CS
2
/NMP extraction of immature source rock concentrates
Guo Shaohui *, Li Shuyuan, Qin Kuangzong
State Key Lab of Heavy Oil Processing, University of Petroleum, 102200, ChinaAbstract
Eleven Chinese immature source rock concentrates from the immature oil formations in four dierent depressions were extracted ultrasonically with a mixture of CS2andN-methyl-2-pyrrolidinone (CS2/NMP) at room temperature.
The samples were also extracted with CHCl3and a mixture of methanol/acetone/chloroform (MAC) for comparison.
The solvent system CS2/NMP is very ecient for the extraction of immature source rock concentrates, giving much
higher extraction yields than CHCl3or MAC. The composition of the extracts using dierent solvent mixtures is also
dierent. Model compound tests indicate that no chemical reactions have taken place between the NMP and the sub-strates in the extraction. These results suggest that there are abundant non-covalent bond interactions in the organic matter of the immature source rock concentrates. The fact that CS2/NMP mixed solvent extracts more than MAC and
CHCl3is not only because it can dissolve higher molecular weight fractions, but also because it has stronger ability to
disrupt the complex interactions existing between the soluble and insoluble fractions. Biomarker distributions in the saturated hydrocarbon fractions are dierent for dierent solvent systems, suggesting that care should be taken when comparing the biomarker parameters in source rocks when using dierent solvents for extraction.# 2000 Elsevier Science Ltd. All rights reserved.
Keywords:Source rock concentrate; Immature source rock; Solvent extraction; CS2/NMP; Methanol/acetone/chloroform; Chloroform
1. Introduction
Although characterization of solvent extracts is a promising approach to the clari®cation of organic structure in geological macromolecules, the extraction yields by conventional solvents under mild conditions is often too low to get valuable information. It is of great value to determine the quantity of extractable substances in source rocks and coals, which is closely related with the debatable concept of the mobile phase (component) and rigid phase (network) in coal structures (Derbyshire et al., 1989). Many studies on solvent extraction have been reported (Pullen, 1981), but they have been mostly by Soxhlet extraction carried out using conventional solvents at temperatures near the boiling point of the solvent. Chemical reactions such as decomposition and oxidation, may occur in certain cases, especially for solvents with high boiling points. A ternary azotrope of chloroform, methanol, and acetone (MAC, 47:23:30 by
weight, bp 57.5C) has been used for extracting biological
markers and other soluble materials from coal macerals and other organic sediments (Allan et al., 1977; Given, 1984). Allan et al. (1977) used this mixture to extract 9± 17% by weight of eight vitrinite concentrates (77±84% C, daf) and 2% from two vitrinites of high rank coals (85.5, 86.7% C). The yield of extract from 10 sporinite concentrates was 4±6%. Solvent extraction under mild conditions, which causes little chemical change to sub-strates extracted, provides valuable information on their structures through characterization of the extract and residue and clari®cation of the extraction mechanisms. There are few studies of extraction of source rocks at room temperature, since it usually gives low yields. It has been found that a mixed solvent of CS2
andN-methyl-2-pyrrolidinone (CS2/NMP) is a powerful
solvent at room temperature for the extraction of bitumi-nous coals (Iino et al., 1988) to give extracts characteristic of the original organic macromolecules.
A number of reports indicate thatN -methyl-2-pyrro-lidinone is chemically inert during extraction and does not react with the extracted coals (Iino, et al., 1988;
0146-6380/00/$ - see front matter#2000 Elsevier Science Ltd. All rights reserved. P I I : S 0 1 4 6 - 6 3 8 0 ( 0 0 ) 0 0 1 2 6 - 1
www.elsevier.nl/locate/orggeochem
Seehra et al., 1988; Cagniant et al., 1990; Pajak et al., 1994; Chervenick and Smart, 1995). Cagniant et al. (1990) concluded that NMP did not react with extracted coals, and a series of model compounds, using boiling NMP (202C) under a nitrogen atmosphere. These
model compounds are representative of the main com-pounds containing functional groups in coals: they include dibenzyl ether, phenyl and benzyl benzoates, dibenzofuran, diphenyl sul®de, diphenyl methane and phenol. Alternatively, Cai and Smart (1993) concluded that NMP is reactive with coal and is responsible for breaking carbon±oxygen bonds when used at its boiling point under nitrogen. They also concluded that during NMP-coal extraction, it may be able to break carbon± oxygen bonds when used at its boiling point. White et al. (1997) suspected that a hydroperoxide may be an intermediate in the reaction of NMP with oxygen to yield N-methylsuccinimide and that the hydroperoxide could initiate the oxidation of other substrates during extraction at the boiling point of NMP. Although it cannot be stated explicitly that there are no reactions between NMP and the substrates extracted under certain extraction conditions, no one has reported, until now, that chemical reactions occur between NMP and the organic substrates extracted under room temperature conditions. Use of this mixed solvent for source rock and coal extraction thus seems to be an eective means of increasing the yield without disrupting the chemical structure of the macromolecules.
