biomarker compounds as indicators of paleoenvironments

Vol. 11 No. 1 CHINESE JOURNAL OF G E O C H E M I S T R Y 1992

Biomarker Compounds as Indicators of Paleoenvironm ents

F u JIAMO (~,J'~i/A~), SHENG GUOYING ( ~ [ ~ ) , XU JIAYOU (~:~TJ~)

JIA RONGFEN ( ~ : ) , FAN SHANFA ( ~ ) , PENG PING ~ q ( ] ~ )

( Institute of Geochemistry, Academia Siniea, Guangzhou ,510640 )

G.EGLINTON AND A. P . GOWAR ( Organic Geochem~try Unit, University of Bristol, U .K • )

Abstract

In an attempt to assess the paleoenvironments of terrestrial sediments, some twenty-two representa- tive Chinese non-marine sediment samples were studied using the. molecular organic geochemistry method. The sediment samples studied include oilshale, shale, mudstone and glauber salt from Tertiary to Cretaceous in age. Judging from geological/geochemical data and paleosalinity data, the samples studied are of lacustrine sedimentary origin and can be divided into three different types: fresh water, brackish and saline /hypersaline lake sediments. The aliphatic fractions were separated from the extracts of the samples and analysed by means of GC and GC /MS instruments, giving a number of parameters such as relative abundances of aikanes and cycioalkanes as shown in the mass chromatograms •

The origin of organic matter in crude oils and related sediments has long re- ceived considerable attention of petroleum geochemists. In recent years the distribution characters of biomarker compounds have been successively applied in paleoenvironment assessment, particularly in distinguishing between marine and terrestrial oils and oil-generating rocks. In some cases such information may be help- ful to subdivision of the sedimentary environments into distinct types (freshwater, hypersaline lakes or marine carbonates or deltaic environments) .

Biomarker compounds in a series of sediments collected from terrestrial basins in eastern China were studied in this paper by using molecular organic geochemical techniques so as to shed light on their paleoenvironment characters.

Samples and Their Geologic Settings

M aj or oil occurrences discovered up to now in China are predominantly of terrestrial origin, ranging from Carboniferous to Eogene in age. The oil-generating rocks were studied and compared with respect to their types and hydrocarbon pro- duction potential based on characteristics of the sedimentary basins from which they were formed (Fu Jiamo and Sheng Guoying, 1986).

In terms of salinities of the depositional basins, the terrestrial oil-generating rocks in China can be assigned to lacustrine sediments formed in fresh, subbrackish and hypersaline waters. Particular care was taken to guarantee fresh- ness of the samples (in most cases drill cores were used)and to make sure that

* This project was financially supported by the National Natural Science Foundation of China.

2 CHINESE JOURNAL OF GEOCHEMISTRY Vol.ll

they have approximately the same degree of maturity and show no sign of supergenic biodegradation.

Sample Nos . 1 - 9 are fresh water sediments, such as oil-generating rocks of deep water and shallow water bog facies from the Damintun Depression in the Liaohe Basin. Green algae characteristic of fresh water environments are the dominant algal species in the deep water sediments.

Sample Nos. 1 0 - 1 7 are oil-generating rocks from the Songliao Basin formed in fresh to subbrackish waters. Paleosalinities range from 0. 5 - 5 . 0 %0, slightly higher than those of fresh water , and may come up to 5. 0--10. 0 %0 for deep-water sediments. Glauconite was found in Cretaceous sediments in the Songliao Basin. According to Yang Wanli et a l . ( 1 9 8 5 ) , the glauconite is an authigenic product formed in subbrackish lakes, instead of a marine mineral (Yang Wanli et a l . , 1985).

Sample Nos . 1 8 - 22 were formed in subbrackish or hypersaline lakes. For example, the hypersaline character of oil-generating rocks in the Qianjiang Basin is evidenced by the common presence of gypsum and halite. Considerably high abundance of chlorine ( 3 - 7 %0 ) is found in pelic rocks. Some salty mudstones may contain as much as 2 0 - 6 0 %0 chlorine and therefore they are almost devoid of any organic fossils (Jiang Jigang, 1982).

