- Email: [email protected]
Characteristics of amorphous kerogens fractionated from terrigenous sedimentary rocks
IKll6-7077/X4/$1
C;,wc t,,,ni< ii iv ~‘r,rmoch,m~~~o .I<,u Vol. 48. pp. 243-249 :Cl Pergdmtm Press Ud 19X4. Pruned in U.S.A.
00 + 00
Characteristics of amorphous kerogens fractionated from terrigenous sedimentary rocks NORIYUKI
SUZUKI
Department of Geology. Faculty of Science, Shimane University. Nishikawazu, Matsue. Shimane. 690, Japan (Received March 11, 1983; accepted in revised_li,rm November 2. 1983) Abstract-A preliminary attempt to fractionate amorphous kerogens from terrigenous bulk kerogen by a benzene-water two phase partition method under acidic condition was made. Microscopic observation revealed that amorphous kerogens and structured kerogens were fractionated effectively by this method. Characteristics of the amorphous and structured kerogens fractionated by this method were examined by some chemical analyses and compared with those of the bulk kerogen and humic acid isolated from the same rock sample (Haizume Formation. Pleistocene, Japan). The elemental and infrared (IR) analyses showed that the amorphous kerogen fraction had the highest atomic H/C ratio and the lowest atomic N/C ratio and was the richest in aliphatic structures and carbonyl and carboxyl functional groups. Quantities of fatty acids from the saponification products of each geopolymer were in agreement with the results of elemental and IR analyses. Distribution of the fatty acids was suggestive that more animal lipids participate in the formation of amorphous kerogens because of the abundance of relatively lower molecular weight fatty acids (such as Cj6 and Cl8 acids) in saponification products of amorphous kerogens. On the other hand, although the amorphous kerogen fraction tends to be rich in aliphatic structures compared with bulk kerogen of the same rock samples, van Krevelen plots of elemental compositions of kerogens from the core samples (Nishiyama Oil Field, Tertiary, Japan) reveal that the amorphous kerogen fraction is not necessarily characterized by markedly high atomic H/C ratio. This was attributed to the oxic environment of deposition and the abundance of biodegraded terrestrial amorphous organic matter in the amorphous kerogen fraction used in this work.
it is necessary to fractionate the structured and amorphous kerogen from bulk kerogen for further detailed organic geochemical investigations of the amorphous kerogens. Although the characteristics of kerogens associated with clay fraction of mudstone have been reported (S~JZUKI and TAGCJCHI, 1982), the amorphous kerogens in bulk kerogen mixture have not been fractionated and analyzed to date, so that the precise relation between the morphologic characteristics of the amorphous kerogens and their chemical characteristics have not been well known. In this work, a preliminary attempt to fractionate amorphous kerogens from tenigenous bulk kerogen based on a two-phase partition method has been made. The two-phase partition method was developed for the fractionation of macromolecular materials and minute particles from their mixtures according to surface characteristics. This method can be used to separate amorphous kerogens from kerogen mixtures. Though aqueous polymers such as polyethylene glycol and dextran have been commonly used for the two phase system, a benzene and acidic water system was chosen as the partition system in this study because of simplicity of operation and of minimal influence on the original structure of the kerogen. This fractionation method is based upon the amorphous kerogens functioning as surface active agents which stabilize the benzene emulsion formed after sufficient agitation. Therefore, most amorphous kerogens are concentrated on the surface of benzene emulsions and recovered with them. The characteristics of amorphous kerogens fractionated by this method are discussed based on ele-
INTRODUCTION KEROGEN, the organic matter in sediments which is insoluble in organic solvents, is roughly classified into structured and amorphous fractions under a microscope. Structured kerogens are morphologically classified further under a transmitted-light microscope and by means of scanning electron microscope (STAPLIN, 1969; BURGESS, 1974; MASRAN and POCOCK, 1981). The structured kerogens are largely derived from phytoclast, spore, pollen, etc., and most of the amorphous kerogens, which are practically unidentifiable organic remains under a microscope, are believed to have originated from condensations of biopolymers and biomonomers, and biodegradations of algal and terrestrial organic matter. Amorphous kerogens among others are being observed by petroleum geologists and geochemists, because it has been speculated that most of amorphous kerogens are type I or II which are relatively rich in hydrogen and produce much oil during diagenesis (TISSOT and WELTE, 1978). On the other hand, the presence of both hydrogen-rich and hydrogen-poor amorphous kerogens has also been pointed out (POWELL et al., 1982). It is most important for organic geochemists to clarify the chemical characteristics of amorphous kerogens. Hitherto information about the chemical characteristics of amorphous kerogens has been obtained from specific samples (e.g. Green River Shale, Tasmanite, Torbanite) which have a great abundance of amorphous kerogens (TISSOT and WELTE, 1978; HUNT, 1979; DURAND, 1980). Bulk kerogen is usually a mixture of structured and amorphous kerogens; therefore, 243
244
N. Suzuki
A marine mudstone sample (outcrop) from the Haizume Formation of Pleistocene age and 10 core samples of marine mudstones from YASUDA R-7 of Pliocene-Miocene age, Nishiyama Oil Field, Japan were used for the study. Location map and generalized stratigraphic column for Nishiyama Oil Field have been shown in SUZUKI and TAGUCHI (I 982).
ogen surface play an important role in stabilizing the benzene emulsion; that is, amorphous kerogen acts as a surface active agent under both acidic and alkaline conditions. Whether or not this explanation is true or whether another one is required cannot be known until further study is done. However, observation under a microscope revealed that amorphous kerogens were fractionated effectively by this method. Judging from the results of the tests described above, the fractionation of amorphous kerogen under acidic conditions (pH 3) was repeated 3 times for further purification. For evaluating the extent of separation and purification of amorphous kerogens. recovered kerogens were observed under a microscope before further chemical analyses. In this work, a fraction which is composed of more than 95% of unidentifiable micro-organic remains was defined as amorphous kerogen fraction.
Preparution of kerogen
Saponification of krrqen
mental composition, IR spectra and composition of saponification products, and compared with those of bulk kerogen, structured kerogens and humic acids isolated from the same sediment samples. METHODS
Samples
Crushed mudstone (2-5 mm in diameter) was agitated by shaking in the mixture of 0.1% Na4P205. IOH* and 0.1 N NaOH for 2 days for the dispersion and the extraction of alkaline soluble organic matter (humic acid and free organic acids). After dispersion had been completed, the material was dried and exhaustively extracted with C$H,/MeOH (9:1) by ultrasonication (I 5 min. 3 times). Residual material was demineralized with 6 N HCl/47% HF (l:l, 2 days. 60°C) and then washed with distilled water to pH 7. A part of the isolated kerogen was stored in the HC1 solution (pH 3, 5°C) and used for the fractionation. Another part was centrifuged, dried and stored in a desiccator. All kerogen and humic acid samples were exhaustively extracted again with C,H,/MeOH (9: I) by ultrasonication (I 5 min. 3 times) before further chemical analyses.