This study is concerned with an investigation of the CHCl3, methanol/acetone/chloroform (MAC, 23:30:47
by weight) and CS2/NMP (1:1 by volume) mixed solvent
extractability, and the solvent-soluble components, of the organic matter (OM) in Chinese immature source rocks. As oil exploration continues, more and more immature oil has been recovered. Currently, immature oil production accounts for nearly 10% of the total production in China. Although several mechanisms for the formation of immature oil have been proposed (Lane and Jackson, 1980; Snowdon and Powell, 1982; Orr, 1986; Khorasani, 1987; Khorasani and Michelsen, 1991; Snowdon, 1991), there are still many questions to be answered. Therefore, only immature source rocks were studied in this work. Extraction of model compounds with NMP was also carried out to test if chemical reactions take place.
2. Experimental
2.1. Materials and pretreatment
Eleven Chinese immature source rock samples were selected from immature oil formations in four dierent depressions. Three source rock samples were taken from well Dai-6 (D-6), well Wangsi 8-2 (WS8-2) and well
Zhouxie 22-1 (ZX22-1) in the Qianjiang depression of the Jianghan basin. One sample was from well Tong-29 (T-29) in the Dongying depression. Three samples from the East-Liaohe depression were from wells To-12 (To-12), To-16 (To-16) and To-20 (T-20). Four samples from the West-Liaohe depression were taken from wells Du-22 (D-22), Gao-60 (G-60), Lei-15 (L-15) and Lei-36 (L-36). Source rock samples were ground to <100 mesh, and treated with HCl and HF to remove minerals and to concentrate the organic matter before extraction.
2.2. CS2/NMP extraction of model compounds
To check if any chemical reactions were involved using CS2/NMP extraction, experiments were carried out on
model compounds known to represent the main functional groups of sedimentary organic matter. The models used in this study included diethyl ether, dibutyl ether,n -butyl-pentyl ether, di-butyl-pentyl ether, methyl phenoxide, diphenyl ether, dibenzyl ether; methyl benzoate,p-hydroxybenzoic acid and phenol. The mixtures of the model compounds and CS2/NMP (1:1 by volume) were sonicated at room
temperature. The reaction was followed immediately by gas chromatography, on samples taken at 30, 60 and 120 min. Decane was used as internal standard.
2.3. Gas chromatographic analysis of the model compounds
The samples (1 mL) were analyzed quantitatively by capillary g. c., under the following conditions: HP-5 fused silica capillary column (25 m0.25 mm0.1mm); temperature program, 50C (3 min) to 300C (20 min) at
4C minÿ1; injection by split/splitless mode,T=280C;
FID detector, T=320C; argon was used as carrier.
2.4. Extraction of source rock concentrates
Mixed solvent CS2/NMP (1:1 by volume) extraction
of immature source rock concentrates was carried out ultrasonically (38 kHz) for 1 h at room temperature. The extraction vessel containing ca. 0.3 g sample and 10 ml mixed solvent was sealed to prevent any loss of CS2. After
centrifugation at 1500 rpm for 60 min the supernatant was separated by decantation. Fresh mixed solvent was added to the residue, which was again extracted ultra-sonically for 30 min. These procedures were repeated until the supernatant became colorless (about 4±7 times). The residue was then washed ultrasonically with 10 ml acetone (three times, 15 min) to remove any CS2
and NMP.
The supernatant was ®ltered using a membrane ®lter with an average pore size of 0.5mm and solid remaining on the paper, if any, was combined with the residue described above. After removing CS2 (stripped below
aqueous HCl per 10 ml ®ltrate. The precipitated extract was separated under vacuum, using a membrane ®lter. The solid extract was water-washed, dried in vacuo at 80C for 8 h and weighed. The extraction yield was
determined from the weight of the extracts.
