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IR classification of kerogen type, thermal maturation, hydrocarbon potential and lithological characteristics
Journal of Southeast Asian Earth Sciences, Vol.5, Nos 1-4,pp. 19-28,1991
0743-9547/91$3.00+ 0.00 PergamonPresspie
Printedin Great Britain
IR classification of kerogen type, thermal maturation, hydrocarbon potential and lithological characteristics H . H . GANZ* and W. KALKREUTHt *Technische Universit/it Berlin, SFB 69, Ackerstr. 71-76, D-1000 Berlin 65, F.R.G. and ~'Institute of Sedimentary and Petroleum Geology, 3303 33rd Street NW, Calgary, Alberta T2L 2A7, Canada Abstract--In the present study, maturation levelsand kerogen types were definedby infrared spectroscopy.The spectra display distinct peaks for aliphatic, carboxyl/carbonyland aromatic compounds which can be used to classify kerogen types similar to the traditional H/C~)/C van Krevelendiagram and to estimate the maturation levelsby means of a vitrinitereflectanceequivalentgrid. Additionalinformationabout the maturation is provided by the shift of the aromatic C-------Cbands of kerogens towards a minimum wavenumber W~, and the IR aromaticity of kerogens and bitumen. Parameters obtained from infrared spectroscopyalso allow the prediction of oil and gas potentials. A combination of Rock-Eval pyrolysis and infrared spectroscopyappears to be an effectivetool to group sourcerocks into oil- and/or gas-pronecategories.Also mineralogicaland sedimentological characteristics are determined by a new infrared method. Quantitative contents of quartz, dolomite, calcite, kaolinite, chlorite, illite, smectite and pyrite are calculated in the original homogenizedsample in less than 15 min.
INTRODUCTION
ANALYTICAL M E T H O D S
TODAYthere is no doubt that natural oil and gas deposits derive from the organic material existing in sediments; the kerogen. Varying amounts of oil, gas, or both, may be generated during burial of sediments according to the type of kerogen and its maturity. Through chemical analysis it is possible to distinguish four types of kerogen by plotting atomic H/C vs O/C on a van Krevelen diagram. For petroleum exploration it is vital to determine not only the kerogen types of oil and gas source rocks but also their maturities--both are important parameters in explorers' basinal models. There have been many attempts to develop techniques to replace elemental analysis which is both time-consuming and very sensitive to kerogen admixtures. Pyrolysis techniques, e.g. Rock-Eval, are routine today for the screening of drill core and cuttings, because this automated procedure provides a great sample handling capacity. It has been discovered, however, that the products of pyrolysis depend substantially upon the mineral composition of the sediment as well as upon the organic carbon content. In practice, although pyrolysis is an inexpensive and useful screening technique, kerogen classification is sometimes unreliable, and the results must be checked with elemental analysis and optical measurements. The latter require considerable palynological training and experience. Recently a geochemical routine for the determination of petroleum source rock characteristics based on infrared spectroscopy and utilizing a rapid bitumen separation method for gas chromatography has been developed. Several geochemically significant parameters were determined with high precision in a short time for preparation and analysis.
Sample preparation
IR spectroscopy is very sensitive to grain size effects due to Rayleigh's law of scattering (see Valasek 1960) and the so-called Christiansen effect (Christiansen 1884). Scattering and the Christiansen effect can be eliminated when the particle size decreases to a very small diameter compared with the wavelength of the light. Thus, prior to the several analytical steps, proper grinding of the samples is necessary. About 9 g of a sample (drill core or cutting, handpicked and washed with distilled water) are treated for 10 s in a tempered chrome steel mortar of a vibratory cup mill (Fa. Siebtechnik; other mills, e.g. the "Fritsch pulverisette 9", are very similar and may also be used). After 10 s the mortar is opened and the sample powder is brushed carefully from the side wall cover and the grinding set of the mortar. Then the grinding is repeated again for 10 s. Analyses of the grain size distribution ("Zilas" laser granulometer) reveal that the sample is now equivalent to 250 mesh ( < 6 3 #). After brushing and homogenizing the sample is manually spliced and 7 g are taken away for the following bitumen extraction and kerogen isolation. The remaining 2 g of sample are ground again 2 x 10 s in the same manner described above. After the final (4th) treatment the sample size is proven to be at least smaller than 10 p. Detailed investigations revealed that further grinding destroyed the internal structure of minerals. This was indicated by an increasing incorporation of OH-bonds and subsequent decrease of the extinction coefficients. After drying at 125°C for 3h, 0.5-0.7mg of this 19
20
H . H . GANZ and W. KALKREUTH
pre-ground and homogenized sample was carefully distributed in 200mg KBr (Merck No. 4907) using a microspoon. For the proper weighing a five decimal high precision balance is essential (Sartorius research). The homogenization was completed by shaking the mixture in the same carrier bag of alumina foil which had already been used for the weighing and distribution process. The mixture is now evacuated and pressed at 10 t c m -2. This rapid homogenization nearly prevents the sample disc from water contamination and the very small amounts of water which might be present are completely removed after a final heating of the pellet at 125°C. The pellet is now scanned using a Perkin Elmer IR 598. After IR analysis the pellet is stored in the alumina foil for later purposes. Previous IR methods for compositional analysis suffered from the interference by OH-bond frequencies due to water (e.g. Flehmig and Kurze, 1973). Their pellets became cloudy during analysis. With the aid of the modified KBr pellet technique which protects the pellet from water contamination the pellet stays clear and translucent all the time. Since the IR spectra of smectite and illite differ only in respect to the content of water which is incorporated in the crystal lattice of smectite, not only the contents of quartz, calcite, dolomite, kaolinite and chlorite are determined quantitatively but also the yields of smectite and illite. Using the modified KBr pellet technique described above, with care in preparation, spectra can be repeated with < 5% variation. Bitumen extraction Soxhlet extraction with CH2C12 is time-consuming (24-48 h) and large amounts of organic solvent have to be evaporated. This often causes loss of the light nalkanes. Also the following asphaltene/non-asphaltene fractionation and column chromatography are very time-consuming methods. These processes are replaced by an extraction with CH2CI: by ultrasonic agitation and centrifuging. Three to seven g of the sample (depending on the TOC content), which was already treated 2 x 10 s with the vibratory cup mill, are filled in a 100 ml glass tube of a centrifuge (Heraeus Christ, Labofuge) and ultrasonically treated with 30 ml of CH2C12 for 5 min. After centrifuging (4 min at 4000 min -1) and decanting of the solvent, the treatment is repeated at least two times (reservoir rocks with high amounts of migrated bitumen more than five times) to remove all soluble bitumen until the liquid remains clear. The bitumen is usually collected in ordinary laboratory beakers, slightly covered with alumina foil, and placed under the hood in order to allow slow evaporation of the solvent. The extracted sediment is transferred into 100 ml centrifuge plastic tubes, and ready for the HC1/HF treatment of the kerogen isolation procedure. After 36 h the weight of the dried bitumen is calculated and the extract is transferred to small 5 ml vessels. After the final evaporation of the solvent the bitumen is ready for further analyses. For IR analysis the bitumen is dissolved in a few drops of CH2C12, taken up by a pipette and concentrate in the
middle of a sodium chloride crystal. A few seconds later the solvent is evaporated and the crystal is mounted in a holder and placed in the beam of the IR spectroscope. Usually for bitumen extraction 24-48 samples are treated in succession. Thus a total working time of less than 15 min/sample is necessary. For GC analysis leaching of the de-volatilized extract with Frigen (CCI3F) leads to the separation of the saturate fraction within a few seconds (Ganz et al. 1987). Kerogen isolation After bitumen extraction and transfer of samples into plastic tubes some drops of methanol are added in order to enhance their wettability. Careful treatment with HCI (10%) is used for the destruction of carbonates. The solution is decanted after centrifuging and the sample is treated with conc. HF (about 15 ml). In order to obtain a good suspension the mixture is stirred carefully. Up to 24 beakers are placed in a water bath at 70°C for about 48 h (Dinkelberg Laktotherm waterbath). Each day the beakers are filled up with additional HF and distilled water. The solution is decanted after centrifuging and treated with hot HC1 (20%) to remove fluorides which might have been formed during the dissolution of the minerals. The amount of newly formed fluorides depends mainly on the content of clay minerals and might be totally prevented if centrifuging and washing of the samples is practiced every 12 h. After the HC1 treatment the samples are finally washed twice with hot distilled water and dried after decantation at 60°C in an ordinary laboratory oven. Since up to 24 samples (or 48 samples if using 2 waterbaths) are isolated at one time, the total working time for the kerogen isolation procedure is less than 15 min. Comparison with cold acid techniques used in an early stage of the studies (Ganz 1986) indicates that the hot acid treatment and the drying at elevated temperatures for a limited time does not influence the composition of the kerogen. After drying of the kerogen it is ground by hand in an achate mortar for 2 min. Measurements of the grain size distribution revealed that the diameter of the particles is at least smaller than 10/~. The kerogen is now ready for IR analysis. The pellet preparation is essentially that described for the mineralogical determinations except for the heating of the pellet at 125°C. Once the kerogen KBr pellet is pressed, no further heating is necessary.
