Kerogen carbon aromaticity—its determination and significance
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Journal of Southeast Asian Earth Sciences, Vol. 5, Nos 1-4, pp. 81-86, 1991 0743-9547/91 $3.00 + 0.00 Printed in Great Britain Pergamon Press pie
Kerogen carbon aromaticity its determination and significance
University of Petroleum, P.O. Box 902, Beijing, People's Republic of China
AMtract--Based on the assumption that oil and gas are primarily generated from the aliphatic portion of kerogen, a general formula for estimating oil and gas potential P = 1.2K c Cor s has been derived from the aromatic and aliphatic carbon balance equations, where Cot s is the abundance of organic carbon and K c is termed "carbon quality factor". It can be determined by carbon aromaticity of kerogens before and after the degradation process. Five methods of spectroscopy and thermal analyses including solid state C-13 NMR, XRD, IR, DSC and TG for measuring the carbon aromaticity of kerogen are commented on. Carbon aromaticity of oil shale and lignite along with their thermal-treated kerogen concentrates of different aromaticity (0.20-0.94) have been measured using these five methods, and among them the solid state C-13 NMR spectroscopy seems to be the most reliable but complex in operation. Thermal analyses DSC and TG consume less than 10 mg of kerogen sample and are simple in operation, but are probably less accurate.
GEOCHEMICAL SIGNIFICANCE OF KEROGEN CARBON AROMATICITY
Kerogen carbon aromaticity and model of hydrocarbon formation
THE HYDROCARBON potential of kerogen depends intrinsi- cally upon its physical and chemical structures. One of the most important chemical structural parameters, car- bon aromaticity, is generally defined as the aromatic carbon fraction of organic carbon content in kerogen (Car~Co,g). From different thermal degradation tests of kerogen many investigators have found that the oil and gas potentials of kerogen are directly proportional to the moiety of the aliphatic structure, but not to that of the aromatic (Miknis et al. 1982, Qin et al. 1985). It implies that the aromatic carbons are rather inert during the oil and gas generation stage of kerogen degradation. Based on the analysis of the products obtained from pyrolysis tests, a chemical reaction model of hydrocarbon formation related to kerogen aromaticity has been proposed and is shown as Fig. 1.
Kerogen carbon aromaticity and "carbon quality factor"
As an approximation, we take aliphatic carbon as the matrix of oil and gas. Two material balance equations for aromatic and aliphatic carbons can be given:
Wo(Corg)o(far)o = Wi(Corg)i(far) i (1)
P, = Wo(Cors)o(C,,)o- W,(Corg),(f,l), (2)
W = weight of the source rock; Corg = organic carbon content of the source rock,
wt %; fa~ = fraction of aromatic carbon; fal = fraction of aliphatic carbon; P, = carbon converted to oil and gas;
o, i = subscripts before and after hydrocarbon conver- sion stage.
From equations (1) and (2), we have:
Pc = W,(Corg)i[(fal)o(far),/(far)o -- (fa,),]" (3) Putting
K,. = [(fal)o(far)i/(far)o -- (fal)i], (4)
equation (3) can be written as
P, = W,(Cor0iK,. (5)
Taking 0.83 as the average carbon content of oil and gas, the quantity of oil and gas generated per unit weight of source rock can be calculated by:
P, = 1.2gc(Co,g)i. (6)
If the carbonyl and carboxyl carbon content of kero- gen may be neglected as compared with the aromatic and aliphatic carbons, then
Lr+L~ = 1,
and the term K, can be simplified as
K,. = [(f~r),/(far)o] -- 1. (7)
thermal ~ thermal depolymerize ' ~oitumer~ degradate=
py rol ysis . i
condensize : .- residues
Fig. 1. Thermal degradation model of organic carbons in kerogens.
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In equation (6), (Corg)i represents the abundance of organic carbon of source rock, whereas Kc represents the quality of the carbon content. It can be measured by equation (7), i.e. by the carbon aromaticity of kerogen before and after the process of oil and gas generation. The term K~ is termed the "carbon quality factor" of kerogen.
In equation (7), the carbon aromaticity (far)o is related to a large extent to the type of kerogen before the stage of oil and gas generation. For immature kerogens of Type I, it ranges from 0.20 to 0.30, yet up to 0.50-0.60 for that of Type III. The carbon aromaticity (f~r)i is closely related to the maturity of kerogen, and unity will be its maximum. The upper limit of Kc for Type I kerogen nearly equals 4, while that for Type III kerogen may be less than 0.7. The "carbon quality factor" K, is a comprehensive quality index for oil and gas potential of kerogen, and it has taken both kerogen type and maturity into account. K, is not an empiric parameter estimated by geological experience, but can be actually determined by measuring the aromaticity of properly selected kerogen samples.