The CHCl3and methanol/acetone/chloroform (MAC,
23:30:47 by weight) extraction of immature source rock concentrates was carried out in a Soxhlet apparatus.
2.5. Fractionation of the extracts
The asphaltene fraction was precipitated with pentane. The maltene fraction (ca. 20±50 mg) in ca. 0.2 ml solvent was adsorbed onto 0.2 g of silica gel and the solvent was removed under nitrogen. The adsorbed sample was then added to the top of a glass column (30 cm0.75 cm i. d.) packed with a hexane slurry of 3 g of silica gel in the lower part of the column and 2 g of neutral alumina in the upper, and sequentially eluted with following solvents: (1) 20 mln-hexane to elute the saturate (aliphatic hydro-carbon) fraction; (2) 15 ml mixed hexane and CH2Cl2(1:2
v/v) to elute the aromatic fraction; and (3) 10 ml ethanol and then 10 ml CHCl3to elute the resin fraction. The
ali-phatic, aromatic and resin fractions were con-centrated by rotary evaporation and transferred to vials, and the remaining solvent was evaporated under vacuum, at 30C
for 2 h and weighed. The four fractions of the solvent extracts were stored under N2for further analysis.
3. Results and discussion
3.1. CS2/NMP extraction of model compounds
N-Methyl-2-pyrrolidinone (a polar and aprotic solvent) has the ability to solubilize coals attributed to oxygen and nitrogen atoms with non-bonding electron pairs. A large synergistic eect was also observed using CS2/
NMP mixtures at room temperature (Iino et al., 1988). Although the high extraction yields could be, a priori, attributed to some reactions of NMP with coals, this hypothesis was not discounted by our experimental. Indeed, CS2/NMP extraction of 10 model compounds
(ether, dibutyl ether,n-butylpentyl ether, dipentyl ether, methyl phenoxide, diphenyl ether, dibenzyl ether; methyl benzoate, p-hydroxyl benzoic acid and phenol) under the conditions described above provided, as sole products, the starting model compounds, which were recovered quantitatively.
3.2. CS2/NMP extraction of immature source rock
concentrates
Geochemical data of the samples are listed in Table 1. The extraction yields using dierent solvents are shown in Table 2 and Fig. 1. The extraction yields from the
source rock concentrates are nearly the same as the yields obtained from untreated source rocks. Here, we only present data of solvent extracts of the concentrates. Extraction yields with CHCl3 are the lowest whereas
with CS2/NMP extraction yields are the highest.
Extraction yields with CS2/NMP mixed solvent for
immature source rock concentrates are so high (Table 2 and Fig. 1) that more than 400 mg/g TOC of extracts were obtained from these immature source rock con-centrates except for sample To-12 (386.51 mg/g TOC); these values are about 2 to 9 times the CHCl3extraction
yields. For samples To-16 and D-6, the extraction yields of up to 743 and 728 mg/gTOC, respectively, were obtained. Such high extraction yields from exinitic organic matter, under mild extraction conditions, have never been reported. The extractability of the MAC is higher than CHCl3, but signi®cantly lower than CS2/
NMP. The MAC extraction yields are from 1.4 to 2.7 times the CHCl3extraction yield.
CS2/NMP extraction of model compounds suggests
that no signi®cant chemical reaction occurs between source rock concentrates and CS2/NMP, therefore, this
extraction process provides valuable information on quan-tities of extractable substances in source rock concentrates. The extracts obtained using CS2/NMP contained no
particles visible to a necked eye, suggesting that the mixed solvent does solvate the extractable molecules. Cody et al. (1997) have reported that the physical con-sequences of CS2/NMP mixed solvent extraction are
real solutions, signi®cantly dierent from those using pyridine extraction. By ®ltering the CS2/NMP extract
through a 0.5mm ®lter, we found that about 10% of extracted molecules cannot pass through the ®lter.
Our further studies indicated that more than 75% of the CS2/NMP extracts could be dissolved in CHCl3. The
amount of CHCl3-soluble molecules in the CS2/NMP
extract is greater than that of the CHCl3extract of the
source rock concentrates.
In order to reveal dierences between the extracts obtained with dierent solvents, all the extracts were fractionated into saturate, aromatic, resin and asphal-tene fractions. Results of the fractionation of the extracts are shown in Table 3 and Figs. 2±4.