Fractionation qf kerozen The fractionation of amorphous kerogen was carried out in a test tube with a tenon-lined screw cap. A 6 ml portion of dispersed kerogen (cu. 1 wt’%) was placed in the test tube, a 4 ml portion of benzene was added and then shaken sufficiently by hand. The benzene emulsions coated by kerogens and the water were separated into two phases in the test tube on standing. The benzene phase was recovered and placed over pure water. Kerogens in the benzene phase passed into the water after evaporation of the benzene. Kerogens were recovered from the water phase by centrifugation. To test the pH dependence of partition of amorphous kerogen by this method, a HC1 suspension of dispersed kerogen was adjusted to various pH conditions (pH 3-12) by the titration of I N KOH while monitoring with a pH meter. Each suspension of various pH conditions was subjected to the same fractionation test mentioned above. These tests revealed that more amorphous kerogens were partitioned preferentially onto the surface of the benzene emulsion under both acidic and alkaline conditions. The neutral condition (pH 6-8) was not suitable for the fractionation. X-ray diffraction analysis of each kerogen fraction recovered from benzene emulsion phase under both acidic (pH 3) and alkaline (pH 12) conditions revealed that sulfide (pyrite. Fe&) was the sole inorganic mineral accompanying the kerogen under acidic conditions, although neoformed fluoride (ralstonite, NaMgAI(F,OH), . H20) was identified with pyrite in the kerogen fraction under alkaline conditions (pH 12). These results suggest that neoformed fluoride is liable to be partitioned mto the upper benzene emulsion phase under alkaline conditions, while most structured kerogens were flocculated and recovered from the water phase under the acidic conditions. These phenomena are due to the amorphous kerogens functioning as a stabilizer which tends to collect at the interface between the benzene phase and water phase to form a coating on the surface of a benzene emulsion. In addition. it is considered that organophilic functional groups such as aIky1 chains and acidic or basic functional groups on an amorphous ker-
About 5 mg of each kerogen was heated under reflux for 2 hours with a solution of 0.5 N KOH in MeOH (I 00 ml). After saponification, the reaction mixture was filtered, and the residue was washed with 0.5 N KOH solution. The combined filtrates were diluted with water, acidified to pH I with 12 N HCI, and extracted with n-hexane/EtOEt (l:l). The extracts were methylated by 14% BFs in MeOH and analyzed by CC and CC-MS.
E.xtructwn and preparation (falkanc The same rock samples used for the kerogen analyses were powdered (~200 mesh) and extracted in a homogenizer with C6H,/MeOH (9: 1). The extract was evaporated and subjected to column chromatography on silicic acid with elution by nhexane to isolate saturated hydrocarbon. Normal alkanes were separated by urea adduction from branched and cyclic alkanes. Normal alkanes. pristane. phytane and steranes were analyzed by CC and CC-MS.
Instrumental M-MS. The methylated saponification products and each hydrocarbon fraction were analyzed by JEOL QH-100 Gas Chromatograph-Mass Spectrometer (CC-MS) with a 2 m X 3 mm i.d. glass column packed with 2% OV-1 on Uniport HP (100-120 mesh). with a G-SCOT Dexsil 300 CC column (40 m X 0.28 mm i.d.) or with a WCOT fused silica capillary column coated with OV-101 (25 m X 0.25 mm i.d.). The mass spectrometer was operated at an electron energy of 70 eV, ion source temperature of 250°C and a separator temperature of 300°C. Typical RIC chromatograms and mass fragmetograms of each hydrocarbon fractions are shown in Fig. I. IR. Infrared (IR) spectra of the finely crushed kerogens were obtained on a Hitachi 260-50 Infrared Spectrophotometer by the KBr disc method. Assignment of absorption bands was based upon ROUXHET ef al. (1980).
RESULT
Haizume
AND
DISCUSSION
mudstone
Most of our knowledge of chemical characteristics of amorphous kerogens has been obtained from specific samples containing mainly amorphous kerogens. Therefore, it is important to clarify the chemical characteristics of amorphous kerogens obtained from terrigenous sediment samples presumed to be rich in structured kerogens. A marine mudstone from the Haizume Formation of Pliocene age was used for this purpose. The Haizume Formation is represented by
Terrigenous kerogen
24s Teradomari Formation (Yasuda R-7, 1714 “I, Nishiyamn Oil Field, Tertiary, Japan]
RIG (m/z 20-350) branched and cyclzc hydrocarbon fract10n I
r
Triterpanes
,
m/z 217 --+
-__
lr,creaslnq
temperature
-I___)
FIG. 1. The revised ion current (RIC) chromatograms and mass fragmentograms of each hydrocarbon fractions. The WCOT fused silica capillary column (25 m x 0.25 mm i.d.) coated with OV-101 was used. The oven was heated from 130°C to 290°C at 4”C/min. Pr = pristane; Ph = phytane; a = (20R) - 5a(H), I4cu(H),1‘loi(H)- cholestane; b = @OR)- 5n(H), 14~$H),I7a(H) - ergostane: c = (20s) &Y(H).14&(H),17Ly(H)-stigmastane; d = (20R)-5a(H), 14a(H), 177a(H)-sligmastane.
neritic deposits and consists mainly of gray siltstones and pale gray calcareous siltstones in the Nishiyama Oil Field. X-ray analysis of clay minerals in the <2 pm fhction of the sample used has shown smectite as the principal clay mineral with small amounts of illite and chlorite. Fundamental data obtained from the Haizume mudstone are follows: Organic Carbon content = 0.68%, Extractable Organic Matter content = 0.077 g/gO.C., pristane/phytane = 3.9, Sa-stigmastane/5n-cholestane = 3.3, 2OS-/2OS- + 20R-Sa-stigmastane = 0.0. These data show that the Haizume mudstone was deposited under relatively oxic conditions accompanied by much terrigenous organic matter, according to DIDYK ez al. (1978), HUANG and ME~NSCHEIN(1979) and SUZUKI and SHIMADA (1982). The extent of isomerization of Sa-stigmastane indicate immaturity with regard to degree of organic maturation (MACKENZIE et al., 1980).