DETERMINATION OF KEROGEN AROMATICITY
Aromaticity is one of the most often measured par- ameters in solid fossil fuel studies. It has been measured by a variety of techniques, but to date there is no universal or standard method. In this paper, with regard to kerogens as the measuring sample, spectrometric methods including C-13 nuclear magnetic resonance (NMR), X-ray diffraction (XRD) and infrared (IR); thermal analyses of differential scanning calorimetry (DSC) and thermogravimetry (TG) will be discussed.
Solid state C-13 NMR (CP /MAS)
Solid state C-13 NMR provides the only direct measurement for the aromaticity of solid fossil fuels. It has been effectively applied in the study of coal chemistry (Wilson and Vassallo 1985) and of kerogen characteriz- ation as well (Barwise et al. 1984).
Figure 2 presents C-13 NMR spectra of three Chinese immature kerogens of the Tertiary period. The Fushun oil shale kerogen is classified as Type I, kerogen of Well Han No. ! from the third member of Shahejie For- mation (Es3) of Linqing Depression as Type II and a lignite from Huangxian as Type III. The C-13 NMR spectra of thermal-treated Fushun oil shale kerogen concentrates are shown in Fig. 3. All these NMR experiments with cross polarization and magic angle spinning (CP/MAS) techniques were performed by a Bruker MSL-300 NMR spectrometer, operating at C-13 frequency of 75.46 MHz (6.9T) and rotor spinning rate of 4 kHz. The selected re-cycle time was 2 s and the contact time, 1.5 ms, fulfilling the Hartmann-Hahn con- dition, was followed by four timed 180C pulses of TOSS echo sequence to refocus the spinning sidebands of aromatic carbon band into the isotropic peak (Dixon
_LL I L i 510 i i
200 150 100 0 ppm
Fig. 2. Solid state C-13 NMR spectra of immature kerogens. (a) Type I; (b) Type II; (c) Type 1II.
1982). The integrated intensity from the chemical shift 0-70 ppm is assigned to the aliphatic bands and 100-160 to the aromatic.
As shown in Fig. 2, the aromaticity of immature kerogens varies from 0.25 for Type I to 0.60 for Type III. In Fig. 3, the carbon aromaticity increases from 0.25 to 0.85 as the final thermal treatment temperature rises up to 510C, inferring that the aromaticity is a quantitative function of kerogen maturity.
From the high resolution solid state C-13 NMR spectra of kerogens, lots of additional structural infor- mation can be extracted and correlated with the hydro- carbon generation process (Qin and Wu 1988). For example, in the spectrum of lignite, the amount of aliphatic and aromatic carbon combined with oxygen may be estimated from the intensity of the chemical shift ranges 50-70ppm and 150-165ppm respectively; the carboxyl carbon is in the range of 165-190 ppm; and the carbonyl carbon, 190-220 ppm. Recent approaches of NMR techniques have opened exciting new areas for kerogen structural investigation (Burgar et al. 1985, Trewhella et al. 1986), and is popularizing rapidly.
As concerns the sampling of solid state C-13 NMR measurement for kerogen, the following points have to be noted:
Since the transients needed to accumulate the signal in a NMR measurement are directly related to the C-13 content of the sample, and the C-13 abundance is only
Kerogen carbon aromaticity 83
i t l l l l , ~ , , l , , , , L i r l l l l , , , I , , , , I
200 100 0
. . . . I)
chemical shift, pore
Fig. 3. Solid state C-13 NMR spectra of Fushun oil shale and its thermal treated kerogen concentrates.
about 1.1% of the total carbon content; for a con- ventional measurement of 5000-20000 transients, the amount of sample used is usually about 50-150mg, which may sometimes become a limitation for kerogen concentrates prepared from the source rock of drilling cores. NMR is a non-destructive process, and the sample can be re-used for the analyses of other purpose after NMR tests.
Conventional de-mineralization using HC1 and HF diminishes the paramagnetic ions and improves the quality of NMR spectra. The removal of pyrites is usually unnecessary, probably due to their homogeneous distribution in kerogen, thus influencing both aliphatic and aromatic carbons at the same extent.
For the NMR measurements of low-rank coals such as peak or lignite, a received sample may have a very high moisture content. The sample must be dry enough to allow tuning within the range of NMR probe, other- wise erroneous intensities and aromaticity values will result (Farnum et al. 1986).