From these data we can see that as the extraction yield increases with MAC and CS2/NMP, hydrocarbon
(saturate+aromatic), resin and asphaltene fractions increase, especially for the CS2/NMP extraction. It is
clear that as the extractability of the solvent increases, the amount of asphaltenes extracted increases greatly.
Saturate fractions of the CHCl3 and CS2/NMP
extracts were analyzed by GC-MS, and the results are listed in Tables 4±6. Fig. 5 shows the TIC of the saturate from CHCl3and CS2/NMP extractions of ZX22-1. Figs.
6±8 show the TIC, m/e 191 and m/e 217 mass fragmen-tograms of the T-29 saturates from the CHCl3and CS2/
the saturates from CHCl3and CS2/NMP are dierent.
CS2/NMP extracts more higher molecular weight
hydrocarbons and isoalkanes than does CHCl3. All the
parameters including CPI, OEP, Pr/Ph, hopane and sterane distribution are dierent in the saturates from dierent solvent extractions (Tables 4±6).
CPI and OEP have been proposed to represent the maturity of petroleum and source rock extracts (Bray and Evans, 1961; Scalan and Smith, 1970). The ratio of Pr/Ph has been considered as an indicator of sedimen-tary environments but other possible sources of Pr and Ph are tocopherols and archean lipids (Brooks et al., 1969; Powell and McKirdy, 1973; Didyk et al., 1978). Sedimentary environments may be reducing if the ratio of Pr/Ph is less than 1, and may be oxidative if it is greater than 1 (Didyk et al., 1978). Although not very sensitive to maturity, the Pr/Ph ratio will increase (Alexander et al., 1981) and the ratio of Pr/C17and Ph/
C18 will decrease (Haven et al., 1987) as maturity
increases. The gammacerane index, which was ®rst con-sidered to be an indicator of lacustrine environments, has been extended to hypersaline/evaporite deposits, but has also been shown to be maturity sensitive (Rohrback, 1983; Moldowan et al., 1985; Fu Jiamo et al., 1986; Moldowan et al., 1992). Hopane and sterane stereo-chemistry has been used to indicate the maturation, migration and source of petroleum (Seifert and Moldo-wan, 1978; Mackenzie et al., 1980).
The CPI and OEP values in some CS2/NMP extracts
(D-6 and W8-2) are greater, while in other CS2/NMP
extracts (D-22, T-29, ZX22-1 and To-16), these values are lower than that in the corresponding CHCl3
extracts. Pr/Ph values in CS2/NMP extracts are greater,
and the ratios of Pr/n-C17 and Ph/n-C18 are smaller,
than that of the CHCl3extracts.
In the CHCl3 extracts of D-6, W8-2 and ZX22-1,
gammacerane is the most abundant compound in the m/e
Table 1
Organic geochemical data of immature source rock concentrates
No. Sample C% H% O% H/C O/C Ea Va Type Ro (%) depth (m)
1 D-6 48.05 6.61 4.49 1.65 0.07 96 4 I 760 2 WS8-2 39.13 5.08 4.95 1.56 0.09 97 3 I 0.30 1235 3 D-22 49.29 6.10 7.52 1.49 0.19 I2 1338
4 To-12 61.24 7.15 5.75 1.40 0.07 I 1625 5 T-29 27.14 3.05 4.30 1.34 0.12 93 7 I2 0.38 1976
6 ZX22-1 33.12 4.49 6.21 1.63 0.14 97 3 I 0.40 1520 7 To-16 55.26 6.42 5.31 1.43 0.07 96 4 I 0.605 2312 8 L-15 52.61 5.92 3.58 1.35 0.05 91 9 I 0.588 2430 9 L-36 32.97 3.53 3.78 1.28 0.09 92 8 I2 0.609 2637
10 G-60 50.21 5.56 6.21 1.33 0.09 94 6 I2 0.614 2155
11 To-20 45.13 4.66 5.58 1.04 0.09 96 4 II1 0.642 2570 a E, exinite; V, vitrinite.
Table 2
Extraction yields of source rocks from dierent solvents No. Sample Extraction yield (mg/g C)
CHCl3 MAC CSs2/NMP
1 D-6 370.8 505.9 728.6 2 WS8-2 142.1 251.2 477.6 3 D-22 126.8 274.2 462.6 4 To-12 139.5 202.2 386.5 5 T-29 98.4 163.6 517.7 6 ZX22-1 258.2 375.4 642.4 7 To-16 163.1 295.3 743.9 8 L-15 55.1 133.4 497.2 9 L-36 119.8 253.9 613.9 10 G-60 203.7 295.6 580.4 11 To-20 53.4 142.9 468.9
Fig. 1. Extraction yield of the source rocks with solvents of CHCl3, MAC and CS2/NMP. The number in the ®gure is the
191 mass fragmentogram with C30abhopane the second.