Experiments and Analytical Method

The samples were thoroughly cleaned before pulverized. Then organic matter was Soxhlet-extracted with benzene-methanal. The extracts were separated into alkanes, aromatic hydrocarbons and polar fractions, followed by GC and C-GC-MS analyses. The compounds were identified by comparing with standard diagrams or available data in literature. The calculations of molecular organic geochemical parameters were based on GC-MS fragment ion data obtained under identical laboratory conditions. N-alkane parameters were calculated from m R 85. Ratios of isoprenoid alkanes and fl-carota.nes to n-alkanes were obtained from RIC diagrams. Parameters of regular steranes and 4:me-steranes were based respectively on m/~ 217 and m,~ 231 GC-MS and those of hopanes and ~,-cerane on m/z 191. For detailed information on the laboratory conditions ,please refer to Fu Jiamo (1990) .

Results

Molecular parameters, including those of acyclic and cyclic alkanes, which are dependent on source and sedimentary environment, are presented in Tables 1 and 2. All of them were calculated from the peak areas and the relative intensities as measured from GC-MS diagrams. The details are given in Table 3.

Fresh water lacustrine sediments

These sediments were collected from the Langgu Depression, the Liaohe Basin, the North Jiangsu Basin and the Maoming area. With the exception of M aoming oilshales which are represented by outcrop samples, all the others are from drill cores of Eogene age. High odd carbon number predominance is noticed in these samples, with CPI values lying between 1.1 and 2 . 9 . The nC3~/nC17

No.1 C H I N E S E J O U R N A L O F G E O C H E M I S T R Y 3

ratios are, with a few exceptions, greater than unity with a maximum of 6 .8 . Major carbon peaks are at nC27 o r nC29 . Except in rare instances, pristane~hytane ratios greater than , or close to, unity characterize the distribution of isoprenoids. In most c a s e s , P r / h C l 7 and Ph/hCls ratios are greater than unity. Minor iC25 and iC30 compounds were detected in a few samples. No fl-carotane was found (Table 1) .

The most characteristic feature of biomarkers in these sediments is the distribution of steroids and terpenoids. Abundant 4-me-sterane is striking in most samples and 4-me-sterane index as high as 3.4 is noticed in some individual samples. The hopaneAterane ratios are high, generally between 3 and 6 , with a maximum of 12.8 noted in sample No. 6. As shown by m/~ 191 GC:MS, the C34 and C35 homohopanes are low or even beyond the limit of detection (Table 2 ) .

Table 1 . N-aikane and isoprenoid a~ane parameters *

~ample Envi- N-alkane Pr Ph iCts iC25+C30 No. ron- main peak C31/Ct7 CPI Pr/Ph Pr/nCl7 Ph/fiCz8 (%) (%) < ( % ) (%)

merit

i 1 nC29 1.29 2.13 0.23 1.55 11.10 13.16 75.25 11.59 0.00 2 1 nC23 ;nC27 0.86 1.76 0.78 1.70 2.83 30.28 50.10 19.62 0.00 3 1 nC25 ;nC27 0.45 1.63 1.34 1.24 1.26 38.35 35.37 26.29 0.00 4 1 nC25;nC27 1.79 1.13 0.47 1.23 2.16 19.94 55.87 24.19 0.21 5 1 nC27 ;nC29 2.67 2.37 0.43 1.97 3.55 24.78 65.94 9.29 0.00 6 1 nC21 ;nC23 1.01 1.3l 1.47 1.76 0.71 42.52 35.26 22.22 0.00 7 1 nC27 2.48 2.28 1.20 3.06 2.94 38.21 37.07 24.72 0.00 8 1 nC27 ;nC29 6.81 i.13 0.81 3.17 2.61 33.52 51.95 14.53 0.25 9 1 nC27 1.13 2.88 1.22 2.03' 2.00 41.62 43.11 15.27 0.00

10 2 nCJ7 0.19 1.46 1.37 0.73 0.81 41.88 30.67 27.46 0.00 11 2 nC23 ;nC25 0.82 1.07 1.11 0.38 0.28 35.24 31.61 33.15 0.00 12 2 nCl9 ;nC25 0.31 1.09 1.13 0.15 0.11 30.65 27.02 42.33 0.00 13 2 aCt.9 ;nC25 0.44 1.08 1.39 0.39 0.27 39.02 21.74 39.23 0.00 14 2 nC2i ;nC27 0.70 1.16 1.79 0.31 0.15 25.88 38.87 35.25 0.00 15 2 nC23 0.80 0.95 0.89 0.42 0.47 32.49 30.37! 37.15 0.03 16 2 nC23 0.49 1.21 1.52 0.72 0.41 39.09 25.79 35.12 0.00 17 2 nC21 ;nC27 0.56 1.16 1.73 0.91 0.47 40.02 23.23i 36.75 0.00 18 3 nC30 ;nC~0 1.54 1.60 0.53 1.18 2.02 26.63 57.54 15.83 0.1 19 3 nC24 ;nC28 1.08 1.03 0.32 2.42 5.84 17 .41 68.23 14.36 0.06 20 3 nC,s ;nC2s 0.54 0.85 0.28 1.70 3.13 10.20 85.71 4.10 0.55