INFRARED SPECTROSCOPY
IR kerogen analysis During kerogen isolation all minerals but pyrite and heavy minerals are dissolved. Often heavy liquids are used for their separation, but these techniques also cause density fractionation of the kerogen. Moreover, the removal is never complete, because pyrite is coated by kerogen. Elemental analyses for kerogen classification suffer from such kerogen impurities.
IR classification Additional time-consuming determination of iron content is often necessary for the evaluation of the amount of pyrite. Similar problems are caused by ralstonitelike fluorides (Na, Mg, A1, (F, OH), H20) which are sometimes formed during kerogen isolation. The differentiation of the several types of kerogen and a certain evaluation of the stage of thermal maturation finally succeeds in the van Krevelen atomic H/C and O/C diagram, if the kerogen was sufficiently pure enough. Besides the conventional elemental analysis, Rock-Eval pyrolysis was established over the past few years. The main advantage of this method is that no kerogen isolation is necessary prior to analysis. Kerogen type, maturity and hydrocarbon generating potential are determined very quickly. Reliability of the data however, is sometimes not very high. Especially, the determination of maturation depends highly upon kerogen type and TOC content, as well as the rates of associated clay minerals (Katz 1981, Peters 1986, Tissot et al. 1987). Even the typing of the kerogen is sometimes not dependable. Thus optical studies, vitrinite reflectance for the determination of maturation, and maceral analysis for kerogen classification, are always achieved for control. In typical oil-prone lithologies additional spectral fluorescence measurements have to be performed (Ottenjahn 1981/82) since vitrinite reflectance values are generally lowered in alginite-rich source rocks (Hutton et al. 1980, Kalkreuth and Macauley 1984). It is well known that the evolution of kerogen structure during burial is reflected by the infrared spectrum (Tissot and Deroo 1978). The spectra (see Fig. 1) typically display distinctive peaks at 2860 and 2930 cm(CH2 and CH 3 aliphatic groups), at 1710 cm- l (carboxyl and carbonyl groups) and at 1630 cm- ~(aromatic C-------C bands). With increasing maturation the aliphatic peaks initially increase while the carboxyl/carbonyl peak decreases. As maturation continues to increase, the aliphatic peaks decrease, while there is no apparent change in the peaks representing the aromatic C-----C bands. Plotting the relative ratios of the intensities of aliphatic/
21
aromatic bands (A-Factor) against the ratios of carboxyl and carbonyl/aromatic bands (C-Factor) in a diagram results in an excellent differentiation of the organic matter. A-Factor =
C-Factor =
(2860 + 2930 cm -~) (2860 + 2930 + 1630 cm -l) (1705 cm- ~) (1705 + 1630 cm-I)"
The resulting plot (Fig. 2) is comparable to the van Krevelen diagram (Ganz and Robison 1985, Ganz 1986). Infrared analysis of a large number of kerogens of different types and thermal maturities resulted in the development of a vitrinite reflectance equivalent grid which allows the precise determination of R0 % (Ganz et al. 1987). Comparison with standard techniques indicate that in typical oil-prone source rock lithologies, infrared spectroscopy is a more reliable maturity indicator than the measured optical reflectance values. The validity of these determinations was confirmed by a great number of samples, for example oil shales from Canada which are directly overlain by a coal seam. Microscopically determined vitrinite reflectance showed values of 0.4% whilst the coal itself had 0.95% reflectance. Infrared spectroscopy on these samples, however, determined more than 0.9% for both oil shales and coal. More detailed recent investigations revealed that the IR spectra yield still more information. Looking at IR spectra of kerogens of different maturations, it is obvious that with increasing maturation there is an increase in the intensities of the aromatic C-H bands in the range between 700 and 900 cm -1 (see Fig. 3). This is reflected by the ratio of the intensity at 865 cm- i against the more or less stable intensity of the aromatic C------Cbands. The resulting curve (Fig. 4) is very similar to the well known methyl-phenanthrene-index (MPI) of Radke et al. (1982), but with a much wider range of application, of up to 4% vitrinite reflectance and no separation of the different kerogen types. That means IR kerogen Ivlaxi=100 O0T
I
f
i
~ /OnD35'00 30{)0 2BIo0 2000
I1 BIO
Min =O00T
1000
5~(]Cm -¶
Fig. I. Determination of the intensities of aliphatic, carboxyl/carbonyl and aromatic bands with the help of the baselines.
22
H . H . GANZ and W. KALKREUTH
113
rinite - -/ - vit . •reftectonce equivalent grid
O~ tI
II
09
/
I
ii i
I
/
! /
evolution path ,of type 1
I
/
-- 0.8 T E t~ ÷
/ /
07 •
i
/
t' /
o.6
,,:'
";E 0.5
x x \ \~ *
/
!
I
o
+
/
I I I
o
i /t
i
I
I
evolution pQth of type II
/
I /
I iI I
cold seam ,
/
1 I
/ I I I
O.t
evolution path of type Ill 0.2 i
0.2
type IV 0.1
0
0.1
~ 0.2 03 0.4 05 05 0.7 C- factor (1710cm't)/(1710*1630cm")
0B
0.9
Fig. 2. Classification of kerogen types and maturation according to Aand C-Factors and the vitrinite reflectance equivalent grid obtained from the IR diagram.
aromaticity seems to be facies independent. The entitled question is how to distinguish between the upper and lower part of the curve in the case of single samples. The interpretation of more than 100 IR spectra of different
kerogens showed that there is another very characteristic feature: with increasing maturation the aromatic C------C bands are shifted towards a minimum wavenumber. In Fig. 5 the minimum wavenumber, Wmin, is plotted against the vitrinite reflectance values of the standard samples. Note that all samples, including those which are affected by weathering, correlate along this curve. Since the standard deviation of that curve is rather high (about 0.4% R0), Wminis not used for the proper determination of Ro % but for the decision as to which part of the IR aromaticity diagram should be used. For example, Wmi, of 1600 indicates that we are still on the lower limb of the IR aromaticity diagram within the oil window. A Wmi. of 1585, however, indicates that we are already in the gas zone and the upper limb of the IR aromaticity diagram should be used for the proper Ro calculation. The availability of three IR indicators for the determination of maturity is a great advantage of the IR technique. In practice, the vitrinite reflectance equivalent grid of the IR diagram is the most reliable indicator of maturation. But the effects of weathering have great impact on the A- and C-Factors. To study this influence a core was drilled horizontally into a coal seam up to about 40 m below the surface. Because all samples were considered to be of identical primary composition, any difference could be related to weathering effects. Piittmann et al. (1987) described the contents of GC-MS-determined polar and aromatic compounds in relation to the distance from the surface. Nearly constant values were obtained at a depth of about 30 m. The results of the IR studies on the same samples showed that the coals at the surface are clearly oxidized and consist of transitional type IV kerogen. The vitrinite reflectance equivalent grid indicated a maturation of 0.4% R0. The real values of 1.2% were nearly obtained
IR-KEROGEN-AROMATICITY IT
Intens£y (C-H"/C-HIC=C)
'
I
I
I
I
2600
1500
1400
1300
.
I t200 ©S - 1
.
.
L.., t tO0
.
.
/
I
i
I
I
lO00
900
!100
7O0
Fig. 3. Determination of the intensity of the aromatic band at 865 cm- ~.
IR classification
23
I R - KEROGEN-AROMATICITY DOW - Plot
4.5
~
~ \
." \
.
3
Kel'ogentyl)e I+Ii zzz zv
"\
\ \~+
4
o ¢
\\ \
"\\
\
.
"
I
~
/
i li f
.~I~.
\-
+ \ o
1 I ~o' ~O5.-i-. _~ °o°o7 o~ ~
oil
--o
"%
~e
"
WINDOW. i *"
0\
\
-WET GAS
.
OiL
.
\ ~o ~" o8~'\\
.