Transmitted-light photomicrographs of three kerogen fractions are shown in Fig. 2. It is clearly rec-
ognized that structured and structureless kerogens are present as a mixture in the bulk kerogen (A in Fig. 2). B and C in Fig. 2 are photomicrographs of the structured and amorphous kerogen fractions, respectively. Both were fractionated from bulk kerogen by the benzene-water two phase partition method and these actual samples were used for the various analyses in this work. These photomicrographs show that structured kerogens, which are presumed to be derived from higher plants, and amorphous kerogens, with structures too fine to identify under a transmitted-light microscope, are fractionated effectively by the benzene-water two phase partition method.
Elementul und IR anul_vses The elemental compositions of each kerogen fraction and of humic acid from the same Ha&me mudstone sample are shown in Table 1. The amorphous kerogen is characterized by the highest atomic H/C ratio and the lowest atomic N/C ratio, while the humic acid has a relatively higher oxygen content and the highest atomic N/C ratio.
N. Suzuki
FIG. 2. Transmitted-light photomicrographs of three kerogen fractions which are used actually for analyses. A. Showing bulk kerogen from Haizume mudstone. B. Showing structured kerogens fractionated from bulk kerogen of Haizume mudstone. C. Showing amorphous kerogens fractionated from bulk kerogen of Haizume mudstone.
The IR spectra of kerogen fractions and the humic acid are shown in Fig. 3. Amorphous kerogens. which have relatively intense absorption bands at 17 10 cm-’ and 2900 cm-‘. are considered to be the richest in aliphatic structures and carbonyl and carboxyl functional groups. Structured kerogen can be estimated to be the richest in aromatic and quinonic structures based on the relatively intense absorption band at 1620 cm-i. Absorption bands at 1650 cm-’ and 3600 cm-’ which are characteristic of neoforrned fluoride (ralstonite) are recognized in the IR spectra of structured kerogen. The existence of ralstonite is also confirmed by X-ray diffraction. Its association with structured kerogen is probably due to the fact that it tends to be concentrated in the acidic water phase from which structured kerogens are recovered. The neoformed fluoride could not be eliminated completely by HCl retreatment (at 60°C). The presence of ralstonite is considered to affect the elemental analysis because of its water of crystallization. However, the structured kerogen fraction has the lowest hydrogen content. It is believed that it has little effect on the discussion in this paper. In an IR spectrum of humic acid, relatively intense absorption is observed at 17 10 cm-’ due to carbonyl and carboxyl functional groups but C-H absorption at 2900 cm-’ is very weak. In addition, the absorption at 3500 cm-’ due to OH functional groups is also larger. The IR spectrum of bulk kerogen shows an intermediate abTable 1. qeopolymer
Elemental fraction
elemental C(%)
*
ash
free
basis:
Fatty acids in sapokjication producls of’kerogen Results of elemental and IR analyses have indicated distinct differences in chemical structure among those geopolymers fractionated. In order to confirm these differences by an additional method, fatty acids recovered from the saponification products of geopolymer fractions have been analyzed also. Mass fragmentograms of m/z 74 of fatty acid methyl esters obtained from each saponification products are shown in Fig. 4. Recoveries of fatty acids per unit organic carbon are as follows: bulk kerogen, 208.2 pg/gC; structured kerogens, 113.4 pg/gC; amorphous kerogens, 7 16.4 pg/gC: humic acid, 369.4 pg/gC. It is difficult to compare quantitative details because the hydrolysis is not completed after 2 hours refluxing with 0.5 N KOH in methanol. However, the fact that the most fatty acids have been recovered from the saponification products of amorphous kerogens may show the aliphatic character of amorphous kerogen. This result is confirmed by the results of elemental and IR analyses. The fact that fatty acids from the saponification products of the structured kerogen are the richest in
composition of each from iia~aume mudstone
Sample
Bulk kerogen Amorphous kerogens Structured kerogens Hwnc Acid
sorption pattern between amorphous and structured kerogens. The same is true for elemental analysis. These observations mentioned above support the higher hydrogen content in the amorphous kerogen.
62.5 58.1 64.8 52.7 ** by
H
(:;I 4.2 4.6 3.9 3.1
difference;
compos~tron*
ash(%)H/C N($l 0+1$(%) 30.1 15.2 0.81 2.6 2.0 2.8 2.5 ***
35.3 28.5 40.8 abundance
4.3 31.1 7.3
0.95 0.72 0.77
in bulk
(")***
N/C O.Oif’ --0.010 0.038 0.050
keroaen
28 72 ---
247
Terrigenous kerogen
1500
3000
2500
2000
wave
1800 Number
lh””
14w
1x10
llW”
HO”
icm-l1
FIG. 3. Comparison of infrared spectra of each geopolymer fraction. * in IR spectrum of structured kerogens shows the absorption bands of neoformed fluoride (ralstonite).
relative abundance of higher molecular weight fatty acids (~2~~) indicates that structured kerogens are derived from higher plants rich in higher molecular weight fatty acids. This agrees with the visual observation under a microscope. A mass fragmentogram of fatty
Bulk
acid methyl esters from the amorphous kerogen saponification product (Fig. 3) shows that the major constituents of n-fatty acids in the amorphous kerogen saponification product are relatively lower molecular weight C,6 and Cl8 acids which are thought to be mainly
kelmgen
saponification
ml-z 740
Str’dct”led
hmorphous kerocI?“s
keroqens
saponlfIc‘ltlon pr"d"cts
saponlflcatlon products
ml2 74.0
m/z 74 0
FIG. 4. Mass fragmentograms of fatty acid methyl esters in saponification products of each geopolymer fraction. The 2 m X 3 m id. glass column packed with 2% OV-I on Uniport HP (100-120 mesh) was used. The oven was heated from 130°C to 280°C at 4”C/min. The carbon numbers of the fatty acids are indicated by the arabic numbers.
248
N. Suzuki
FIG. 5. Elemental analysis of bulk kerogens and amorphous kerogens from Nishiyama Oil Field, Tertiary, Japan. The solid lines in atomic H/C vs. atomic O/C diagram show evolution paths of the principal types of kerogen (TISSOTand WELTE, 1978).
derived from phytoplankton and zooplankton. In addition, it is also recognized that higher molecular weight fatty acids are present in significant quantities in the amorphous kerogen saponification product though their relative abundance is comparatively small. The higher molecular weight fatty acids may be derived from biodegraded higher plant debris, such as biodegraded terrestrial kerogen (MASRAN and POCOCK, 198 1 ), which have been partitioned into the benzene emulsion phase because of the surface characteristics similar to the amorphous kerogens of marine algal source or biopolymers and biomonomers formed by condensation. In addition, it is also probable that lipids of higher plant origin take part in the formation of amorphous kerogen. In either case, the fact that saponification products of amorphous kerogens are richer in C,, and Cl8 acids is remarkable and suggestive that the animal lipids participate more in formation of amorphous kerogens than the lipids of higher plant origin even when it occurs in the terrigenous organic rich sediments such as Haizume mudstone. Only trace amounts of higher molecular weight fatty acids are recognized in the humic acid saponification product. This suggests that lipids of higher plant origin participate only slightly in the formation of humic acid.