74.66 0.19 21 3 nCls 0.70 1.15 0.18 0.72 2.78 15.49 ! 9.86 22 3 nC24 ;nC22 0.14 0.58 0.15 1.79 4.55 8.98' 83.40 7.62 0.07

#/aC17

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.04 0.00 0.03 0.03

* For calculations and implications of the parameters, see Table 3.

Subbrackish lacustrine sediments

Sediments of this type are Cretaceous mudstones collected from the Songliao Basin. N-alkanes show low carbon number predominance with insignificant even-odd predominance and less-than-unity nC3t / n C t 7 . Pr /Ph ratios are greater than unity while Pr/nC17 and PhAaCls ratios are apparently low. No iC25 and iC30 isoprenoid alkanes and fl-carotane were discovered (Table 1 ) .

In most of the samples rearranged steranes are commonly contained but 4-me-steranes are absent. The regular steranes are composed of C27, CES and C29 compounds, with dominant C27 in some samples and C29 in few cases.

4 C H I N E S E J O U R N A L O F G E O C H E M I S T R Y V o l . l l

~ample[ No.

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18 19 20 21 22

Envi- ron- ment

1 0.00 1 0.00 1 0.05 1 0.19 1 0.00 1 0.00 1 0.00 1 0.20 1 0.04 2 0.15 2 0.71 2 0.05 2 0.27 2 0.27 2 0.05 2 0.44 2 0.29 3 0.04 3 0.00 3 0.00 3 0.05 3 0.00

Table 2 . Sterane and terpane p a r m t ~ r s

Rearranged 4-me- Regular sterane (%) 4-me-sterane (%) Hopaue/Homohopa- sterane sterane index index C27 C2s C29 C2s C29 C~0 Sterane ne index

3.43 35 19 46 27 16 57 4.21 0.40 34 21 45 52 12 36 2.99 0.59 34 28 38 36 20 14 3.70 n.d. 22 27 51 n.d. n . d . n . d . 1.27 n.d. 34 29 37 n.d. n . d . n . d . 3.44 0.24 25 33 42 32 9 59 12.78

27.35 38 19 42 n.d. n . d . n . d . 7.15 n.d. 30 26 44 n.d. n . d . n . d . 6.10 0.81 32 15 53 36 18 47 5.01 0.22 38 22 40 30 24 30 2.85 n.d. 62 23 15 n.d. n . d . n . d . 16.75 0.21 55 14 31 n.d. n . d . n . d . 5.13 m.d. 36 25 38 n.d. n . d . n . d . 9.14 n.d. 54 21 25 n.d. n . d . n . d . 29.38 0.45 43 18 39 36 18 46 1.81 n.d. 37 22 42 n.d. n . d . n . d . 4:96 n.d. 57 15 28 u.d. n . d . m . d . 6.07 0.18 31 24 45 13 41 45 0.81 0.03 38 21 41 11 46 43 0.41 0.02 25 20 55 12 54 34 0.17 0.06 9 37 54 11 38 52 0.57 0.03 36 16 47 15 29 56 0.20

parameters, see Table

Gammaceran index

0.00 0.00 0.89 0.00 0.00 0.00 0.00 0.46 1.10 0.00 0.71 0.00 0.00 0.00 0.57 0.48 0.00 0.26 0.77 0.00 0.54 0.00 0.89 0.11 0.41 0.00 0.62 0.20 0.71 0.17 0.65 0.44 0.27 0.12 4.20 0.30 1.39 1.55 0.84 2.65 2.60 0.19 2.77 0.51

* For calculations and implications of the 3.