In contrast, in the m/e 191 mass framentogram of the CS2/
NMP extracts, the most abundant component is the C30ab
hopane. The ratios of the C30abhopane/gammacerane in
the CHCl3extracts are 0.93, 0.89 and 0.71, respectively.
These values increase from 1.77 to 1.90 and 2.11 in the corresponding CS2/NMP extracts, and the gammacerane
index decreases signi®cantly. The ratios of hopanes C31to
C3422S/(22S+22R) increase, and the ratios of hopane C29
bb/(bb+ab+ba) decrease, in the CS2/NMP extracts.
Compared with CHCl3extracts, the gammacerane index
and the ratio of C29bb/(bb+ab+ba) hopane in CS2/NMP
extracts of T-29, To-16, D-22 and G-60 decrease, and the 22S/(22S+22R) ratio of C31,C33and C34hopane increases.
Table 3
Bulk composition of the extracts using dierent solvents
No. Sample Solvent Saturate (mg/gC) Aromatic (mg/gC) Resina(mg/gC) Asphaltene (mg/gC)
1 D-6 CHC3 60.00 46.09 142.35 122.36
MAC 64.20 62.28 152.74 226.61 CS2/NMP 76.72 68.78 182.52 400.60
2 WS8-2 CHCl3 47.12 15.70 42.74 36.53
MAC 58.38 18.89 53.86 120.08 CS2/NMP 84.74 59.80 85.88 247.18
3 D-22 CHCl3 9.95 6.06 19.48 91.31
MAC 20.23 12.42 36.68 204.82 CS2/NMP 74.80 15.63 61.71 310.43
4 To-12 CHC13 21.84 11.60 28.08 78.04
MAC 23.85 16.78 53.77 107.75 CS2/NMP 85.88 23.19 89.52 187.92
5 T-29 CHCl3 20.09 9.00 25.64 43.65
MAC 28.53 14.17 36.60 84.30 CS2/NMP 66.42 16.88 102.04 332.36
6 ZX22-1 CHCl3 56.37 52.06 78.30 71.48
MAC 71.44 59.20 90.06 154.71 CS2/NMP 102.78 66.42 120.90 352.29
7 To-16 CHCl3 16.42 9.85 53.07 83.71
MAC 33.40 16.60 81.42 163.91 CS2/NMP 112.63 38.54 107.95 484.83
8 L-15 CHCl3 8.29 4.77 10.98 31.08
MAC 12.41 9.26 25.12 86.64 CS2/NMP 79.31 17.25 44.60 356.07
9 L-36 CHCl3 33.59 14.56 33.59 38.08
MAC 39.76 19.27 49.28 146.84 CS2/NMP 122.78 34.93 91.90 364.28
10 G-60 CHCl3 43.36 14.73 23.23 122.43
MAC 49.39 24.32 38.66 183.19 CS2/NMP 76.14 28.67 77.65 397.89
11 To-20 CHCl3 12.94 7.77 14.89 17.80
MAC 21.30 14.11 39.06 68.46 CS2/NMP 46.89 23.40 60.20 338.38 a By dierence. Resin=extraction yieldÿ(saturate+aromatic+asphaltene).
The sterane parameters of CHCl3 and CS2/NMP
extracts are also dierent. As seen in Table 6, all the values of C27and C29aaa(S/S+R), (S+R)bb/(bb+aa)
and the C29percentage in the C27, C28, C29distribution
in the CS2/NMP extracts are greater than in the CHCl3
extracts. The increase of these values diers with the samples. The C29 aaa(S/S+R) ratio in CS2/NMP
extracts of T-29, D-6, To-16 and D-22 is 5.73, 5.30, 4.49 and 4.42 times of that in CHCl3extracts, which increases
from 0.06, 0.05, 0.10 and 0.03 in CHCl3extracts to 0.34,
0.25, 0.45 and 0.12 in CS2/NMP extracts respectively. The
C27aaa(S/S+R) ratio in CS2/NMP extracts of To-16,
T-29 and D-22, C27and C29(S+R)bb/(bb+aa) in all
the CS2/NMP extracts also increase signi®cantly.