Analytical results mentioned above show not only that characteristics of amorphous kerogens are clearly different from those of structured kerogens but that amorphous kerogens, which are unidentifiable organic remains, are not necessarily characterized by markedly high atomic H/C ratio. Amorphous kerogens in the Haizume mudstone correlated to type III kerogen (DURAND and I&PITAI.I& 1976) in a van Krevelen diagram (Fig. 5). This is in contrast with the prediction that most amorphous kerogens belong to type I or type 11 kerogen. In order to confirm this htct. amorphous kerogens in core samples from Nishiyama Oil Field, Japan were also analyzed in the same manner. Results of organic geochemical analyses of core samples (YASUDA R-7) are shown in Table 2. These data indicate that the mudstones are deposited under relatively oxic condition and are relatively rich in terrigenous organic matter based on the pristane/phytane ratio and 5a-stigmastane/Sn-cholestane ratio. The deeper samples seem to be matured sufficiently to generate hydrocarbons based on CPI value, 2OS/2OS + 20R of Sn-stigmastane, and n-alkane concentration. Van Krevelen plots of both amorphous kerogens and bulk kerogens in core samples are shown in Fig. 5. These results indicate that amorphous kerogens tend to have high atomic H/C ratios and low atomic N/C ratios in comparison with bulk kerogen. Although amorphous kerogens have relatively higher atomic H/C ratios than bulk kerogens. all of these plot near the region of type III kerogen, as shown by Fig. 5. These data suggest that hydrogen-poor amorphous kerogens, which may be partially composed of biodegraded terrestrial kerogen, are formed under relatively oxic depositional conditions in marine environments. In addition, it can be considered that the abundance of terrigenous organic matter corresponds to the abundance of biodegraded terrestrial kerogen which is presumed to be poor in hydrogen, Consequently. amorphous kerogens which are too fine for structure identification under a transmitted-light microscope may tend to be hydrogen poor when they are associated
Terrigenous
with much terrigenous
organic matter deposited
under
oxic condition. CONCLUSIONS A satisfactory procedure using a benzene-water two phase partition system for fractionating amorphous kerogens has been developed and applied. The results show that bulk kerogen is a mixture of various kerogens with different origins and chemical characteristics and that amorphous kerogen fractions tend to be richer in aliphatic structures than bulk kerogen. However, elemental composition of core samples reveals that amorphous kerogens are not necessarily characterized by markedly high H/C atomic ratio. Elemental composition of amorphous kerogens from core samples plot between type II and type III kerogens on a van Krevelen diagram. It is considered that the amorphous kerogens may tend to be hydrogen poor when they are in a sediment accompanied by much terrigenous organic matter that developed under oxic conditions. The observations described above suggest that amorphous kerogens do not necessarily have excellent oil source potential. Therefore, if most petroleum hydrocarbons are derived from kerogens, it is necessary for oil-source rock evaluation to include an investigation of the paleoenvironment of deposition or of the chemical nature of the amorphous kerogens in addition to the measurement of the amorphous kerogen content. The amorphous kerogen fractions defined in this paper still may be complicated mixtures of various types of amorphous kerogens with different origins and chemical characteristics. Therefore, a more precise investigation of amorphous kerogens in natural samples should be made. For that purpose, a special fractionation technique is required, because amorphous kerogens are practically unidentifiable under a microscope. The more improved partition method will make it possible to fractionate the amorphous kerogens further in the future.
Acknon,/edRmmts-The author thanks Dr. K. Kasuga and Dr. Y, Kubo of the chemistry department of Shimane Uni-
249
kerogen
versity for considerable assistance in performing elemental and IR analyses; and Japan Petroleum Exploration Co., Ltd. for providing the core samples. Thanks are due to Dr. K. Taguchi of Tohoku University and Prof. Shimada of Shimane University for making pertinent suggestions during the course of this work. This work was supported financially in part by a Grant-in-Aid for Scientific Research from the Ministry of Education of Japan. REFERENCES BURGESS.I. D. (I 974) Carbonaceous materials as indicators of metamorphism. In G.S.A. Special Paper 153, pp. l930. DURANDB. (1980) Kerogen. Editions Technip. DURAND B. and ESPITALI~J. (1976) Geochemical studies on the organic matter from the Douala Basin (Cameroot+ II. Evolution of kerogen. Geochim. Cosmochim. Acta 40, 801-808. HUANG W.
Y. and MEINCHEINW. G. (1979) Sterols as ecological indicators. Geochim. C’osmochim. Acta 42, I39 I 1396.
HUNT J. M. (1979) Petroleum Geochemistry and Geology. W. H. Freeman and Company. MACKENZIEA. S.. PATIENCER. L., MAXWELLJ. R., VANDENBROU~KEM. and DURAND B. (I 980) Molecular parameters of maturation in the Toarcian shales. Paris Basin, France. I. Isoprenoid alkanes. steranes and triterpanes. Geochim.
Cosmochim.
Acta 44, 1709- 172
1.
MASRANTH.C. and PO~OCK A. S. (198 I) The classification of plant-derived particulate organic matter in sedimentary rocks. In Organic Maturation Studies and Fossil Fuel Exploration (ed. J. BROOKS), pp. 145-l 75. Academic Press. POWELL T. G., CREANEY S. and SNOWD~N L. R. (1982) Limitations of use of organic petrographic techniques for identification of petroleum source rocks. Amer ASSOL..PC,trol. Geol. Bull. 66, 430-435. ROUXHETP. G.. ROBINP. L. and NICAISE G. (1980) Characterization of kerogens and of their evolution by infrared spectroscopy. In Kerogec’n(ed. B. DURAND). pp. 163- 190. STAPLINF. IL. (I 969) Sedimentary organic matter. organic metamorphism and oil and gas occurrence. BUN. Can. Petrol. Geol. 17, 47-66. SUZUKIN. and SHIMADAI. (1982) Steroid hydrocarbons (501C2,, CZ8,Czs steranes) in sedimentary rocks-relation between their compositions and paleoenvironments. Mem. Fat. .%I, Shimane Univ. 16. 125-142. (in Japanese with English abstract). SUZUKIN. and TACI~JCHI K. (1983) Characteristics and diagenesis of kerogens associated with clay fractions of mudstone. In Advances in Organic Geochemistr,v 1981 (ed. M. BJOR@Y), pp. 607-6 12. Wiley. TISSOT B. P. and WELTE D. H. (1978) Petroleum Formation and Orcurrww. Springer-Verlag.