The validity of aromaticity measurements by NMR (CP/MAS) has been reviewed by Wilson and Vassallo (1985). The main troubles are associated with quantifi- cation using cross polarization techniques. During the cross polarization process, magnetization is transferred from protons to carbons at a rate governed by a time
constant Tc.; however, the decay of polarization of nuclei occurs at a rate determined by the proton rotating frame spin lattice relaxation time constant TtpH, which is independent of the rate of cross polarization. If T]pH is much smaller than the time needed for the most weakly coupled carbons to cross polarize, these carbons will be reduced in intensity. The rate of cross polariz- ation depends on the proximity of protons to carbons and on molecular motion, hence carbons remote from protons with long Tc. may not be observed. For the samples such as high maturity kerogens or anthracites, carbons in large polycyclic aromatic clusters distant from neighbouring protons may cause the result to be semi-quantitative and sometimes underestimated to some extent. In general, a conventional solid state C-13 NMR measurement for kerogen aromaticity is sufficient to meet the validity requirements in most geological applications.
XRD spectroscopy used for measurement of pet- roleum asphaltenes was suggested by Yen et al. (1961) and has been extended to coals and oil shales (Kwan and Yen 1976). This method is based on resolving and comparing the area of the gamma band (d = 4.5-4.8/~) arising from aliphatic structures with that of the 002 band (d = 3.5-3.7 ~,) from a stack of aromatic clusters.
The study on XRD spectroscopy with model com- pounds by Ebert et al. (1983) revealed some potential difficulties in the determination of aromaticity by XRD. Although it is assumed that the area of the 002 band reflects the aromatic content, the other 00L peaks may contribute to the gamma band region thereby causing an over-estimation of aliphatic content. In addition, the paraffinic carbons contribute to the gamma band range, but the naphthenic carbons to the range of 5.2-5.4 A.
Qin et al. (1987) have found that the aromaticity of immature kerogens and lignites assessed by XRD often gives negative deviations compared with those measured by solid state C-13 NMR spectroscopy, probably be- cause the aromatic rings in the immature kerogens are not presented as the stacking structures. Corre- spondingly, Huang et al. (1987) plotted the kerogen aromaticity determined by XRD vs the factor 1460 cm-1/1600cm -1 from IR spectra; the correlation curve does not pass the origin as the XRD measured aromaticity is towards zero.
Nevertheless, using XRD as a semiquantitative esti- mation method for the kerogen aromaticity, coupling with another XRD parameter stacking height Lc has been applied in the study of kerogen structure evolution during the process of hydrocarbon generation (Qin et al. 1987).
Infrared spectroscopy is a traditional and convenient analytical means and is in much more popular use for
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characterization of kerogens than that of NMR or XRD. Fourier transform infrared (FTIR) techniques provide rapid averaging with computer-performed spec- tral substraction and least squares fitting to enhance the resolution and sensibility that was previously unattain- able. Many investigators have attempted to quantify the aromaticity measures of coals and kerogens by FTIR (Schenk et al. 1986, Riesser et al. 1984). Solomon et al. (1982) proposed a quantitative measure of aliphatic and aromatic CH content of coals by using the area under the peaks between 3000 and 2800 cm- ~ for aliphatic CH and between 900 and 700cm -~ for aromatic CH. Although the Beer-Lambert equation gives a simple quantitative formula for IR analysis, it is complicated to determine a reliable and universal absorption coefficient for relating IR band intensity to the corresponding concentration of aromatic and aliphatic CH groups in such a complex structure as kerogen. This intrinsic deficiency of IR analysis for kerogen remains unsolved to date.
It seems to be more convenient in geochemical practice to use semiquantitative factors from the IR absorption bands for characterization of kerogens. Ganz and Kalkreuth (1987) suggested the "A factor" (2930 cm -~ + 2860 cm-l)/(2930cm -~ + 2860cm -~ + 1630cm-~); Huang et al. (1987) suggested the value of 1460cm m/1600cm ~. These parameters give good correlations with oil and gas potentials of the source rock. It agrees well with the viewpoint that the aliphatic fraction in kerogen is the main matrix of oil and gas.
It is well known that the ignition point of coal in combustion rises with increasing carbon aromaticity. Two distinct bands are generally presented in the ther- mogram from a controlled combustion test of coal performed by differential scanning calorimetry. By com- paring the thermogram with those of model compounds, Benson and Schobent (1982) and Kube et al. (1984) concluded that these two bands primarily arose from the aliphatic and aromatic portions of the coal respectively. It remains true for the case of either oil shales or kerogen concentrates from source rocks (Lu et al. 1986, Qian and Chen 1987).
The DSC experiment provides for temperature pro- grammed combustion of a 1-1.5 mg sample of -100 mesh coal or kerogen in the atmosphere of air or oxygen. The sample is heated at 10-20Cmin -~ in the range 150-600C. The first exothermic band appears at nearly 300-350C and is assigned to the combustion of ali- phatics, and the second band at nearly 420-470C is to that of aromatics. The instrument response is a thermogram plot...