II
÷ o
ill + .
i~I
~+ -+- ..I
~"
"~
+~;°+"b~+-
. u V " " / " ~/- - ~ ~ 0 .0." ~. -. . ~- :~'-" 0,5 . ~ +0~o o+I..---,.,~,~,''~ e ..... --." """ DIAGENESIS
.......-'"
0
0
O,2 I~
O,4 + W rain era-l)
m-ll / I l~
Fig. 4. Correlation of vitrinite reflectance and IR aromaticity of kerogens (DOW-plot: indication of diagenesis, oil window and wet gas zone).
at about 30m. As the A/C factor of the weathered samples, the IR aromaticity is highly influenced by the degradation process and could not be used in this case
as a proper indication of maturity. The minimum wavenumber Wm~,,however, was unchanged by weathering effects and reflected the real maturation for the
Minimal Wavenumber (W rain) ~
of wet ~
~ae
4.5 ~r0iintyoe I+II III IV
+ 0 .
/#
/
iII/~ I
/?//
3.5
i 0
iI //, , I ~//I
3
J:
//'/i/
t
WET GAS Z O N E
2
o
OIL WINDOW
/11"//"//I-.. /0 l / - , / ,$..~%*11 . ~B.-/~. I/I/ /
~.~/_~ J l :
"Y
. . . . . . . .
.*++~o
t640
o $ ...-~'a i~6a0
r
, 1600 W,,a~
~
, t580
I
t56~
~em-D
Fig. 5. Correlation of vitrinite reflectance and the shift of the aromatic C~-------Cbands towards a minimum wavenumber, 14/mm.
24
H . H . GANZ and W. KALKREUTH
complete core. Thus IR kerogen analysis is also a valuable tool for the identification of weathering effects. The amount of IR determined aliphatic hydrocarbons in kerogen (A-Factor) can be used to define the hydrocarbon potentials of organic rich rocks. It could be shown that the A-Factor multiplied by the total organic carbon content x l0 correlates well with results obtained from Fischer Assay test and Rock-Eval St + $2 pyrolysis (Ganz and Kalkreuth 1987). Including all types of kerogen the samples were clearly divided according to their oil- and gas-prone characters. In Fig. 6 some of the values of the percentage of liptinite are plotted. The resulting oil/gas ratio may help to evaluate the percentage of oil- and gas-prone organic material in sediments. IR bitumen analysis
Once the bitumen is extracted it is very easy to obtain IR spectra on it. The first investigations revealed that the IR diagram might also be used for the separation of bitumen derived from oil- and gas-prone kerogens (Ganz 1986). Much more important, however, is the typing of bitumen rather than the determination of maturation. Most standard methods suffer from the dependence of facies, and reliable parameters are scarce and expensive. Besides the contents of aliphatic bands, IR spectra on bitumen yield valuable information about the aromatic composition (see Fig. 7). Using the same ratio as for kerogen aromaticity, a comparable IR bitumen aromaticity is obtained (Fig. 8). Rather surprising is the identification of facies independancy which was also observed at the kerogens, than the validity of that curve
for bitumen up to values of 4% vitrinite reflectance. Since the wet gas zone is defined to appear up to 2% only (Tissot and Welte 1984) it is believed that the bitumen obtained above that level is highly asphaltic, kerogenlike material which is left behind in the sediment after migration of the light paraffin fraction. Although the absolute amounts of bitumen in these highly mature samples are extremely low, the determination of the IR aromaticity in most cases is excellent. In principle we have the same problem as for the kerogen DOW plot (diagenesis-oil window-wet gas zone) to determine which limb of the curve is valid. But there is enough information in the IR spectrum to offer a solution. It was found that the diagenetic stage of the bitumen was indicated by relatively high amounts of long chained CH, bands at 720 cm -~ which disappear continuously with the beginning of the oil window. Thus all samples of the diagenetic stage plot in a field of relatively high contents of long chained CH-bands (see Fig. 9). The beginning of the wet gas zone of the bitumen is marked by a significant increase of a special aromatic band at 830cm -z relative to a band at 810cm -~. All samples of the wet gas zone with R~ > 1.3% are spread in the wet gas field of Fig. 10. Thus the reliable determination of the maturation of total bitumen is possible through the main stages of organic metamorphism. Lithology
Beside these techniques, infrared spectroscopy is also applied for the fast qualitative and quantitative
HYDROCARBON
- POTENTIAL
~ Li~
OIL / GAS 1.0
RATIO 0.7
0.1
15o NO g
m
lOO
m
98/26
3
0.0
4
G ~S ?IP'O~ 30 2o l0 I
0
/
100
I
I
2oo
i
I
3oo
I
400
A-FK/~rx TOC x I0 Fig. 6. Diagram illustratingoil and gas potentials from Roek-Evalvs source rock potentials obtained from IR analysis. Numbers indicate the percentage of liptinite of some source rocks and coals,
IR classification
25
m - B I T U M E N -AROMATIClTY i
|
i
i
i
i
i
i
i
i
i
DOW-PLOT : I ( 8 6 5 cm-1 / 8 6 5 cm-1 + C'C) DIAGENESIS : I ( 7 2 0 c m - 1 / 7 2 0 + 7 5 0 c m - 1 ) WET GAS ZONE : I ( 8 3 0 cm-1 / 8 3 0 + 8 1 0 cm-1)
~
AROMATC I C-H
-.
-+v i 1700
1600
1500
'// I
r
t.400
1300
mi
- I
1200
liO0
t
l
l
tO00
I 750 I
I
i
900
lO0
- ~
700
Cl -1
Fig. 7. Determination of the intensities of different aromatic bands for the calculation of IR aromaticity of bitumen.
determinations of the mineralogical composition in sediments. Previous IR methods for compositional analysis suffered from interference by OH-bond frequencies due to water. With the aid of the modified KBr pellet technique which protects the pellet from con-
tamination, quantitative analysis of kaolinite, illite, chlorite, smectite, quartz, calcite and dolomite can be carried out in less than 15 min. No sample preparation other than grinding to a grain size < 10# is needed.
IR- BITUMEN-AROMATICITY DOW -Plot
4
\\\N%+~"~"\\ \ N
3.5
,X\\
Bitumenof kerogentypeIIll+ll o* IV
.
N
~n,
.
>
WET
.
OIL WINDOW
l
0.5
....,. 0
o
......-.---"
o
......
3~
+
.... ~ ""
***~...o~..-o J:4+"o"T'o + +"
oN
\\
X
o
\
~
-
o~. °
...L.--""
o~,..,.,~'"~
~
........
0.~.0 ----" • " "
__
.
\
~
.....
_,.o-o.-"r
-.~.---'n' ' ° ~ °
DIAGENESIS I
0.00
*
.... ---" " ' ~
I
.-- "
GAS
i
0.20
i
0.40
i 486S am-D/! (865 + W ,,~, o~-])
Fig. 8. Correlation of vitrinite reflectance and IR aromaticity of bitumen (DOW-plot: indication of diagenesis, oil window and wet gas zone). SEAES 5 I - 4 ~ (
26
H.H. GANZ and W. KALKREUTH
IR - B I T U M E N - A R O M A T I C I T Y ie,Sk~km of (Mqlm~ic Mt-mea I
Bitumen of kee0gentyDe I÷II III IV
0.9
4' 0
0.8 DIAGENESIS
f
0.7
J J
0.6
O
o
O
J
J
4-
.
O
O +
0.5
+
o
191
+
o
+
~ O
0,4
O
++
+ lilo
o 4-
4"
o +
o
y
4-
oB
o
0.3
o
0 O
O
o
0.2
0.I +
0
t
0
O~
0,2
i
0,3
0.4
i (865 cm -]) / ! (865 + W mJ~ cm-U Fig. 9. indication of the diagenetic stage of maturation of bitumen by means o f aliphatic/aromatic bands at 720 and
750 c m - ~. Calibration was achieved with the help of vitrinite reflectance measurements on the associated kerogens.
IR - B I T U M E N - A R O M A T I C I T Y I m l k ~ k ~ of wet ~
0,6 Bituun of kenogentypm I+II .
+
III
o
IV
.
0,5 WET
i
GAS
0.4
ZONE
+
9
i
+
o0
0.3
o ,
\
o
.°"
O.o
+
~
@
o
o~
+
+ o ' ~ + ' ' ~ 4-
,,+ o,~
÷
°° . °
÷ +
+ 4
0.1
~
÷
+ +
+
+
o 0
: +
O
Oa
0,2 I (865 cm -1~ / 118C~ + W m b
0,3
0.4
cm--B
Fig. 10. Indication of the stage of the wet gas zone of bitumen by means of aromatic bands at 830 and 810 c m - t. Calibration was achieved with the help of vitrinite reflectance measurements on the associated kerogens.