C;,wc t,,,ni< ii iv ~‘r,rmoch,m~~~o .I<,u Vol. 48. pp. 243-249 :Cl Pergdmtm Press Ud 19X4. Pruned in U.S.A.
00 + 00
Characteristics of amorphous kerogens fractionated from terrigenous sedimentary rocks NORIYUKI
SUZUKI
Department of Geology. Faculty of Science, Shimane University. Nishikawazu, Matsue. Shimane. 690, Japan (Received March 11, 1983; accepted in revised_li,rm November 2. 1983) Abstract-A preliminary attempt to fractionate amorphous kerogens from terrigenous bulk kerogen by a benzene-water two phase partition method under acidic condition was made. Microscopic observation revealed that amorphous kerogens and structured kerogens were fractionated effectively by this method. Characteristics of the amorphous and structured kerogens fractionated by this method were examined by some chemical analyses and compared with those of the bulk kerogen and humic acid isolated from the same rock sample (Haizume Formation. Pleistocene, Japan). The elemental and infrared (IR) analyses showed that the amorphous kerogen fraction had the highest atomic H/C ratio and the lowest atomic N/C ratio and was the richest in aliphatic structures and carbonyl and carboxyl functional groups. Quantities of fatty acids from the saponification products of each geopolymer were in agreement with the results of elemental and IR analyses. Distribution of the fatty acids was suggestive that more animal lipids participate in the formation of amorphous kerogens because of the abundance of relatively lower molecular weight fatty acids (such as Cj6 and Cl8 acids) in saponification products of amorphous kerogens. On the other hand, although the amorphous kerogen fraction tends to be rich in aliphatic structures compared with bulk kerogen of the same rock samples, van Krevelen plots of elemental compositions of kerogens from the core samples (Nishiyama Oil Field, Tertiary, Japan) reveal that the amorphous kerogen fraction is not necessarily characterized by markedly high atomic H/C ratio. This was attributed to the oxic environment of deposition and the abundance of biodegraded terrestrial amorphous organic matter in the amorphous kerogen fraction used in this work.
it is necessary to fractionate the structured and amorphous kerogen from bulk kerogen for further detailed organic geochemical investigations of the amorphous kerogens. Although the characteristics of kerogens associated with clay fraction of mudstone have been reported (S~JZUKI and TAGCJCHI, 1982), the amorphous kerogens in bulk kerogen mixture have not been fractionated and analyzed to date, so that the precise relation between the morphologic characteristics of the amorphous kerogens and their chemical characteristics have not been well known. In this work, a preliminary attempt to fractionate amorphous kerogens from tenigenous bulk kerogen based on a two-phase partition method has been made. The two-phase partition method was developed for the fractionation of macromolecular materials and minute particles from their mixtures according to surface characteristics. This method can be used to separate amorphous kerogens from kerogen mixtures. Though aqueous polymers such as polyethylene glycol and dextran have been commonly used for the two phase system, a benzene and acidic water system was chosen as the partition system in this study because of simplicity of operation and of minimal influence on the original structure of the kerogen. This fractionation method is based upon the amorphous kerogens functioning as surface active agents which stabilize the benzene emulsion formed after sufficient agitation. Therefore, most amorphous kerogens are concentrated on the surface of benzene emulsions and recovered with them. The characteristics of amorphous kerogens fractionated by this method are discussed based on ele-
INTRODUCTION KEROGEN, the organic matter in sediments which is insoluble in organic solvents, is roughly classified into structured and amorphous fractions under a microscope. Structured kerogens are morphologically classified further under a transmitted-light microscope and by means of scanning electron microscope (STAPLIN, 1969; BURGESS, 1974; MASRAN and POCOCK, 1981). The structured kerogens are largely derived from phytoclast, spore, pollen, etc., and most of the amorphous kerogens, which are practically unidentifiable organic remains under a microscope, are believed to have originated from condensations of biopolymers and biomonomers, and biodegradations of algal and terrestrial organic matter. Amorphous kerogens among others are being observed by petroleum geologists and geochemists, because it has been speculated that most of amorphous kerogens are type I or II which are relatively rich in hydrogen and produce much oil during diagenesis (TISSOT and WELTE, 1978). On the other hand, the presence of both hydrogen-rich and hydrogen-poor amorphous kerogens has also been pointed out (POWELL et al., 1982). It is most important for organic geochemists to clarify the chemical characteristics of amorphous kerogens. Hitherto information about the chemical characteristics of amorphous kerogens has been obtained from specific samples (e.g. Green River Shale, Tasmanite, Torbanite) which have a great abundance of amorphous kerogens (TISSOT and WELTE, 1978; HUNT, 1979; DURAND, 1980). Bulk kerogen is usually a mixture of structured and amorphous kerogens; therefore, 243
244
N. Suzuki
A marine mudstone sample (outcrop) from the Haizume Formation of Pleistocene age and 10 core samples of marine mudstones from YASUDA R-7 of Pliocene-Miocene age, Nishiyama Oil Field, Japan were used for the study. Location map and generalized stratigraphic column for Nishiyama Oil Field have been shown in SUZUKI and TAGUCHI (I 982).
ogen surface play an important role in stabilizing the benzene emulsion; that is, amorphous kerogen acts as a surface active agent under both acidic and alkaline conditions. Whether or not this explanation is true or whether another one is required cannot be known until further study is done. However, observation under a microscope revealed that amorphous kerogens were fractionated effectively by this method. Judging from the results of the tests described above, the fractionation of amorphous kerogen under acidic conditions (pH 3) was repeated 3 times for further purification. For evaluating the extent of separation and purification of amorphous kerogens. recovered kerogens were observed under a microscope before further chemical analyses. In this work, a fraction which is composed of more than 95% of unidentifiable micro-organic remains was defined as amorphous kerogen fraction.
Preparution of kerogen
Saponification of krrqen
mental composition, IR spectra and composition of saponification products, and compared with those of bulk kerogen, structured kerogens and humic acids isolated from the same sediment samples. METHODS
Samples
Crushed mudstone (2-5 mm in diameter) was agitated by shaking in the mixture of 0.1% Na4P205. IOH* and 0.1 N NaOH for 2 days for the dispersion and the extraction of alkaline soluble organic matter (humic acid and free organic acids). After dispersion had been completed, the material was dried and exhaustively extracted with C$H,/MeOH (9:1) by ultrasonication (I 5 min. 3 times). Residual material was demineralized with 6 N HCl/47% HF (l:l, 2 days. 60°C) and then washed with distilled water to pH 7. A part of the isolated kerogen was stored in the HC1 solution (pH 3, 5°C) and used for the fractionation. Another part was centrifuged, dried and stored in a desiccator. All kerogen and humic acid samples were exhaustively extracted again with C,H,/MeOH (9: I) by ultrasonication (I 5 min. 3 times) before further chemical analyses.