These biomarker parameters in the CS2/NMP extracts
thus point to a higher maturity than in the CHCl3
extracts. These results therefore reveal that care should be taken when we compare the biomarker parameters in source when using dierent extraction solvent.
Compounds released upon extraction are linked to the macromolecules via non-covalent bonds. The above results indicate that such bonds are more abundant than previously thought. Moreover, dierent kinds of non-covalent linkages must be considered in these macro-molecules. Some of these linkages are so strong that CHCl3and/or MAC cannot disrupt them. Some of the
CHCl3-soluble components in the mixed solvent extracts
have strong interactions with other macromolecules and cannot be extracted by CHCl3, or MAC, from the original
samples. After the mixed solvents have weakened these
Fig. 4. Bulk composition of the CS2/NMP extracts.
Table 4
Some alkane parameters of the saturates from CHCl3and CS2/NMP extractiona
Sample CPI(1) CPI(2) OEP(1) OEP(2) Pr/Ph Pr/C17 Ph/C18 D-6, A 0.90 0.78 0.78 0.70 0.06 0.81 4.74 D-6, B 0.97 1.06 1.12 0.85 0.28 0.23 0.38 W8-2, A 0.94 0.89 0.84 0.89 0.15 1.02 4.27 W8-2, B 1.15 1.008 1.15 0.88 0.81 0.34 0.37 D-22, A 4.76 4.38 3.52 5.43 0.05 1.17 4.05 D-22, B 1.25 1.21 0.71 1.17 0.80 0.24 0.30 T-29, A 2.09 1.71 1.41 1.64 0.31 0.42 0.84 T-29, B 1.32 1.23 1.17 1.01 0.71 0.31 0.37 ZX22-1, A 1.59 1.39 1.24 1.45 0.07 0.58 5.00 ZX22-1, B 1.43 1.30 1.17 1.11 0.88 0.38 0.40 To-16, A 2.22 2.41 2.40 2.86 0.09 0.59 2.55 To-16, B 1.23 1.08 1.05 0.95 0.48 0.52 0.43
a A, CHCl
3extracted; B, CS2/NMP extracted.
CPI 1 1 2
C25C27C29C31C33 C26C28C30C32C34
C25C27C29C31C33 C24C26C28C30C32
h i
CPI 2 2 C23C25C27C29 C222 C24C26C28C30
h i
OEP 1 C256C27C29
4C224C24
OEP 2 C256C27C29
4C264C28
interactions, they become soluble in CHCl3. The
mechanism of mixed solvent extraction is not clearly understood at present. Mae et al. (1997) argued, in their study of coals, that CS2has the ability to destroy the
electron transfer interactions in coal structure, hence to destroy the associated structure of the coal. NMP has the ability to destroy the hydrogen bonds in coal, and to dissolve the structural units having aromatic clusters as in the basic structure.
From the above results we conclude that there are complex interactions between the soluble molecules and the unextractable macromolecules in the source rock concentrates. The fact that CS2/NMP gives more extract
than does MAC and CHCl3, MAC giving more extracts
than CHCl3, is not only because the CS2/NMP can
dis-solve higher molecular weight fractions, but also
because it has the stronger ability to disrupt the complex interactions existing between solvent soluble and inso-luble fractions. High extraction yields by CS2/NMP
suggest that there are abundant non-covalent interac-tions in the macromolecule, and the associate structure is formed by interactions of these non-covalent bonds. The CS2/NMP insoluble macromolecules may be part of
a network structure interlinked with covalent bonds, but weaker forces such as hydrogen bonds, also, charge transfer and polarization forces may exist extensively in the soluble molecules and between the soluble and insoluble macromolecules. The associate `network' is formed by these molecular interactions. Extractable biomarkers may be trapped in these associate networks. The biomarker distribution in the associate networks is uneven, resulting in the dierent values of these
Table 6
Sterane parameters of some saturated hydrocarbons from CHCl3and CS2/NMP extractiona
Sample aaa(S/S+R) (S+R)bbb/(bb+aa) Sterane distribution
C27 C29 C27 C29 C27(%) C28(%) C29(%)
D-6, A 0.15 0.05 0.09 0.23 58.47 20.92 20.61 D-6, B 0.20 0.25 0.12 0.31 47.73 22.43 29.84 W8-2, A 0.27 0.28 0.06 0.22 34.61 26.80 38.59 W8-2, B 0.27 0.30 0.10 0.26 30.51 25.31 44.18 D-22, A 0.21 0.03 0.04 0.22 16.72 18.82 64.46 D-22, B 0.32 0.12 0.25 0.26 15.54 18.20 66.26 T-29, A 0.24 0.06 0.10 0.24 27.33 43.67 29.00 T-29, B 0.42 0.34 0.34 0.37 28.94 23.23 47.83 ZX22-1, A 0.22 0.12 0.03 0.19 53.93 21.96 24.11 ZX22-1, B 0.26 0.31 0.12 0.34 40.78 23.82 35.40 To-16, A 0.22 0.10 0.14 0.22 20.36 34.34 45.30 To-16, B 0.45 0.45 0.39 0.42 30.66 22.22 47.12
a A, CHCl
3extracted; B, CS2/NMP extracted.