IR classification
27 Mox',=100 O0T
IIte ~ooo 3(oo
3ooo
2(oo
? 1500
zooo
5~)0 cm -I
1000
Fig. 1 I. Diagnostic bands of several minerals and their determination with the help of the baselines.
The absorption of monochromatic infrared radiation by finely dispersed mineral particles in the KBr pellet is governed by the Lambert-Beer law: log ~ =
EcL
I0 is the intensity of incident light, I is the intensity of transmitted light, E is the extinction coefficient of the minerals, c is mineral concentration, and l is the length of the sample cell. Wavelengths selected from infrared spectra of pure minerals (Fig. 11) allow the determination of mineral concentrations with minimum interferences. Extinction coefficients for each mineral at its own diagnostic wavelengths have been determined by calibration curves using pure minerals (Ganz et al. 1987). Correction coefficients have also been calculated where interferences are present. Table 1 shows the extinction and correction coefficients of dolomite, calcite, quartz, kaolinite, chlorite, illite and smectite at their diagnostic wavelengths. The diagnostic band of pyrite at 420 cm- l is not sensitive enough to allow accurate determination of pyrite in the presence of most other minerals. If gypsum is absent, as indicated by the lack of a strong band at 670cm -~, pyrite is calculated by the sulfur content, which is determined simultaneously with TOC using a LECO C/S analyzer. Note that only 1 mg of homogenized sample is necessary and for example contents as low as 1% kaolinite can be analyzed without problems, thus also allowing the
quantitative characterization of reservoir rock lithologies. The accuracy of the method was demonstrated by analyzing synthetic mixtures and by comparison with standard techniques (Ganz et al. 1987). All determinations are quantitative, + 10 relative standard deviations, except chlorite, illite and smectite which are semiquantitative. The further developments which are now under investigation, including more varieties of clay minerals, are expected to lead to fully quantitative determination. The reliability of the new methods have been improved on a great number of samples. The results are usually shown in the IR screening log (Ganz et al. 1987, Ganz 1987a,b).
CONCLUSIONS (1) The organic materials contained in source rocks, oil shales and coals can be classified into kerogen types by using parameters derived from infrared spectroscopy. (2) The stage of thermal maturation ofkerogen can be predicted by the vitrinite reflectance equivalent grid of the IR diagram, the IR aromaticity and the shift of the aromatic C--C bands towards the minimum wavenumber, Wmin. Since the latter is not influenced by weathering effects, Wmi, offers a valuable tool for the study of such processes. (3) Oil and gas potential of source rocks, oil shales and coals can be predicted by the A-Factor derived from
Table 1. Extinction and correction coefficients of minerals at their distinctive wavelengths Extinction
Ilk
Correction coefficient
(cm-~)
Dolomite
Calcite
Quartz
Chlorite
Montmor.
Illite
Kaolinite
Dolomite Calcite Quartz Chlorite Montmor, Illite Kaolinite
728 712 798 3560 3420 3620 3700
0.330 ~ -0.012 ---.
-0.350 - 0.0--.0~ ---. .
-0.120 -0.200 1.690 ---.
-0.121 0.114 0.052 0.240 0.821 0.120 .
0.094 0.149 -0.033 0.133 0.I12 0.886
-0.061 -0.069 -0,015 0.067 0.214 0.176 0.010
-0.230 0.114 0.086 0.050 0.071 1.455 0.820
28
H . H . GANZ and W. KALKREUTH
infrared spectroscopy. Combined analysis of Rock-Eval pyrolysis and infrared spectroscopy appears to be effective to group source rocks into oil- and gas-prone, and to determine the percentage of oil- and gas-prone compounds present. (4) The stage of thermal maturation of total bitumen can be predicted by the IR aromaticity. The end of the stage of diagenesis and the beginning of the wet gas zone are marked by the appearance of distinct aromatic bands. The differentiation is indicated by the position in related diagrams. Screening gas chromatography of the Frigen-fraction of the ultrasonic-extracted bitumen provides additional valuable information. (5) Infrared spectroscopy is well suited for the rapid quantitative determination of bulk mineralogical composition of sediments. The contents of quartz, calcite, dolomite, kaolinite, chlorite, illite, smectite and pyrite and calculated in the original homogenized sample in less than 15 min. The accuracy is comparable with X-ray diffraction data. (6) All data provided by the IR analysis are presented in an infrared screening log in a similar way to RockEval data. The reliable typing of source rocks, however, is possible even though only a few samples of a core are available. Since the new IR method combines both high precision and a short time of analysis, the time savings which result from these new techniques compared to standard analytical methods sum up to about 90%. Acknowledgements--The authors thank the Organic Geochemistry Laboratory at ISPG for handling Rock-Eval pyrolysis and F. B6ttcher, M. Bussman and F. Oner (SFB 69) for analytical assistance. This study was financially supported by the German Research Foundation.
REFERENCES Christiansen, C. 1884. Untersuchungen fiber die optischen Eigenschaften von fein verteilten Kfrpern. Wiedemann Ann. Phys. 23, 298-306. Flehmig, W. and Kurze, R. 1973. Die quantitative infrarotspektroskopische Phasenanalyse von Mineralgemengen. N. ,lb. Miner. Abh. 119, 101-112.
Ganz, H. 1986. Organisch- und anorganisch-geochimische Untersuchungen an/igyptischen Schwarzschiefer/Phosphoritsequenzen-methodenentwicklung und genetisches modell. Berliner geowiss. Abh. 70, 113. Ganz, H. 1987a. Geochemical evaluation of hydrocarbon source rock characteristics and facies analysis--methods and application. Berliner geowiss. Abh. 75, 669~90. Ganz, H. 1987b. Petroleum geochemistry of oils and source rocks from the Ras Gharib and Abu Rudeis area, Gulf of Suez. 13th Int. Mtg Org. Geochem., 21-25 September, Venezia, p. 251 (abstract). Ganz, H. and Kalkreuth, W. 1987. Application of infrared spectroscopy to the classification of kerogentypes and the evaluation of source rock and oil shale potentials. Fuel 66, 708-711. Ganz, H., Kalkreuth, W., Oner, F., Pearson, M. J. and Small, J. S. 1987. Enhanced geochemical screening analysis--accurate determination of petroleum source rock characteristics and comparison with standard techniques. In: Petroleum Geology of North West Europe (Edited by Brooks, J. and Glennie, K.), pp. 847-851. Graham and Trotman Ltd, London. Ganz, H. and Robison, V. D. 1985. Newly developed infrared method for characterizing kerogen type and thermal maturation. 13th Int. Mtg Org. Geochem. 16-20 September 1985, Jiilich, p. 96. (abstract). Hutton, A. C., Cook, A. C., Kanstler, A. J. and Mckirdy, D. M. 1980. Organic matter of oil shales. APEA J. 20, 44457. Kalkreuth, W. and Macautey, G. 1984. Organic petrology and geochemical (Rock-Eval) studies on oil shales and coals from Pictou and Antigonish Areas, Nova Scotia, Canada. CSPG Bull. 77, 895-908. Katz, B. J. 1981. The limitations of Rock-Eval pyrolysis for typing organic matter. AAPG Bull. 65, 944-955. Ottenjahn, K. 1981/82. Verbesserungen bei der mikroskopphotometrischen Fluoreszenzmessung an Kohlemaceralen. Zeiss Informat. 26, 40-46. Peters, K. 1986. Guidelines for evaluating petroleum source rock using programmed pyrolysis. AAPG Bull. 70, 318-329. Piittmann, W., Steffens, K. and Kalkreuth, W. 1987. Chemical characteristics of natural weathering in coal. In: 19871nternational Conference on Coal Science (Edited by Moulijn, J. A. et al.), pp. 411-414. Elsevier, Amsterdam. Radke, M., Welte, D. H. and Willsch, H. 1982. Geochemical study on a well in the Western Canada Basin: relation of the aromatic distribution pattern to maturity of organic matter. Geochim. Cosmochim. Acta 46, 1-10. Tissot, B. P. and Deroo, G. 1978. Geochemical study of the Uinta Basin: formation of petroleum from the Green River Formation. Geochim. Cosmochim. Acta 42, 1469-1485. Tissot, B. P., Pelet, R. and Ungerer, Ph. 1987. Thermal history of sedimentary basins, maturation indices, and kinetics of oil and gas generation. AAPG Bull, 71, 1445-1466. Tissot, B. P. and Welte, D. H. 1984. Petroleum Formation and Occurrence, 2nd Edn. Springer, Berlin. Valasek, J. 1960. Theoretical and Experimental Optics. Wiley, London.