Table 3 • Bioatarkera : ctdetfltfion aml exphmation of their p~ttmeters

Parameter Description Calculation Indication General explanation Reference

Major The most RIC or Organic input Ci7 main peak indicates Brasscl1,1978 peak abundant m/z 85 algal input . Single peaks Tissot et a l , 1977 carbon n-alkane of C27 ,C29 or C3t imply Caldieoff and Eglinton,

higher plant input. 1973 Bimodal distribution is interpretated a s mixing S O U r C e S •

C3I/CI7 n - C31/n- CI7 m/z 85 Organic input ; values less than 0.5 indicate ex- tensive algae input while values greater than 2 reflect higher plant input •

Generally, algae are rich in n-C17 and higher plants contain abundant n-C31 •

Brassell et a l , 1978 Gelpi et a1,1970 Eglinton et d ,1962

Pr/Ci7 Pristane / RIC n- Cl7

Values greater Pristane is thought to Blumer et a l . 1971 than 1 imply be of algal input,gen- Blamer and Snyder, algal source .But erally originated from 1965 the value may be chlorophyll and dehydro Forsberg and Bjory, reduced with vitamin E .The parame- 1983

ter may be influenced Mackenzie et a1,1983 inc~asing degra- by maturity and the dation, rearrangement of Leytheauscr and

hydrocarbons. Sehwarzkopt, 1986

N o . l C H I N E S E J O U R N A L O F G E O C H E M I S T R Y 5

Table 3 Continued


Ph/Cl8 Phytane / RIC Organic input; High ratios are always Risatti et a1,1984

n- Cl8 values> 1 are indicative of strongly Didyk et a1,1978 regarded as to reducing environments. be of ancient bacteria source

Pr/Ph Pristane/ RIC Ratios less than The parameter is source- Blumer and Snyder, Phytane 1 indicate ancientdependent .Pr and Ph 1965

reducing environ- may have been derived Prahl et al, 1985 Didyk et al, 1978

ments, l~om aquatic organisms Pratt et a/,1986 and bacteria in addition Ten Haven et a1,1985 to chlorophyll. Powell and Mckirdy,

1983

iC2~+iC30 The percentage m/z 85 Organic input: of C2s and C30 the two compoun- isoprenoids in ds are thought relation to total to be methanoge- isopresoids nic.

6, i 0,15,19-pentamethyi eicosane is methanogenic.

Brassell et al, 1981 Risatti et al, 1984 De Rose et at, 1983 Waples et a1,1974

/fiC~7 fl- carotane / RIC Environment Abundant fl- carotane is Hall and Dauglas, 1983 nCi7 favored by reducing or Jiang and Fowler,1986

hypersaline environments

(227 / T EC27/E Sterane Organic input ; Maturity dependent Seifert et al , 1983 C2s / T Y~C28/E Sterane m/z 217 algal input is Williams and Dauglas, C29/T E C29/r. Sterane indicated by 1983

large C27/T. Huang and Meischein, C29/T ratio is 1979 a criterion of higher plant Volkman, 1986

input or other algal species

20S 20S% C29 ~ t ~ - - m/z 217 Increasing with 20S and ct /~ fl

20S+ 20R increasing ma- increase with tiff% C29 ~ #fl 20 (S+ R) m/z 217 turity .Equili- increasing maturity

aflfl+ ~tcta 20 (S+ R) brium value 55%

Mackenzie et al, 1980

4-me-index EC30-4-methyl/ m/z 231/ Organic input; 4-me-steranes are Robinson et a1,1984 1229 (20S+20R) m/z 217 4-me-sterane is diagenetic products Wolff et al, 1986a

contributed by of 4-me-sterol con- Woltt et al, 1986b sterane methanogenic centrated in methano-

algae, genic algae. Boon et al, 1979

Rearranged 20R+ 20SC27 Under diagenetic Clay may have acted sterane rearranged m/z 217 conditions high as a catalyzer for the index sterane ratios may be in- rearrangement of

dicative of an steranes to form 20R+ 20SC29~aat environment of rearranged steranes •

sterane abundant clay, Also maturity- or , otherwise, dependent. carbonate environment.