Table 5
Hopane parameters of some saturated hydrocarbons from CHCl3and CS2/NMP extractiona
Sample GI 22S/22S+22R) C29 C30 C29/C30
C31 C32 C33 C34 C35 [bb/bb+sb+ba)] (ba/ab)
D-6, A 0.54 0.44 0.49 0.55 0.52 0.53 0.20 0.11 0.15 D-6, B 0.24 0.54 0.54 0.61 0.55 0.51 0.07 0.09 0.36 W8-2, A 0.40 0.56 0.55 0.53 0.48 0.46 0.11 0.18 0.41 W8-2, B 0.20 0.54 0.59 0.60 0.50 0.44 0.09 0.11 0.44 D-22, A 0.11 0.40 0.63 0.60 0.49 1.00 0.29 0.16 0.39 D-22, B 0.13 0.55 0.59 0.61 0.64 ± 0.11 0.14 0.35 T-29, A 0.08 0.36 0.58 0.41 0.50 0.35 0.19 0.15 0.40 T-29, B 0.06 0.56 0.60 0.62 0.65 1.00 0.10 0.14 0.39 ZX22-I, A 0.62 0.45 0.50 0.44 0.46 0.44 0.32 0.17 0.36 ZX22-1, B 0.20 0.58 0.61 0.53 0.59 0.51 0.12 0.08 0.42 To-16, A 0.24 0.43 0.81 0.37 0.40 0.33 0.41 0.11 0.18 To-16, B 0.07 0.56 0.50 0.63 0.82 1.00 0.05 0.09 0.49
a A, CHCl
parameters in the extracts when using dierent solvent extraction.
Recently, signi®cant contributions of noncovalent interactions in coal macromolecules such as hydrogen bonding (Larsen et al., 1985), p±p interactions and charge transfer interactions (Sanokawa et al., 1990; Nishioka et al., 1991; Liu et al., 1993) have been reported. Physical cross-links by locally cooperative non-covalent interactions for the formation of three-dimensional cross-linked structures have been indicated in synthetic
and biological polymers. A recently proposed model considers coal to be an associated structure, held together by non-covalent bond interactions acting in some unde-®ned cooperative manner (Nishioka et al., 1991; Nish-ioka, 1991a,b, 1992; Takanohashi et al., 1995). Cody et al. (1993) suggested that coal is indeed a macromolecule but the network consists essentially of entangled rather than covalently linked chains. This view was based on some seminal and very important experiments on the mechanical response of swollen coal particles. Such a
model is somewhat dubious because it would suggest that there should be a solvent that could completely dissolve coal, but no such solvent has been found. There are dierent kinds of non-covalent bonds in coal, such as hydrogen bonds (Painter et al., 1987), charge transfer interactions (Nishioka, 1991a), and van der Waals forces due to p±p interactions (Nishioka and Larsen, 1990), etc. Breaking such non-covalent bond interactions must be properly considered to maximize the solubility (extractability) of the source rock concentrates and coal. High covalent bond cross-linking density, and abundant relatively strong secondary interactions, will result in diculty of extraction.
Although these non-covalent bond interactions are individually much weaker than covalent ones, their cumulative eects have a great impact on the properties of macromolecules. Such associated `network' structures may dissociate under lower thermal stress, contributing to immature oil formation.
