbance units (AU). Panels c and e are of the same scale, and b, d and f are of the same scales.

We performed linear chemical mapping along a transect through funginite into adjacent vitrinite in region C in the same coal block to examine (i) chemical differences among vitrinite, the funginite rim and the interior exsudatinite impregnation and (ii) to evaluate the influence of funginite on neighbouring vitrinite (Fig. 8a). The linear map covers the adjacent vitrinite (points C1 and C2), the exterior funginite rim (points C2 and C4), and exsudatinite impregnation (point C3). The vitrinite spectra at both ends of the transect similarly display high intensities of aliphatic CHx stretching groups (Fig. 8b). The aliphatic signal decreases abruptly upon reaching the more aromatic funginite rim. The interior exsudatinite impregnation in funginite features the most intense aliphatic CHx stretching bands and the weakest aromatic C=C stretching at ∼1600 cm−1 (Fig. 8c and d). The adjacent vitrinite presents a higher intensity of OH stretching at ∼3300 cm−1 compared with funginite and inner exsudatinite. A comparison of vitrinite adjacent to funginite versus more distant vitrinite yielded no distinct differences in aliphatic CHx and C=C intensities and, therefore, did not corroborate the hypothesis that funginite exerts a chemical influence on adjacent vitrinite. We note, however, that the funginite walls were too thin for the resolution of micro-FTIR to examine the possible chemical influence of exsudatinite impregnation and associated vitrinite on the funginite rim.

Figure 8.

Line map of region C featuring a cross section of the same funginite maceral as shown in Fig. 7. Points C1 and C5 are in the adjacent vitrinite; points C2 and C4 are on the exterior funginite rim; point C3 is on the exsudatinite impregnation. (a) Micro-FTIR microscopic image observed through a dry 15× objective; a mapping sequence is indicated by yellow dots; (b) 3D-image of absorbance of aliphatic CHx stretching bands at 2800–3000 cm−1 across a transect through funginite into adjacent vitrinite; (c) integrated area of aliphatic CHx stretching at 2800–3000 cm−1 as a function of distance from point C1; (d): integrated area of aliphatic CHx stretching at 1550–1650 cm−1 as a function of distance from point C1. The aliphatic characteristic is weakest in the funginite wall but strongest in the interior exsudatinite. vit, vitrinite; Ex, exsudatinite; Fu, funginite in panels b–d.

Discussion

Chemical properties of resinite and funginite

The micro-FTIR data confirm that resinite has the strongest aliphatic character and funginite expresses the most pronounced aromatic nature among the three studied macerals. The aliphatic CHx stretching at 2800–3000 cm−1 is reported to be roughly proportional to shales’ oil yields and can be used to estimate the aliphatic hydrogen contents in kerogen (Solomon & Miknis, 1980). Resinite displays the strongest absorbance in this region, which is in agreement with the higher hydrogen contents and more aliphatic characteristics of liptinite macerals relative to other macerals (van Krevelen, 1961). Resinite's higher CH2/CH3 ratio reflects its longer-chained and less branched molecular structure. The degree of branching and chain length of aliphatic side groups affect bond dissociation energies that determine the hydrocarbon generation kinetics of kerogen (Lin & Ritz, 1993). Therefore, CH2/CH3 ratios can be used to assess the oil or gas potential. The ‘A’ factor is a widely used proxy for the hydrocarbon generation potential. High CH2/CH3 ratios and ‘A’ factors indicate that resinite is the most hydrocarbon-prone kerogen among the three macerals studied. Resinite serves as a major source of liquid hydrocarbons in many coals and source rocks, such as in northwestern Canada, Indonesia and Australia (Powell & Snowdon, 1983; Goodarzi & McFarlane, 1991).

Alkyl-substitution of hydrogen atoms on cycloalkane rings in the resinite structure may be responsible for resinites’ relatively low CH2/CH3 ratio in comparison to other liptinite macerals such as alginite (Lin & Ritz, 1993; Guo & Bustin, 1998). The alkyl substitutions on cycloalkanes introduce numerous terminal CH3 groups to the resinite structure and lower the CH2/CH3 ratio in resinite (Lin & Ritz, 1993). The strongly variable chemistry of resinites in different coals warrants caution when extrapolating the results to resinite macerals from other regions. Aliphatic chains in resinite is estimated to average only 5–12 carbon atoms based on Lin and Ritz's (1993) use of n-alkanes as a model for aliphatic structures in resinite.

The intense aromatic C=C ring stretching and large out-of-plane deformation peaks in funginite spectra corroborate funginite's highly aromatic nature. The high aromaticity, low ‘A’ factor and low C=O/C=C ratio of funginite are in agreement with our previous results on other macerals of the inertinite group (Chen et al., 2012). The low 870 cm−1/815 cm−1 ratios in funginite indicate a low degree of alkyl-substitution in aromatic rings in funginite relative to resinite and vitrinite.

The influence of resinite and funginite on adjacent vitrinite

Although the influence of liptinite and inertinite on the chemistry of adjacent macerals has been addressed in several studies (Hutton & Cook, 1980; Mastalerz et al., 1993a, b; Suárez-Ruiz et al., 1994; Stankiewicz et al., 1996; Iglesias et al., 2006), no consensus has been reached about underlying mechanisms. Three possible causes have been suggested to explain alterations of vitrinite's chemistry in the proximity of liptinite and inertinite macerals: (1) botanic precursors and the depositional environment; (2) coalification history of particular samples and (3) intermaceral reactions during the early peat-forming stage or during later coalification.