Machkenzie et al , 1982

Rubinstein et al , 1975

Rullkotter, 1983

Hopane / C30~fl hopae / m/z 191 / High ratios The ratio is sensitive to HotSnan et a1,1984 Sterane ~C29 sterane m/z 217/ (> 10) indicate the relative proportion Mackenzie et a1,1982

abundant of algae and terrestrial Cassani and Eglinton, organisms. It may also

terrestrial inputs, be affected by bacterial 1986 reworking. Moldowan et al, 1985

#~At# #a moretane / m/z 191 Maturity May also be affected by Ensminger et a1,1977 a // hopane source material. Seifert and Moldwan,

1980

6 C H I N E S E J O U R N A L O F G E O C H E M I S T R Y Vol. I1

Table 3 Continued


22S% C32~/~22S/22S+ m/z 191 Maturity ;varia- The 22R biological stru- Mackenzie et a1,1980 22R tion range: cture would be changed Ensminger et a1,1977

0 - 6 0 into the more stable 22S structure with increasing maturity.

Gammacer- Gammacerane/ m/z 191 A source indi- Recognizable in all en- Hills et a l . , 1966 ane index ~tflC30 hopane cator vironments but high Brassell and Egtinton,

abundances are found 1983 in brackish and subbra- Shie t a1,1982

ckish basins. Fu et al,1986, 1982

Homohopa- C35 (22S+ 22R) ne index homohopane /

C32 (22S+ 22R) homohopane

m/z 191 High ratios are The distribution of C35> Ten Haven et ah

associated with C34>C33 is commonly 1985,1988 hypersaline noticed in hypersaline Fu et ah 1986,1988 sediments, environments. Brassell et a1,1988

Mello et a1,1988

Hopane/~terane ratios are high, generally in excess of 5, and may be as high as 29.4 in extreme cases. The homohopanes are composed of compounds from C3a to C35 (C36 was detected in a few samples), but no C3~>C34>C33 distribution was observed. Gammacerane was commonly found but in small amounts (Table 2 ) .

Hypersaline lacustrine sediments

Drill-core samples of Eogene age collected from the Qianjiang Basin and the Central Hebei Depression were studied. Odd or even carbon number predominance is observable in n-alkane distribution. As a striking feature of isoprenoid constitution, phytane is dominant , constituting the major peaks in alkane RIC diagrams. Pr /Ph ratios are between 0.2 and 0 .5 , with the content of phytane be- ing generally in excess of 60% among the isoprenoid alkanes., iC25 and iC30 isoprenoids were found in all the samples examined and fl-carotane was detected in some samples (Table 1) .

No rearranged steranes were found. C29 predominance is significant in regular steranes. The hopane/~terane ratios are very low (less than uni ty) . The distribution characterized by C35>C34>C33 is apparent for the homohopanes (C31-C35) in m/~ 191 GC-MS. The hopane index is , in most cases, greater than unity. An- other feature is the relatively high content of gammacerane which is apparently higher than C30 at fl hopane and may sometimes constitute the major peak in m/~ 191 GC-MS (Table 2 ) .

Discussion

Maturity of oil-generating rocks

Most of the samples studied range in maturity from a status close to the oil-generating threshold to the major oil-generating phase (liquid window ) , except in few cases where lower maturity is indicated.

No.1 CHINESE JOURNAL OF GEOCHEMISTRY 7

Paleosalinity of the lacustrine environments

Particular care must be taken to avoid interference due to maturity and biodegradation in an attempt to interpret the sedimentary environment on the ba- sis of biomarker information. The samples studied in this paper are all fresh and the hydrocarbons show no sign of biodegradation. Meanwhile, differences in maturity among these samples are insignificant, as evidenced by maturity parameters. In addition, the samples are approximately similar in lithology, composed of mudstone or shale, with salt minerals, sometimes mirabilite, present in sediments formed in salt lake environment.

Distribution and characteristics of biomarker compounds

1. N-alkanes Fresh water environments are characterized by high carbon number n-alkanes, with main peaks at nC27 and nC29 and apparent odd-even carbon number predominance, reflecting terrestrial inputs. Subbrackish basins are dominated by low carbon compounds, w i th nC17 as main peaks for some sam- pies, representing an abundant algal supply. N-alkanes in hypersaline environments are largely concentrated between nC,s-nC2s, showing an even carbon number predominance, with main peaks at nCn , nC24 or nC2s. The predominance of nC2~ and nC,~-nC24 may be attributed to the input of certain kinds of bacteria (Ten Haven et al., 1988) or algae (Tissot et a l . , 1977 ; Mackirdy et al., 1986) or may have been converted from pre-existing n-alkanes under strong reducing conditions in hypersaline circumstances (Sheng Guoying, 1980 ) .