Fractionation qf kerozen The fractionation of amorphous kerogen was carried out in a test tube with a tenon-lined screw cap. A 6 ml portion of dispersed kerogen (cu. 1 wt’%) was placed in the test tube, a 4 ml portion of benzene was added and then shaken sufficiently by hand. The benzene emulsions coated by kerogens and the water were separated into two phases in the test tube on standing. The benzene phase was recovered and placed over pure water. Kerogens in the benzene phase passed into the water after evaporation of the benzene. Kerogens were recovered from the water phase by centrifugation. To test the pH dependence of partition of amorphous kerogen by this method, a HC1 suspension of dispersed kerogen was adjusted to various pH conditions (pH 3-12) by the titration of I N KOH while monitoring with a pH meter. Each suspension of various pH conditions was subjected to the same fractionation test mentioned above. These tests revealed that more amorphous kerogens were partitioned preferentially onto the surface of the benzene emulsion under both acidic and alkaline conditions. The neutral condition (pH 6-8) was not suitable for the fractionation. X-ray diffraction analysis of each kerogen fraction recovered from benzene emulsion phase under both acidic (pH 3) and alkaline (pH 12) conditions revealed that sulfide (pyrite. Fe&) was the sole inorganic mineral accompanying the kerogen under acidic conditions, although neoformed fluoride (ralstonite, NaMgAI(F,OH), . H20) was identified with pyrite in the kerogen fraction under alkaline conditions (pH 12). These results suggest that neoformed fluoride is liable to be partitioned mto the upper benzene emulsion phase under alkaline conditions, while most structured kerogens were flocculated and recovered from the water phase under the acidic conditions. These phenomena are due to the amorphous kerogens functioning as a stabilizer which tends to collect at the interface between the benzene phase and water phase to form a coating on the surface of a benzene emulsion. In addition. it is considered that organophilic functional groups such as aIky1 chains and acidic or basic functional groups on an amorphous ker-
About 5 mg of each kerogen was heated under reflux for 2 hours with a solution of 0.5 N KOH in MeOH (I 00 ml). After saponification, the reaction mixture was filtered, and the residue was washed with 0.5 N KOH solution. The combined filtrates were diluted with water, acidified to pH I with 12 N HCI, and extracted with n-hexane/EtOEt (l:l). The extracts were methylated by 14% BFs in MeOH and analyzed by CC and CC-MS.
E.xtructwn and preparation (falkanc The same rock samples used for the kerogen analyses were powdered (~200 mesh) and extracted in a homogenizer with C6H,/MeOH (9: 1). The extract was evaporated and subjected to column chromatography on silicic acid with elution by nhexane to isolate saturated hydrocarbon. Normal alkanes were separated by urea adduction from branched and cyclic alkanes. Normal alkanes. pristane. phytane and steranes were analyzed by CC and CC-MS.
Instrumental M-MS. The methylated saponification products and each hydrocarbon fraction were analyzed by JEOL QH-100 Gas Chromatograph-Mass Spectrometer (CC-MS) with a 2 m X 3 mm i.d. glass column packed with 2% OV-1 on Uniport HP (100-120 mesh). with a G-SCOT Dexsil 300 CC column (40 m X 0.28 mm i.d.) or with a WCOT fused silica capillary column coated with OV-101 (25 m X 0.25 mm i.d.). The mass spectrometer was operated at an electron energy of 70 eV, ion source temperature of 250°C and a separator temperature of 300°C. Typical RIC chromatograms and mass fragmetograms of each hydrocarbon fractions are shown in Fig. I. IR. Infrared (IR) spectra of the finely crushed kerogens were obtained on a Hitachi 260-50 Infrared Spectrophotometer by the KBr disc method. Assignment of absorption bands was based upon ROUXHET ef al. (1980).
RESULT
Haizume
AND
DISCUSSION
mudstone
Most of our knowledge of chemical characteristics of amorphous kerogens has been obtained from specific samples containing mainly amorphous kerogens. Therefore, it is important to clarify the chemical characteristics of amorphous kerogens obtained from terrigenous sediment samples presumed to be rich in structured kerogens. A marine mudstone from the Haizume Formation of Pliocene age was used for this purpose. The Haizume Formation is represented by
Terrigenous kerogen
24s Teradomari Formation (Yasuda R-7, 1714 “I, Nishiyamn Oil Field, Tertiary, Japan]
RIG (m/z 20-350) branched and cyclzc hydrocarbon fract10n I
r
Triterpanes
,
m/z 217 --+
-__
lr,creaslnq
temperature
-I___)
FIG. 1. The revised ion current (RIC) chromatograms and mass fragmentograms of each hydrocarbon fractions. The WCOT fused silica capillary column (25 m x 0.25 mm i.d.) coated with OV-101 was used. The oven was heated from 130°C to 290°C at 4”C/min. Pr = pristane; Ph = phytane; a = (20R) - 5a(H), I4cu(H),1‘loi(H)- cholestane; b = @OR)- 5n(H), 14~$H),I7a(H) - ergostane: c = (20s) &Y(H).14&(H),17Ly(H)-stigmastane; d = (20R)-5a(H), 14a(H), 177a(H)-sligmastane.
neritic deposits and consists mainly of gray siltstones and pale gray calcareous siltstones in the Nishiyama Oil Field. X-ray analysis of clay minerals in the <2 pm fhction of the sample used has shown smectite as the principal clay mineral with small amounts of illite and chlorite. Fundamental data obtained from the Haizume mudstone are follows: Organic Carbon content = 0.68%, Extractable Organic Matter content = 0.077 g/gO.C., pristane/phytane = 3.9, Sa-stigmastane/5n-cholestane = 3.3, 2OS-/2OS- + 20R-Sa-stigmastane = 0.0. These data show that the Haizume mudstone was deposited under relatively oxic conditions accompanied by much terrigenous organic matter, according to DIDYK ez al. (1978), HUANG and ME~NSCHEIN(1979) and SUZUKI and SHIMADA (1982). The extent of isomerization of Sa-stigmastane indicate immaturity with regard to degree of organic maturation (MACKENZIE et al., 1980).
Transmitted-light photomicrographs of three kerogen fractions are shown in Fig. 2. It is clearly rec-
ognized that structured and structureless kerogens are present as a mixture in the bulk kerogen (A in Fig. 2). B and C in Fig. 2 are photomicrographs of the structured and amorphous kerogen fractions, respectively. Both were fractionated from bulk kerogen by the benzene-water two phase partition method and these actual samples were used for the various analyses in this work. These photomicrographs show that structured kerogens, which are presumed to be derived from higher plants, and amorphous kerogens, with structures too fine to identify under a transmitted-light microscope, are fractionated effectively by the benzene-water two phase partition method.