Although kerogen is often considered to consist of solvent-insoluble, covalently cross-linked networks, and solvent-soluble, low molecular weight substances trapped
in the network, several structural models have been proposed for dierent types of kerogens (Burlingame et al., 1969; Yen, 1976; Spiro and Kosky, 1982; Behar and Vandenbroucke, 1987; Faulon et al., 1990). The model proposed by Behar and Vandenbroucke (1987) is the most representative among them. Most structural models proposed for kerogens only consider the covalent bond structures within the macromolecules, neglecting the non-covalent bond interactions. According to our CS2/
NMP extraction results (Table 2), more than 600 mg/g TOC extracts are obtained from the concentrates of D-6, ZX22-1, To-16 and L-36. Such high extraction yields indicate that the network in immature source rock con-centrates (exinitic kerogen) may consist of covalent bond structures and non-covalent cross-links such as hydrogen bonds, charge transfer interactions, van der Waals forces andp±p interactions. The contribution of non-covalent bonds should be quite important, since extracts are soluble in the solvent while the network, cross-linked by covalent bonds, is thought to be inso-luble. The nature of the cross-links in kerogen is a key factor for the elucidation of kerogen structure, indeed
there are a lot of hetero-atoms and polar functional groups occurring within the solvent soluble molecules and insoluble macromolecules. Dierent kinds of non-covalent bonds may form the associate structures between these polar functional groups.
The concept of an `oil window' has been widely accepted by petroleum geochemists, the threshold of which corresponds directly to the energy required for the disruption of covalent C±C bonds which, organic macromolecules, is nearly 340 KJ/mol. The energy required for cracking the covalent bonds containing hetero-atom such as C±S and C±O may be somewhat lower, but their magnitudes are of the same order (Qin et al., 1997). Non-covalent bonds are much more easily broken. For example, hydrogen bond energy is gen-erally <35 KJ/mol, much lower than the covalent bonding energy. The amount and distribution of these non-covalent bonds depend on the chemical nature of the source rock concentrates and will have a great impact on maturation. Dissociation of the associate structures in sedimentary organic matter may happen under lower thermal stress, resulting in the formation of
immature oil in the early stages of evolution. Oil generation, from the dissociation of associate structures, will start before the threshold of the so-called `oil window'.
CS2/NMP extraction diers from MAC and CHCl3
extractions, not only in the size of yields, but also in the composition of the extracts. As the CS2/NMP extraction
yield increases, all the saturate, aromatic, resin and asphaltene fractions increase simultaneously; this is especially so for the asphaltene fraction (Table 2). Because no signi®cant chemical reaction has happened during CS2/NMP extraction, the increased non-polar
hydrocarbons in these extracts when compared with the CHCl3 extracts, should be trapped in the association
network of the macromolecules. Resins and asphaltenes contain abundant hetero-atoms, association may be formed within these fractions themselves and between these fractions and insoluble macromolecules.
As minerals are the matrix in which sedimentary organic matter is deposited, molecular interactions between minerals and organic molecules have to be included in the concept of composite structures.
4. Conclusion
The structure of organic matter in source rocks depends on covalent cross-linking and associate networks formed by non-covalent cross-links such as hydrogen bonds, charge transfer interactions, van der Waals forces and p±p interactions. High yields from CS2/NMP extraction suggest
that the contribution of covalent bonds in immature source rock concentrates is probably relatively small. The solubi-lity, or extractabisolubi-lity, of the organic matter in source rocks and the composition of the soluble matter depends upon the distribution of non-covalent bonds as well as the dierent kind of solvent used.
Care should be taken when we compare biomarker parameters in source rocks when using dierent solvents for extraction. Extractable biomarkers may be trapped in the non-covalent bond associate networks. The bio-marker distribution in the associate networks is uneven, resulting in dierences of these parameters in the extracts when using dierent solvents
The nature of the associate structures in sedimentary organic matters may have a great impact on maturation
and contribute to the formation of immature oil. Limited data only about non-covalent bond interactions have been so far presented in literature. The type and dis-tribution of non-covalent bonds in immature source rock concentrates are under investigation.
Acknowledgements
This work was supported by the National Science Foundation of China (No. 49672126) and the Fundamental Research Foundation of University of Petroleum (No. ZX9701).
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