2. Acyclic isoprenoid alkanes The pristane/phytane ratios are around unity (with few exceptions of lower values) in fresh water environments and are greater than unity in subbrackish environments. Sediments formed in hypersaline basins are characterized by extremely low pristane/phytane ratios, with phytane main peaks in most alkane RIC spectra. Such prominant predominance of phytane and the excessively low pristane/phytane ratio may reflect an input of methanogenic bacteria and saltophilic bacteria or a strong reducing condition in hypersaline environment. Hypersaline sediments always contain small amounts of 2 , 6 , 10, 15, 19-pentamethyl eicosane (iC25) and squalane (iC~). The former is thought to result from introduction of methanogenic bacteria while the latter may have also been caused by acidophilic and thermophilic bacteria (Brassell et a l . , 1981).

3. Terpenoids and tetraterpenoids Homohopanes have a wide range in composition (generally between C3, and C35, with C36 found occasionally) and low index in fresh water environments but a restrict composition range (C31-C33) and slightly higher index in brackish waters. Hypersaline waters are characterized by very high homohopane~ index, which may be attributed to diagenetic peculiari- ties or to some special kinds of bacteria or blue-green algae (Ten Haven et a l . , 1985; Fu Jiamo et a l . , 1986, 1988).

High hopane,~terane ratios are, in most cases, associated with subbrackish and fresh waters which seem to favor the formation of bacterial hopanes but not th~ reservation of steranes. As suggested by Maokenzie et al . (1982) , the high hopane/~terane ratios may also be related to the input of higher plants into

8 CHINESE JOURNAL OF GEOCHEMISTRY Vol.11

terrestrial fresh water basins. Gammacerane was found in all the environments, increasing with the salinity

of the water body, with the maximum content occurring in hypersaline environments, where gammacerane, instead of C30 • fl hopane, constitutes the main peak at m/~ 191. The gammacerane is considered to have been derived from primary, or saltophilic organisms. The presence of gammacerane in large amounts is now widely accepted as an indicator of hypersaline environments (Ten Haven et a l . , 1985, 1988; Fu Jiamo et a l . , 1986, 1988).

fl-carotane is present in small amounts in samples from hypersaline environments. However, significant amounts of fl-carotane and its derivatives were found in Carboniferous-Permian oil-generating rocks in the Dzungaria Basin, Xinjiang, which is thought to be related to extremely strong reducing conditions as a result of volcanic sedimentation.

4. Steroids No significant difference has been found in the distribution and composition of regular steranes from environments of different salinities. For all of them, C29 and C27 are the major constituents, indicating an input of higher plants and algae. What is worthy of special consideration is that fresh water sediments are conspicuous for their high contents of 4-me-sterane, although it can be always detected in subbrackish and hypersaline basins. 4-me-sterane is considered to be derived from methanogenic algae (Boon et a l . , 1979). Thus , it can be said that its prosperity would only be favored by fresh water environment. As shown in'Fig. 1, the 4-me-steranes in salt lakes, in contrast to those in the other two environments, are particularly rich in C29 and C30 compounds. This compositional difference may be indicative of different species of methanogenic algae in environments of different salinities.

C2s ( 100 % )

el "2

C29( 100% ) C3o(i00 0~ )

Fig.l. 4-rnc-stcrane distribution in sediments fl'om the different environments. I. Fresh water; 2. subbrackish water or flesh water; 3. saline or hypcrsaline water.

5. The thrcc types of lacustrine environment as indicated by biomarker compounds The principal biomarker parameters used in paleoenvironment


assessment and their implications are summarized in Table 3. As may be seen, the hypersaline environment can be readily distinguished from the other two by using these parameters as long as the sediments are free f rom biodegradation and are approximately similar in maturity. Major characters of hypersaline sediments arc: the excessively low Pr /Ph ratio, abundant gammacerane, abnormal distribution of homohopanes (C3~>C3,>C33), occasional presence of even-carbon number predominance in n-alkanes and the unique composition of 4-me-steranes (Fig. 1 ) . It i§ clear that samples from salt lakes are well separated from those from the other two environments, clustering on the left hand side in Fig. 2 or in the upper right part (high gammacerane and high homohopane) in Fig.3.