0743-9547/91$3.00+ 0.00 PergamonPresspie
Printedin Great Britain
IR classification of kerogen type, thermal maturation, hydrocarbon potential and lithological characteristics H . H . GANZ* and W. KALKREUTHt *Technische Universit/it Berlin, SFB 69, Ackerstr. 71-76, D-1000 Berlin 65, F.R.G. and ~'Institute of Sedimentary and Petroleum Geology, 3303 33rd Street NW, Calgary, Alberta T2L 2A7, Canada Abstract--In the present study, maturation levelsand kerogen types were definedby infrared spectroscopy.The spectra display distinct peaks for aliphatic, carboxyl/carbonyland aromatic compounds which can be used to classify kerogen types similar to the traditional H/C~)/C van Krevelendiagram and to estimate the maturation levelsby means of a vitrinitereflectanceequivalentgrid. Additionalinformationabout the maturation is provided by the shift of the aromatic C-------Cbands of kerogens towards a minimum wavenumber W~, and the IR aromaticity of kerogens and bitumen. Parameters obtained from infrared spectroscopyalso allow the prediction of oil and gas potentials. A combination of Rock-Eval pyrolysis and infrared spectroscopyappears to be an effectivetool to group sourcerocks into oil- and/or gas-pronecategories.Also mineralogicaland sedimentological characteristics are determined by a new infrared method. Quantitative contents of quartz, dolomite, calcite, kaolinite, chlorite, illite, smectite and pyrite are calculated in the original homogenizedsample in less than 15 min.
INTRODUCTION
ANALYTICAL M E T H O D S
TODAYthere is no doubt that natural oil and gas deposits derive from the organic material existing in sediments; the kerogen. Varying amounts of oil, gas, or both, may be generated during burial of sediments according to the type of kerogen and its maturity. Through chemical analysis it is possible to distinguish four types of kerogen by plotting atomic H/C vs O/C on a van Krevelen diagram. For petroleum exploration it is vital to determine not only the kerogen types of oil and gas source rocks but also their maturities--both are important parameters in explorers' basinal models. There have been many attempts to develop techniques to replace elemental analysis which is both time-consuming and very sensitive to kerogen admixtures. Pyrolysis techniques, e.g. Rock-Eval, are routine today for the screening of drill core and cuttings, because this automated procedure provides a great sample handling capacity. It has been discovered, however, that the products of pyrolysis depend substantially upon the mineral composition of the sediment as well as upon the organic carbon content. In practice, although pyrolysis is an inexpensive and useful screening technique, kerogen classification is sometimes unreliable, and the results must be checked with elemental analysis and optical measurements. The latter require considerable palynological training and experience. Recently a geochemical routine for the determination of petroleum source rock characteristics based on infrared spectroscopy and utilizing a rapid bitumen separation method for gas chromatography has been developed. Several geochemically significant parameters were determined with high precision in a short time for preparation and analysis.
Sample preparation
IR spectroscopy is very sensitive to grain size effects due to Rayleigh's law of scattering (see Valasek 1960) and the so-called Christiansen effect (Christiansen 1884). Scattering and the Christiansen effect can be eliminated when the particle size decreases to a very small diameter compared with the wavelength of the light. Thus, prior to the several analytical steps, proper grinding of the samples is necessary. About 9 g of a sample (drill core or cutting, handpicked and washed with distilled water) are treated for 10 s in a tempered chrome steel mortar of a vibratory cup mill (Fa. Siebtechnik; other mills, e.g. the "Fritsch pulverisette 9", are very similar and may also be used). After 10 s the mortar is opened and the sample powder is brushed carefully from the side wall cover and the grinding set of the mortar. Then the grinding is repeated again for 10 s. Analyses of the grain size distribution ("Zilas" laser granulometer) reveal that the sample is now equivalent to 250 mesh ( < 6 3 #). After brushing and homogenizing the sample is manually spliced and 7 g are taken away for the following bitumen extraction and kerogen isolation. The remaining 2 g of sample are ground again 2 x 10 s in the same manner described above. After the final (4th) treatment the sample size is proven to be at least smaller than 10 p. Detailed investigations revealed that further grinding destroyed the internal structure of minerals. This was indicated by an increasing incorporation of OH-bonds and subsequent decrease of the extinction coefficients. After drying at 125°C for 3h, 0.5-0.7mg of this 19
20
H . H . GANZ and W. KALKREUTH
pre-ground and homogenized sample was carefully distributed in 200mg KBr (Merck No. 4907) using a microspoon. For the proper weighing a five decimal high precision balance is essential (Sartorius research). The homogenization was completed by shaking the mixture in the same carrier bag of alumina foil which had already been used for the weighing and distribution process. The mixture is now evacuated and pressed at 10 t c m -2. This rapid homogenization nearly prevents the sample disc from water contamination and the very small amounts of water which might be present are completely removed after a final heating of the pellet at 125°C. The pellet is now scanned using a Perkin Elmer IR 598. After IR analysis the pellet is stored in the alumina foil for later purposes. Previous IR methods for compositional analysis suffered from the interference by OH-bond frequencies due to water (e.g. Flehmig and Kurze, 1973). Their pellets became cloudy during analysis. With the aid of the modified KBr pellet technique which protects the pellet from water contamination the pellet stays clear and translucent all the time. Since the IR spectra of smectite and illite differ only in respect to the content of water which is incorporated in the crystal lattice of smectite, not only the contents of quartz, calcite, dolomite, kaolinite and chlorite are determined quantitatively but also the yields of smectite and illite. Using the modified KBr pellet technique described above, with care in preparation, spectra can be repeated with < 5% variation. Bitumen extraction Soxhlet extraction with CH2C12 is time-consuming (24-48 h) and large amounts of organic solvent have to be evaporated. This often causes loss of the light nalkanes. Also the following asphaltene/non-asphaltene fractionation and column chromatography are very time-consuming methods. These processes are replaced by an extraction with CH2CI: by ultrasonic agitation and centrifuging. Three to seven g of the sample (depending on the TOC content), which was already treated 2 x 10 s with the vibratory cup mill, are filled in a 100 ml glass tube of a centrifuge (Heraeus Christ, Labofuge) and ultrasonically treated with 30 ml of CH2C12 for 5 min. After centrifuging (4 min at 4000 min -1) and decanting of the solvent, the treatment is repeated at least two times (reservoir rocks with high amounts of migrated bitumen more than five times) to remove all soluble bitumen until the liquid remains clear. The bitumen is usually collected in ordinary laboratory beakers, slightly covered with alumina foil, and placed under the hood in order to allow slow evaporation of the solvent. The extracted sediment is transferred into 100 ml centrifuge plastic tubes, and ready for the HC1/HF treatment of the kerogen isolation procedure. After 36 h the weight of the dried bitumen is calculated and the extract is transferred to small 5 ml vessels. After the final evaporation of the solvent the bitumen is ready for further analyses. For IR analysis the bitumen is dissolved in a few drops of CH2C12, taken up by a pipette and concentrate in the
middle of a sodium chloride crystal. A few seconds later the solvent is evaporated and the crystal is mounted in a holder and placed in the beam of the IR spectroscope. Usually for bitumen extraction 24-48 samples are treated in succession. Thus a total working time of less than 15 min/sample is necessary. For GC analysis leaching of the de-volatilized extract with Frigen (CCI3F) leads to the separation of the saturate fraction within a few seconds (Ganz et al. 1987). Kerogen isolation After bitumen extraction and transfer of samples into plastic tubes some drops of methanol are added in order to enhance their wettability. Careful treatment with HCI (10%) is used for the destruction of carbonates. The solution is decanted after centrifuging and the sample is treated with conc. HF (about 15 ml). In order to obtain a good suspension the mixture is stirred carefully. Up to 24 beakers are placed in a water bath at 70°C for about 48 h (Dinkelberg Laktotherm waterbath). Each day the beakers are filled up with additional HF and distilled water. The solution is decanted after centrifuging and treated with hot HC1 (20%) to remove fluorides which might have been formed during the dissolution of the minerals. The amount of newly formed fluorides depends mainly on the content of clay minerals and might be totally prevented if centrifuging and washing of the samples is practiced every 12 h. After the HC1 treatment the samples are finally washed twice with hot distilled water and dried after decantation at 60°C in an ordinary laboratory oven. Since up to 24 samples (or 48 samples if using 2 waterbaths) are isolated at one time, the total working time for the kerogen isolation procedure is less than 15 min. Comparison with cold acid techniques used in an early stage of the studies (Ganz 1986) indicates that the hot acid treatment and the drying at elevated temperatures for a limited time does not influence the composition of the kerogen. After drying of the kerogen it is ground by hand in an achate mortar for 2 min. Measurements of the grain size distribution revealed that the diameter of the particles is at least smaller than 10/~. The kerogen is now ready for IR analysis. The pellet preparation is essentially that described for the mineralogical determinations except for the heating of the pellet at 125°C. Once the kerogen KBr pellet is pressed, no further heating is necessary.