Elementul und IR anul_vses The elemental compositions of each kerogen fraction and of humic acid from the same Ha&me mudstone sample are shown in Table 1. The amorphous kerogen is characterized by the highest atomic H/C ratio and the lowest atomic N/C ratio, while the humic acid has a relatively higher oxygen content and the highest atomic N/C ratio.
N. Suzuki
FIG. 2. Transmitted-light photomicrographs of three kerogen fractions which are used actually for analyses. A. Showing bulk kerogen from Haizume mudstone. B. Showing structured kerogens fractionated from bulk kerogen of Haizume mudstone. C. Showing amorphous kerogens fractionated from bulk kerogen of Haizume mudstone.
The IR spectra of kerogen fractions and the humic acid are shown in Fig. 3. Amorphous kerogens. which have relatively intense absorption bands at 17 10 cm-’ and 2900 cm-‘. are considered to be the richest in aliphatic structures and carbonyl and carboxyl functional groups. Structured kerogen can be estimated to be the richest in aromatic and quinonic structures based on the relatively intense absorption band at 1620 cm-i. Absorption bands at 1650 cm-’ and 3600 cm-’ which are characteristic of neoforrned fluoride (ralstonite) are recognized in the IR spectra of structured kerogen. The existence of ralstonite is also confirmed by X-ray diffraction. Its association with structured kerogen is probably due to the fact that it tends to be concentrated in the acidic water phase from which structured kerogens are recovered. The neoformed fluoride could not be eliminated completely by HCl retreatment (at 60°C). The presence of ralstonite is considered to affect the elemental analysis because of its water of crystallization. However, the structured kerogen fraction has the lowest hydrogen content. It is believed that it has little effect on the discussion in this paper. In an IR spectrum of humic acid, relatively intense absorption is observed at 17 10 cm-’ due to carbonyl and carboxyl functional groups but C-H absorption at 2900 cm-’ is very weak. In addition, the absorption at 3500 cm-’ due to OH functional groups is also larger. The IR spectrum of bulk kerogen shows an intermediate abTable 1. qeopolymer
Elemental fraction
elemental C(%)
*
ash
free
basis:
Fatty acids in sapokjication producls of’kerogen Results of elemental and IR analyses have indicated distinct differences in chemical structure among those geopolymers fractionated. In order to confirm these differences by an additional method, fatty acids recovered from the saponification products of geopolymer fractions have been analyzed also. Mass fragmentograms of m/z 74 of fatty acid methyl esters obtained from each saponification products are shown in Fig. 4. Recoveries of fatty acids per unit organic carbon are as follows: bulk kerogen, 208.2 pg/gC; structured kerogens, 113.4 pg/gC; amorphous kerogens, 7 16.4 pg/gC: humic acid, 369.4 pg/gC. It is difficult to compare quantitative details because the hydrolysis is not completed after 2 hours refluxing with 0.5 N KOH in methanol. However, the fact that the most fatty acids have been recovered from the saponification products of amorphous kerogens may show the aliphatic character of amorphous kerogen. This result is confirmed by the results of elemental and IR analyses. The fact that fatty acids from the saponification products of the structured kerogen are the richest in
composition of each from iia~aume mudstone
Sample
Bulk kerogen Amorphous kerogens Structured kerogens Hwnc Acid
sorption pattern between amorphous and structured kerogens. The same is true for elemental analysis. These observations mentioned above support the higher hydrogen content in the amorphous kerogen.
62.5 58.1 64.8 52.7 ** by
H
(:;I 4.2 4.6 3.9 3.1
difference;
compos~tron*
ash(%)H/C N($l 0+1$(%) 30.1 15.2 0.81 2.6 2.0 2.8 2.5 ***
35.3 28.5 40.8 abundance
4.3 31.1 7.3
0.95 0.72 0.77
in bulk
(")***
N/C O.Oif’ --0.010 0.038 0.050
keroaen
28 72 ---
247
Terrigenous kerogen
1500
3000
2500
2000
wave
1800 Number
lh””
14w
1x10
llW”
HO”
icm-l1
FIG. 3. Comparison of infrared spectra of each geopolymer fraction. * in IR spectrum of structured kerogens shows the absorption bands of neoformed fluoride (ralstonite).
relative abundance of higher molecular weight fatty acids (~2~~) indicates that structured kerogens are derived from higher plants rich in higher molecular weight fatty acids. This agrees with the visual observation under a microscope. A mass fragmentogram of fatty
Bulk
acid methyl esters from the amorphous kerogen saponification product (Fig. 3) shows that the major constituents of n-fatty acids in the amorphous kerogen saponification product are relatively lower molecular weight C,6 and Cl8 acids which are thought to be mainly
kelmgen
saponification
ml-z 740
Str’dct”led
hmorphous kerocI?“s
keroqens
saponlfIc‘ltlon pr"d"cts
saponlflcatlon products
ml2 74.0
m/z 74 0
FIG. 4. Mass fragmentograms of fatty acid methyl esters in saponification products of each geopolymer fraction. The 2 m X 3 m id. glass column packed with 2% OV-I on Uniport HP (100-120 mesh) was used. The oven was heated from 130°C to 280°C at 4”C/min. The carbon numbers of the fatty acids are indicated by the arabic numbers.
248
N. Suzuki
FIG. 5. Elemental analysis of bulk kerogens and amorphous kerogens from Nishiyama Oil Field, Tertiary, Japan. The solid lines in atomic H/C vs. atomic O/C diagram show evolution paths of the principal types of kerogen (TISSOTand WELTE, 1978).
derived from phytoplankton and zooplankton. In addition, it is also recognized that higher molecular weight fatty acids are present in significant quantities in the amorphous kerogen saponification product though their relative abundance is comparatively small. The higher molecular weight fatty acids may be derived from biodegraded higher plant debris, such as biodegraded terrestrial kerogen (MASRAN and POCOCK, 198 1 ), which have been partitioned into the benzene emulsion phase because of the surface characteristics similar to the amorphous kerogens of marine algal source or biopolymers and biomonomers formed by condensation. In addition, it is also probable that lipids of higher plant origin take part in the formation of amorphous kerogen. In either case, the fact that saponification products of amorphous kerogens are richer in C,, and Cl8 acids is remarkable and suggestive that the animal lipids participate more in formation of amorphous kerogens than the lipids of higher plant origin even when it occurs in the terrigenous organic rich sediments such as Haizume mudstone. Only trace amounts of higher molecular weight fatty acids are recognized in the humic acid saponification product. This suggests that lipids of higher plant origin participate only slightly in the formation of humic acid.