Within a limited region in vitrinite, the absorbance by aliphatic CHx stretching at 2800–3000 cm−1 is significantly enhanced at close proximity to resinite. Among the three above-mentioned possible explanations, the first two are unlikely responsible, because the measured vitrinites are located within a distance of ∼25 μm. The vitrinites have the same maturity, underwent the same coalification process and are almost certainly derived from the same parent material. Intermaceral exchange in the form of diffusion of aliphatic material with low or moderate molecular weight from resinite into the adjacent vitrinite is the only reasonable explanation. Vitrinite can accommodate hydrocarbons generated from liptinite during catagenesis (Hutton & Cook, 1980; Mastalerz et al., 1993b). The close association of resinite and vitrinite can over time lead to transport of chemical moieties across maceral boundaries and subsequent zonal chemical changes in neighbouring macerals. Intermaceral exchange can also explain the locally restricted alteration of vitrinite's chemistry, whereby resinite's influence is manifest only for the resinite–vitrinite boundary zone. The vitrinite reflectance values indicate that the coal samples are at the beginning of the oil window and started to generate hydrocarbons. Heterogeneity in vitrinite chemistry could partially result from vitrinite–liptinite interaction in liptinite-rich coals as a result of coalification, or even at the early stage of deposition.

The described intermaceral influence is spatially limited by the ability of components with low or moderate molecular weight to diffuse across maceral boundaries into vitrinite where morphologically preserved cell walls are slowing lateral transport. Moreover, the extent of the affected zone in vitrinite is strongly dependent on the chemistry of resinite as the source of potentially mobile components. The internal chemical variability of adjacent vitrinite as compared to unaffected vitrinite is most clearly expressed by differences in the intensity of aliphatic CHx stretching (Fig. 6), suggesting that hydrogen enrichment is the most likely mechanism of resinite's influence on vitrinite's chemical properties.

In contrast to localized but distinct intermaceral effects found between resinite and vitrinite, we observed no chemical impact of fungal spores/sclerotia on adjacent vitrinite. No migration of aliphatics is expected from funginite to vitrinite, mainly because funginite contains considerably fewer aliphatics than liptinite (Mastalerz et al., 1993a). There are various types of funginites (Hower et al., 2011a, b; O’Keefe & Hower, 2011) that may express variable chemistries. For example, fungal hyphae are likely to affect the chemistry of surrounding macerals due to their highly activity. Additional chemical mapping will be required on more funginite bodies from different coals. The very small size and low amounts of funginite in most coals will make micro-FTIR a necessary and invaluable tool to probe funginite's influence on other maceral groups.

The successful application of high resolution reflectance micro-FTIR in this study suggests that the technique can be applied for in situ chemical investigations of small and dispersed organic matter in other source rocks, such as shales. Similar to coal, shales’ well-known heterogeneity limits the accuracy of predicting the hydrocarbon-generating potential and storage capacity based solely on bulk information (Ross & Bustin, 2008, 2009). In situ micro-FTIR characterization of shale has been rarely attempted, but promises to become a powerful asset for describing and diagnostically utilizing the heterogeneity of shales.

Conclusions

Cenozoic coal samples from the Amaga Formation in the Sinifana Basin in Colombia with a vitrinite reflectance value of ∼0.55% were characterized by micro-FTIR methods. Resinite, funginite and associated vitrinite were the main maceral targets of this study. The following conclusions can be reached based on the micro-FTIR results:

  • 1Resinite exhibits the highest CH2/CH3 ratios and ‘A’ factor values in response to the longest aliphatic chain lengths and the highest hydrocarbon-generating potential among the three macerals studied. Funginite, in contrast, expresses the strongest aromatic character and shows the highest aromaticity and lowest hydrocarbon-generating potential. Vitrinite generally displays an intermediate characteristic between funginite and resinite.
  • 2Resinite can influence the chemistry of adjacent vitrinite by contributing some of its mobile aliphatic moieties. The zone of influence in the studied samples is narrowly limited to less than 25 μm around the resinite/vitrinite contact.
  • 3No evidence of interactions between highly aromatic funginite (primarily fungal spores and sclerotia) and adjacent vitrinite was found. However, before making assumptions about a supposedly strongly aromatic and chemically nonreactive character of funginite, close attention should be paid to the material that is filling funginite cells. Aliphatic exsudatinite was found abundantly filling funginite cells. As a result, the area occupied by the funginite body becomes much less aromatic than would be expected from the funginite alone.
  • 4This study demonstrates the great utility of nondestructive micro-FTIR, including its chemical mapping potential, for characterization of coal macerals at high resolution. The presented analytical examples of small-size resinite and funginite macerals suggest that this technique could be applied for in situ chemical investigations of organic matter in other rock types. It might be especially applicable for organic matter-bearing shales to address their heterogeneity.

Acknowledgements

Financial support from the China Scholarship Council (CSC) is gratefully acknowledged. This study was partially supported by U.S. Department of Energy, Basic Energy Sciences, Grant No. DE-FG02–11ER16246. The authors also acknowledge the support by National University and Colciencias for coal sources and mining.

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