INFRARED SPECTROSCOPY
IR kerogen analysis During kerogen isolation all minerals but pyrite and heavy minerals are dissolved. Often heavy liquids are used for their separation, but these techniques also cause density fractionation of the kerogen. Moreover, the removal is never complete, because pyrite is coated by kerogen. Elemental analyses for kerogen classification suffer from such kerogen impurities.
IR classification Additional time-consuming determination of iron content is often necessary for the evaluation of the amount of pyrite. Similar problems are caused by ralstonitelike fluorides (Na, Mg, A1, (F, OH), H20) which are sometimes formed during kerogen isolation. The differentiation of the several types of kerogen and a certain evaluation of the stage of thermal maturation finally succeeds in the van Krevelen atomic H/C and O/C diagram, if the kerogen was sufficiently pure enough. Besides the conventional elemental analysis, Rock-Eval pyrolysis was established over the past few years. The main advantage of this method is that no kerogen isolation is necessary prior to analysis. Kerogen type, maturity and hydrocarbon generating potential are determined very quickly. Reliability of the data however, is sometimes not very high. Especially, the determination of maturation depends highly upon kerogen type and TOC content, as well as the rates of associated clay minerals (Katz 1981, Peters 1986, Tissot et al. 1987). Even the typing of the kerogen is sometimes not dependable. Thus optical studies, vitrinite reflectance for the determination of maturation, and maceral analysis for kerogen classification, are always achieved for control. In typical oil-prone lithologies additional spectral fluorescence measurements have to be performed (Ottenjahn 1981/82) since vitrinite reflectance values are generally lowered in alginite-rich source rocks (Hutton et al. 1980, Kalkreuth and Macauley 1984). It is well known that the evolution of kerogen structure during burial is reflected by the infrared spectrum (Tissot and Deroo 1978). The spectra (see Fig. 1) typically display distinctive peaks at 2860 and 2930 cm(CH2 and CH 3 aliphatic groups), at 1710 cm- l (carboxyl and carbonyl groups) and at 1630 cm- ~(aromatic C-------C bands). With increasing maturation the aliphatic peaks initially increase while the carboxyl/carbonyl peak decreases. As maturation continues to increase, the aliphatic peaks decrease, while there is no apparent change in the peaks representing the aromatic C-----C bands. Plotting the relative ratios of the intensities of aliphatic/
21
aromatic bands (A-Factor) against the ratios of carboxyl and carbonyl/aromatic bands (C-Factor) in a diagram results in an excellent differentiation of the organic matter. A-Factor =
C-Factor =
(2860 + 2930 cm -~) (2860 + 2930 + 1630 cm -l) (1705 cm- ~) (1705 + 1630 cm-I)"
The resulting plot (Fig. 2) is comparable to the van Krevelen diagram (Ganz and Robison 1985, Ganz 1986). Infrared analysis of a large number of kerogens of different types and thermal maturities resulted in the development of a vitrinite reflectance equivalent grid which allows the precise determination of R0 % (Ganz et al. 1987). Comparison with standard techniques indicate that in typical oil-prone source rock lithologies, infrared spectroscopy is a more reliable maturity indicator than the measured optical reflectance values. The validity of these determinations was confirmed by a great number of samples, for example oil shales from Canada which are directly overlain by a coal seam. Microscopically determined vitrinite reflectance showed values of 0.4% whilst the coal itself had 0.95% reflectance. Infrared spectroscopy on these samples, however, determined more than 0.9% for both oil shales and coal. More detailed recent investigations revealed that the IR spectra yield still more information. Looking at IR spectra of kerogens of different maturations, it is obvious that with increasing maturation there is an increase in the intensities of the aromatic C-H bands in the range between 700 and 900 cm -1 (see Fig. 3). This is reflected by the ratio of the intensity at 865 cm- i against the more or less stable intensity of the aromatic C------Cbands. The resulting curve (Fig. 4) is very similar to the well known methyl-phenanthrene-index (MPI) of Radke et al. (1982), but with a much wider range of application, of up to 4% vitrinite reflectance and no separation of the different kerogen types. That means IR kerogen Ivlaxi=100 O0T
I
f
i
~ /OnD35'00 30{)0 2BIo0 2000
I1 BIO
Min =O00T
1000
5~(]Cm -¶
Fig. I. Determination of the intensities of aliphatic, carboxyl/carbonyl and aromatic bands with the help of the baselines.
22
H . H . GANZ and W. KALKREUTH
113
rinite - -/ - vit . •reftectonce equivalent grid
O~ tI
II
09
/
I
ii i
I
/
! /
evolution path ,of type 1
I
/
-- 0.8 T E t~ ÷
/ /
07 •
i
/
t' /
o.6
,,:'
";E 0.5
x x \ \~ *
/
!
I
o
+
/
I I I
o
i /t
i
I
I
evolution pQth of type II
/
I /
I iI I
cold seam ,
/
1 I
/ I I I
O.t
evolution path of type Ill 0.2 i
0.2
type IV 0.1
0
0.1
~ 0.2 03 0.4 05 05 0.7 C- factor (1710cm't)/(1710*1630cm")
0B
0.9
Fig. 2. Classification of kerogen types and maturation according to Aand C-Factors and the vitrinite reflectance equivalent grid obtained from the IR diagram.
aromaticity seems to be facies independent. The entitled question is how to distinguish between the upper and lower part of the curve in the case of single samples. The interpretation of more than 100 IR spectra of different
kerogens showed that there is another very characteristic feature: with increasing maturation the aromatic C------C bands are shifted towards a minimum wavenumber. In Fig. 5 the minimum wavenumber, Wmin, is plotted against the vitrinite reflectance values of the standard samples. Note that all samples, including those which are affected by weathering, correlate along this curve. Since the standard deviation of that curve is rather high (about 0.4% R0), Wminis not used for the proper determination of Ro % but for the decision as to which part of the IR aromaticity diagram should be used. For example, Wmi, of 1600 indicates that we are still on the lower limb of the IR aromaticity diagram within the oil window. A Wmi. of 1585, however, indicates that we are already in the gas zone and the upper limb of the IR aromaticity diagram should be used for the proper Ro calculation. The availability of three IR indicators for the determination of maturity is a great advantage of the IR technique. In practice, the vitrinite reflectance equivalent grid of the IR diagram is the most reliable indicator of maturation. But the effects of weathering have great impact on the A- and C-Factors. To study this influence a core was drilled horizontally into a coal seam up to about 40 m below the surface. Because all samples were considered to be of identical primary composition, any difference could be related to weathering effects. Piittmann et al. (1987) described the contents of GC-MS-determined polar and aromatic compounds in relation to the distance from the surface. Nearly constant values were obtained at a depth of about 30 m. The results of the IR studies on the same samples showed that the coals at the surface are clearly oxidized and consist of transitional type IV kerogen. The vitrinite reflectance equivalent grid indicated a maturation of 0.4% R0. The real values of 1.2% were nearly obtained
IR-KEROGEN-AROMATICITY IT
Intens£y (C-H"/C-HIC=C)
'
I
I
I
I
2600
1500
1400
1300
.
I t200 ©S - 1
.
.
L.., t tO0
.
.
/
I
i
I
I
lO00
900
!100
7O0
Fig. 3. Determination of the intensity of the aromatic band at 865 cm- ~.
IR classification
23
I R - KEROGEN-AROMATICITY DOW - Plot
4.5
~
~ \
." \
.
3
Kel'ogentyl)e I+Ii zzz zv
"\
\ \~+
4
o ¢
\\ \
"\\
\
.
"
I
~
/
i li f
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\-
+ \ o
1 I ~o' ~O5.-i-. _~ °o°o7 o~ ~
oil
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~e
"
WINDOW. i *"
0\
\
-WET GAS
.
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.
\ ~o ~" o8~'\\
.