Analytical results mentioned above show not only that characteristics of amorphous kerogens are clearly different from those of structured kerogens but that amorphous kerogens, which are unidentifiable organic remains, are not necessarily characterized by markedly high atomic H/C ratio. Amorphous kerogens in the Haizume mudstone correlated to type III kerogen (DURAND and I&PITAI.I& 1976) in a van Krevelen diagram (Fig. 5). This is in contrast with the prediction that most amorphous kerogens belong to type I or type 11 kerogen. In order to confirm this htct. amorphous kerogens in core samples from Nishiyama Oil Field, Japan were also analyzed in the same manner. Results of organic geochemical analyses of core samples (YASUDA R-7) are shown in Table 2. These data indicate that the mudstones are deposited under relatively oxic condition and are relatively rich in terrigenous organic matter based on the pristane/phytane ratio and 5a-stigmastane/Sn-cholestane ratio. The deeper samples seem to be matured sufficiently to generate hydrocarbons based on CPI value, 2OS/2OS + 20R of Sn-stigmastane, and n-alkane concentration. Van Krevelen plots of both amorphous kerogens and bulk kerogens in core samples are shown in Fig. 5. These results indicate that amorphous kerogens tend to have high atomic H/C ratios and low atomic N/C ratios in comparison with bulk kerogen. Although amorphous kerogens have relatively higher atomic H/C ratios than bulk kerogens. all of these plot near the region of type III kerogen, as shown by Fig. 5. These data suggest that hydrogen-poor amorphous kerogens, which may be partially composed of biodegraded terrestrial kerogen, are formed under relatively oxic depositional conditions in marine environments. In addition, it can be considered that the abundance of terrigenous organic matter corresponds to the abundance of biodegraded terrestrial kerogen which is presumed to be poor in hydrogen, Consequently. amorphous kerogens which are too fine for structure identification under a transmitted-light microscope may tend to be hydrogen poor when they are associated
Terrigenous
with much terrigenous
organic matter deposited
under
oxic condition. CONCLUSIONS A satisfactory procedure using a benzene-water two phase partition system for fractionating amorphous kerogens has been developed and applied. The results show that bulk kerogen is a mixture of various kerogens with different origins and chemical characteristics and that amorphous kerogen fractions tend to be richer in aliphatic structures than bulk kerogen. However, elemental composition of core samples reveals that amorphous kerogens are not necessarily characterized by markedly high H/C atomic ratio. Elemental composition of amorphous kerogens from core samples plot between type II and type III kerogens on a van Krevelen diagram. It is considered that the amorphous kerogens may tend to be hydrogen poor when they are in a sediment accompanied by much terrigenous organic matter that developed under oxic conditions. The observations described above suggest that amorphous kerogens do not necessarily have excellent oil source potential. Therefore, if most petroleum hydrocarbons are derived from kerogens, it is necessary for oil-source rock evaluation to include an investigation of the paleoenvironment of deposition or of the chemical nature of the amorphous kerogens in addition to the measurement of the amorphous kerogen content. The amorphous kerogen fractions defined in this paper still may be complicated mixtures of various types of amorphous kerogens with different origins and chemical characteristics. Therefore, a more precise investigation of amorphous kerogens in natural samples should be made. For that purpose, a special fractionation technique is required, because amorphous kerogens are practically unidentifiable under a microscope. The more improved partition method will make it possible to fractionate the amorphous kerogens further in the future.
Acknon,/edRmmts-The author thanks Dr. K. Kasuga and Dr. Y, Kubo of the chemistry department of Shimane Uni-
249
kerogen
versity for considerable assistance in performing elemental and IR analyses; and Japan Petroleum Exploration Co., Ltd. for providing the core samples. Thanks are due to Dr. K. Taguchi of Tohoku University and Prof. Shimada of Shimane University for making pertinent suggestions during the course of this work. This work was supported financially in part by a Grant-in-Aid for Scientific Research from the Ministry of Education of Japan. REFERENCES BURGESS.I. D. (I 974) Carbonaceous materials as indicators of metamorphism. In G.S.A. Special Paper 153, pp. l930. DURANDB. (1980) Kerogen. Editions Technip. DURAND B. and ESPITALI~J. (1976) Geochemical studies on the organic matter from the Douala Basin (Cameroot+ II. Evolution of kerogen. Geochim. Cosmochim. Acta 40, 801-808. HUANG W.
Y. and MEINCHEINW. G. (1979) Sterols as ecological indicators. Geochim. C’osmochim. Acta 42, I39 I 1396.
HUNT J. M. (1979) Petroleum Geochemistry and Geology. W. H. Freeman and Company. MACKENZIEA. S.. PATIENCER. L., MAXWELLJ. R., VANDENBROU~KEM. and DURAND B. (I 980) Molecular parameters of maturation in the Toarcian shales. Paris Basin, France. I. Isoprenoid alkanes. steranes and triterpanes. Geochim.
Cosmochim.
Acta 44, 1709- 172
1.
MASRANTH.C. and PO~OCK A. S. (198 I) The classification of plant-derived particulate organic matter in sedimentary rocks. In Organic Maturation Studies and Fossil Fuel Exploration (ed. J. BROOKS), pp. 145-l 75. Academic Press. POWELL T. G., CREANEY S. and SNOWD~N L. R. (1982) Limitations of use of organic petrographic techniques for identification of petroleum source rocks. Amer ASSOL..PC,trol. Geol. Bull. 66, 430-435. ROUXHETP. G.. ROBINP. L. and NICAISE G. (1980) Characterization of kerogens and of their evolution by infrared spectroscopy. In Kerogec’n(ed. B. DURAND). pp. 163- 190. STAPLINF. IL. (I 969) Sedimentary organic matter. organic metamorphism and oil and gas occurrence. BUN. Can. Petrol. Geol. 17, 47-66. SUZUKIN. and SHIMADAI. (1982) Steroid hydrocarbons (501C2,, CZ8,Czs steranes) in sedimentary rocks-relation between their compositions and paleoenvironments. Mem. Fat. .%I, Shimane Univ. 16. 125-142. (in Japanese with English abstract). SUZUKIN. and TACI~JCHI K. (1983) Characteristics and diagenesis of kerogens associated with clay fractions of mudstone. In Advances in Organic Geochemistr,v 1981 (ed. M. BJOR@Y), pp. 607-6 12. Wiley. TISSOT B. P. and WELTE D. H. (1978) Petroleum Formation and Orcurrww. Springer-Verlag.