II
÷ o
ill + .
i~I
~+ -+- ..I
~"
"~
+~;°+"b~+-
. u V " " / " ~/- - ~ ~ 0 .0." ~. -. . ~- :~'-" 0,5 . ~ +0~o o+I..---,.,~,~,''~ e ..... --." """ DIAGENESIS
.......-'"
0
0
O,2 I~
O,4 + W rain era-l)
m-ll / I l~
Fig. 4. Correlation of vitrinite reflectance and IR aromaticity of kerogens (DOW-plot: indication of diagenesis, oil window and wet gas zone).
at about 30m. As the A/C factor of the weathered samples, the IR aromaticity is highly influenced by the degradation process and could not be used in this case
as a proper indication of maturity. The minimum wavenumber Wm~,,however, was unchanged by weathering effects and reflected the real maturation for the
Minimal Wavenumber (W rain) ~
of wet ~
~ae
4.5 ~r0iintyoe I+II III IV
+ 0 .
/#
/
iII/~ I
/?//
3.5
i 0
iI //, , I ~//I
3
J:
//'/i/
t
WET GAS Z O N E
2
o
OIL WINDOW
/11"//"//I-.. /0 l / - , / ,$..~%*11 . ~B.-/~. I/I/ /
~.~/_~ J l :
"Y
. . . . . . . .
.*++~o
t640
o $ ...-~'a i~6a0
r
, 1600 W,,a~
~
, t580
I
t56~
~em-D
Fig. 5. Correlation of vitrinite reflectance and the shift of the aromatic C~-------Cbands towards a minimum wavenumber, 14/mm.
24
H . H . GANZ and W. KALKREUTH
complete core. Thus IR kerogen analysis is also a valuable tool for the identification of weathering effects. The amount of IR determined aliphatic hydrocarbons in kerogen (A-Factor) can be used to define the hydrocarbon potentials of organic rich rocks. It could be shown that the A-Factor multiplied by the total organic carbon content x l0 correlates well with results obtained from Fischer Assay test and Rock-Eval St + $2 pyrolysis (Ganz and Kalkreuth 1987). Including all types of kerogen the samples were clearly divided according to their oil- and gas-prone characters. In Fig. 6 some of the values of the percentage of liptinite are plotted. The resulting oil/gas ratio may help to evaluate the percentage of oil- and gas-prone organic material in sediments. IR bitumen analysis
Once the bitumen is extracted it is very easy to obtain IR spectra on it. The first investigations revealed that the IR diagram might also be used for the separation of bitumen derived from oil- and gas-prone kerogens (Ganz 1986). Much more important, however, is the typing of bitumen rather than the determination of maturation. Most standard methods suffer from the dependence of facies, and reliable parameters are scarce and expensive. Besides the contents of aliphatic bands, IR spectra on bitumen yield valuable information about the aromatic composition (see Fig. 7). Using the same ratio as for kerogen aromaticity, a comparable IR bitumen aromaticity is obtained (Fig. 8). Rather surprising is the identification of facies independancy which was also observed at the kerogens, than the validity of that curve
for bitumen up to values of 4% vitrinite reflectance. Since the wet gas zone is defined to appear up to 2% only (Tissot and Welte 1984) it is believed that the bitumen obtained above that level is highly asphaltic, kerogenlike material which is left behind in the sediment after migration of the light paraffin fraction. Although the absolute amounts of bitumen in these highly mature samples are extremely low, the determination of the IR aromaticity in most cases is excellent. In principle we have the same problem as for the kerogen DOW plot (diagenesis-oil window-wet gas zone) to determine which limb of the curve is valid. But there is enough information in the IR spectrum to offer a solution. It was found that the diagenetic stage of the bitumen was indicated by relatively high amounts of long chained CH, bands at 720 cm -~ which disappear continuously with the beginning of the oil window. Thus all samples of the diagenetic stage plot in a field of relatively high contents of long chained CH-bands (see Fig. 9). The beginning of the wet gas zone of the bitumen is marked by a significant increase of a special aromatic band at 830cm -z relative to a band at 810cm -~. All samples of the wet gas zone with R~ > 1.3% are spread in the wet gas field of Fig. 10. Thus the reliable determination of the maturation of total bitumen is possible through the main stages of organic metamorphism. Lithology
Beside these techniques, infrared spectroscopy is also applied for the fast qualitative and quantitative
HYDROCARBON
- POTENTIAL
~ Li~
OIL / GAS 1.0
RATIO 0.7
0.1
15o NO g
m
lOO
m
98/26
3
0.0
4
G ~S ?IP'O~ 30 2o l0 I
0
/
100
I
I
2oo
i
I
3oo
I
400
A-FK/~rx TOC x I0 Fig. 6. Diagram illustratingoil and gas potentials from Roek-Evalvs source rock potentials obtained from IR analysis. Numbers indicate the percentage of liptinite of some source rocks and coals,
IR classification
25
m - B I T U M E N -AROMATIClTY i
|
i
i
i
i
i
i
i
i
i
DOW-PLOT : I ( 8 6 5 cm-1 / 8 6 5 cm-1 + C'C) DIAGENESIS : I ( 7 2 0 c m - 1 / 7 2 0 + 7 5 0 c m - 1 ) WET GAS ZONE : I ( 8 3 0 cm-1 / 8 3 0 + 8 1 0 cm-1)
~
AROMATC I C-H
-.
-+v i 1700
1600
1500
'// I
r
t.400
1300
mi
- I
1200
liO0
t
l
l
tO00
I 750 I
I
i
900
lO0
- ~
700
Cl -1
Fig. 7. Determination of the intensities of different aromatic bands for the calculation of IR aromaticity of bitumen.
determinations of the mineralogical composition in sediments. Previous IR methods for compositional analysis suffered from interference by OH-bond frequencies due to water. With the aid of the modified KBr pellet technique which protects the pellet from con-
tamination, quantitative analysis of kaolinite, illite, chlorite, smectite, quartz, calcite and dolomite can be carried out in less than 15 min. No sample preparation other than grinding to a grain size < 10# is needed.
IR- BITUMEN-AROMATICITY DOW -Plot
4
\\\N%+~"~"\\ \ N
3.5
,X\\
Bitumenof kerogentypeIIll+ll o* IV
.
N
~n,
.
>
WET
.
OIL WINDOW
l
0.5
....,. 0
o
......-.---"
o
......
3~
+
.... ~ ""
***~...o~..-o J:4+"o"T'o + +"
oN
\\
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o
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~
-
o~. °
...L.--""
o~,..,.,~'"~
~
........
0.~.0 ----" • " "
__
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\
~
.....
_,.o-o.-"r
-.~.---'n' ' ° ~ °
DIAGENESIS I
0.00
*
.... ---" " ' ~
I
.-- "
GAS
i
0.20
i
0.40
i 486S am-D/! (865 + W ,,~, o~-])
Fig. 8. Correlation of vitrinite reflectance and IR aromaticity of bitumen (DOW-plot: indication of diagenesis, oil window and wet gas zone). SEAES 5 I - 4 ~ (
26
H.H. GANZ and W. KALKREUTH
IR - B I T U M E N - A R O M A T I C I T Y ie,Sk~km of (Mqlm~ic Mt-mea I
Bitumen of kee0gentyDe I÷II III IV
0.9
4' 0
0.8 DIAGENESIS
f
0.7
J J
0.6
O
o
O
J
J
4-
.
O
O +
0.5
+
o
191
+
o
+
~ O
0,4
O
++
+ lilo
o 4-
4"
o +
o
y
4-
oB
o
0.3
o
0 O
O
o
0.2
0.I +
0
t
0
O~
0,2
i
0,3
0.4
i (865 cm -]) / ! (865 + W mJ~ cm-U Fig. 9. indication of the diagenetic stage of maturation of bitumen by means o f aliphatic/aromatic bands at 720 and
750 c m - ~. Calibration was achieved with the help of vitrinite reflectance measurements on the associated kerogens.
IR - B I T U M E N - A R O M A T I C I T Y I m l k ~ k ~ of wet ~
0,6 Bituun of kenogentypm I+II .
+
III
o
IV
.
0,5 WET
i
GAS
0.4
ZONE
+
9
i
+
o0
0.3
o ,
\
o
.°"
O.o
+
~
@
o
o~
+
+ o ' ~ + ' ' ~ 4-
,,+ o,~
÷
°° . °
÷ +
+ 4
0.1
~
÷
+ +
+
+
o 0
: +
O
Oa
0,2 I (865 cm -1~ / 118C~ + W m b
0,3
0.4
cm--B
Fig. 10. Indication of the stage of the wet gas zone of bitumen by means of aromatic bands at 830 and 810 c m - t. Calibration was achieved with the help of vitrinite reflectance measurements on the associated kerogens.
IR classification
27 Mox',=100 O0T
IIte ~ooo 3(oo
3ooo
2(oo
? 1500
zooo
5~)0 cm -I
1000
Fig. 1 I. Diagnostic bands of several minerals and their determination with the help of the baselines.
The absorption of monochromatic infrared radiation by finely dispersed mineral particles in the KBr pellet is governed by the Lambert-Beer law: log ~ =
EcL
I0 is the intensity of incident light, I is the intensity of transmitted light, E is the extinction coefficient of the minerals, c is mineral concentration, and l is the length of the sample cell. Wavelengths selected from infrared spectra of pure minerals (Fig. 11) allow the determination of mineral concentrations with minimum interferences. Extinction coefficients for each mineral at its own diagnostic wavelengths have been determined by calibration curves using pure minerals (Ganz et al. 1987). Correction coefficients have also been calculated where interferences are present. Table 1 shows the extinction and correction coefficients of dolomite, calcite, quartz, kaolinite, chlorite, illite and smectite at their diagnostic wavelengths. The diagnostic band of pyrite at 420 cm- l is not sensitive enough to allow accurate determination of pyrite in the presence of most other minerals. If gypsum is absent, as indicated by the lack of a strong band at 670cm -~, pyrite is calculated by the sulfur content, which is determined simultaneously with TOC using a LECO C/S analyzer. Note that only 1 mg of homogenized sample is necessary and for example contents as low as 1% kaolinite can be analyzed without problems, thus also allowing the
quantitative characterization of reservoir rock lithologies. The accuracy of the method was demonstrated by analyzing synthetic mixtures and by comparison with standard techniques (Ganz et al. 1987). All determinations are quantitative, + 10 relative standard deviations, except chlorite, illite and smectite which are semiquantitative. The further developments which are now under investigation, including more varieties of clay minerals, are expected to lead to fully quantitative determination. The reliability of the new methods have been improved on a great number of samples. The results are usually shown in the IR screening log (Ganz et al. 1987, Ganz 1987a,b).
CONCLUSIONS (1) The organic materials contained in source rocks, oil shales and coals can be classified into kerogen types by using parameters derived from infrared spectroscopy. (2) The stage of thermal maturation ofkerogen can be predicted by the vitrinite reflectance equivalent grid of the IR diagram, the IR aromaticity and the shift of the aromatic C--C bands towards the minimum wavenumber, Wmin. Since the latter is not influenced by weathering effects, Wmi, offers a valuable tool for the study of such processes. (3) Oil and gas potential of source rocks, oil shales and coals can be predicted by the A-Factor derived from
Table 1. Extinction and correction coefficients of minerals at their distinctive wavelengths Extinction
Ilk
Correction coefficient
(cm-~)
Dolomite
Calcite
Quartz
Chlorite
Montmor.
Illite
Kaolinite
Dolomite Calcite Quartz Chlorite Montmor, Illite Kaolinite
728 712 798 3560 3420 3620 3700
0.330 ~ -0.012 ---.
-0.350 - 0.0--.0~ ---. .
-0.120 -0.200 1.690 ---.
-0.121 0.114 0.052 0.240 0.821 0.120 .
0.094 0.149 -0.033 0.133 0.I12 0.886
-0.061 -0.069 -0,015 0.067 0.214 0.176 0.010
-0.230 0.114 0.086 0.050 0.071 1.455 0.820
28
H . H . GANZ and W. KALKREUTH
infrared spectroscopy. Combined analysis of Rock-Eval pyrolysis and infrared spectroscopy appears to be effective to group source rocks into oil- and gas-prone, and to determine the percentage of oil- and gas-prone compounds present. (4) The stage of thermal maturation of total bitumen can be predicted by the IR aromaticity. The end of the stage of diagenesis and the beginning of the wet gas zone are marked by the appearance of distinct aromatic bands. The differentiation is indicated by the position in related diagrams. Screening gas chromatography of the Frigen-fraction of the ultrasonic-extracted bitumen provides additional valuable information. (5) Infrared spectroscopy is well suited for the rapid quantitative determination of bulk mineralogical composition of sediments. The contents of quartz, calcite, dolomite, kaolinite, chlorite, illite, smectite and pyrite and calculated in the original homogenized sample in less than 15 min. The accuracy is comparable with X-ray diffraction data. (6) All data provided by the IR analysis are presented in an infrared screening log in a similar way to RockEval data. The reliable typing of source rocks, however, is possible even though only a few samples of a core are available. Since the new IR method combines both high precision and a short time of analysis, the time savings which result from these new techniques compared to standard analytical methods sum up to about 90%. Acknowledgements--The authors thank the Organic Geochemistry Laboratory at ISPG for handling Rock-Eval pyrolysis and F. B6ttcher, M. Bussman and F. Oner (SFB 69) for analytical assistance. This study was financially supported by the German Research Foundation.
REFERENCES Christiansen, C. 1884. Untersuchungen fiber die optischen Eigenschaften von fein verteilten Kfrpern. Wiedemann Ann. Phys. 23, 298-306. Flehmig, W. and Kurze, R. 1973. Die quantitative infrarotspektroskopische Phasenanalyse von Mineralgemengen. N. ,lb. Miner. Abh. 119, 101-112.
Ganz, H. 1986. Organisch- und anorganisch-geochimische Untersuchungen an/igyptischen Schwarzschiefer/Phosphoritsequenzen-methodenentwicklung und genetisches modell. Berliner geowiss. Abh. 70, 113. Ganz, H. 1987a. Geochemical evaluation of hydrocarbon source rock characteristics and facies analysis--methods and application. Berliner geowiss. Abh. 75, 669~90. Ganz, H. 1987b. Petroleum geochemistry of oils and source rocks from the Ras Gharib and Abu Rudeis area, Gulf of Suez. 13th Int. Mtg Org. Geochem., 21-25 September, Venezia, p. 251 (abstract). Ganz, H. and Kalkreuth, W. 1987. Application of infrared spectroscopy to the classification of kerogentypes and the evaluation of source rock and oil shale potentials. Fuel 66, 708-711. Ganz, H., Kalkreuth, W., Oner, F., Pearson, M. J. and Small, J. S. 1987. Enhanced geochemical screening analysis--accurate determination of petroleum source rock characteristics and comparison with standard techniques. In: Petroleum Geology of North West Europe (Edited by Brooks, J. and Glennie, K.), pp. 847-851. Graham and Trotman Ltd, London. Ganz, H. and Robison, V. D. 1985. Newly developed infrared method for characterizing kerogen type and thermal maturation. 13th Int. Mtg Org. Geochem. 16-20 September 1985, Jiilich, p. 96. (abstract). Hutton, A. C., Cook, A. C., Kanstler, A. J. and Mckirdy, D. M. 1980. Organic matter of oil shales. APEA J. 20, 44457. Kalkreuth, W. and Macautey, G. 1984. Organic petrology and geochemical (Rock-Eval) studies on oil shales and coals from Pictou and Antigonish Areas, Nova Scotia, Canada. CSPG Bull. 77, 895-908. Katz, B. J. 1981. The limitations of Rock-Eval pyrolysis for typing organic matter. AAPG Bull. 65, 944-955. Ottenjahn, K. 1981/82. Verbesserungen bei der mikroskopphotometrischen Fluoreszenzmessung an Kohlemaceralen. Zeiss Informat. 26, 40-46. Peters, K. 1986. Guidelines for evaluating petroleum source rock using programmed pyrolysis. AAPG Bull. 70, 318-329. Piittmann, W., Steffens, K. and Kalkreuth, W. 1987. Chemical characteristics of natural weathering in coal. In: 19871nternational Conference on Coal Science (Edited by Moulijn, J. A. et al.), pp. 411-414. Elsevier, Amsterdam. Radke, M., Welte, D. H. and Willsch, H. 1982. Geochemical study on a well in the Western Canada Basin: relation of the aromatic distribution pattern to maturity of organic matter. Geochim. Cosmochim. Acta 46, 1-10. Tissot, B. P. and Deroo, G. 1978. Geochemical study of the Uinta Basin: formation of petroleum from the Green River Formation. Geochim. Cosmochim. Acta 42, 1469-1485. Tissot, B. P., Pelet, R. and Ungerer, Ph. 1987. Thermal history of sedimentary basins, maturation indices, and kinetics of oil and gas generation. AAPG Bull, 71, 1445-1466. Tissot, B. P. and Welte, D. H. 1984. Petroleum Formation and Occurrence, 2nd Edn. Springer, Berlin. Valasek, J. 1960. Theoretical and Experimental Optics. Wiley, London.