2.8

2 .4

~ 2 . 0

1.6 g~

~ 1 . 2

1 0 . 8

O 0.4

O

0 0. I

• 1 + 2 o 3

0 • +

÷ +

' _ - 'O.a5 * . * , ~. ' ~ ; ; r . j , : 0 . 3 0 . 7 " 0 .9 1.1 1.3 1 .5 1 .7

P r i s t a n e / P h y t a n e

Fig.2. Pr/Ph-gammacerane index diagram of sediments from the different environments. 1 . Fresh water; 2 . subbrackish water or fresh water; 3 . saline or hypersaline water.

2 .8

2 .4

~ 2 . 0

. 4

~ 1 . 6

E E

~ 0 . 8

0 . 4

• 1

+ 2

o 3

o o+ o

+4- o "1" -$ . . i . ~ t ~ t -

°o - - 1 - i Homohopane

Fig.3. Homohopane-gammacerane diagram of sediments from the different environments. 1 • Fresh water; 2 . subbrackish wateror fresh water; 3. saline or hypersaline water.

10 CHINESE J O U R N A L O F G E O C H E M I S T R Y Vol . l I

No apparent distinction can be made between fresh and subbrackish water sediments ,probably due to insignificant salinity difference between them. In general, sediments from fresh waters contain more abundant 4-me-steranes and have a wider compositional range with respect to homohopanes . These features, together with the n-alkane main peaks at C27 and C29 , the high odd carbon number predominance, high CPI value and slight C29 predominance in regular steranes, all reflect a terrestrial source.

Principal factor analyses based on normalized data from Tables 1 and 2 show that the accumulated characteristic value of the first three principal factors is

40

30

20

10

C n, 0

-10

-20

-30

-40 -40

4-

+ +

+

+

+ + + + •

+ O O

O O

Fig.4. 1. Fresh water ;

o

• 1

o + 2

o 3 o

0 0

o

' ' ' i o ' ' - 0 40 PCt

Pcrrc2 diagram of sediments from the d~rent environments. 2. subbraCkish water or fresh water; 3. saline or hypcrsaline water.

0 . 8

0 . 7

0 . 6

0 . 5

0 .4~

0 . 3

~-~ 0 . 2

O. 1

0

-0 . l

- O . 2

- 0 . 3

- 0 . 4

- 0 . 5 -0 . 5

Fig. 5 •

iCis-t8

• iC 19

w

C~7/ T

C30aft h opane//C29s

ICz8/ T

iC2o

Cz9 / T

-0' 3 1 0 .1 0 . 3 0 . 5 0 . 7 • - 0 I. • i i

PC~

Factor load diagram of sediments from the different environments.


approximately 74% (PC~ 43 .6%, PC 2 19 .6%) . Samples from the different environments can be well distinguished from each other in the PC~-PC2 diagram (Fig. 4 ) . As indicated by Fig. 5 , the relative abundance of straight chain isoprenoids, gammacerane index and homohopane index have a greater contribution to PCt while ratios of normal alkanes (such as nC27/nC~7, etc. ) exhibit greater load on PC 2 .

Concluding Remarks

As shown in this study, the biomarker parameters are of great value in estab- lishing paleosedimentary environments for terrestrial oil-generating rocks. The vari- ous parameters of straight chain and cyclic alkanes were defined and their implications as paleoenvironment indicators were discussed (Table 3 ) . From Tables 1 and 2 and Figs. 3 and 4 , apparent differences can be noticed between the oil-generating rocks formed in lakes of different salinities in eastern China. The PCfPC2 diagram based on PCA has proven to be particularly useful for distinction between sediments formed in the three environments.

Hypersaline mudstones can be easily distinguished from their fresh and subbrackish equivalents by biomarkers based on their characteristically high predominance of phytane, abundant gammacerane and abnormal distribution of homohopanes (homohopane index >> 1 ) . The difference between fresh water mudstones and those formed in subbrackish waters is less clear. Generally, evi- dence of terrestrial higher plant inputs is comparatively more obvious in fresh water sediments, as suggested by high carbon number compounds (C27, C~9) that constitute the main peaks of n-alkanes, the odd carbon number predominance and the dominance of C:9 st~rane.

Biomarkers can also be used in paleoenvironment assessment and genetic classi- fication of terrestrial oils in China. Such attempts have been particularly successful when the oils are approximately similar in maturity and free from biodegradation. Results of these studies will be published elsewhere.

Acknowledgements

This presentation has been benefited from the financial supports provided by the Academia Sinica and British Royal Association for the visiting tours of England for some of the authors. The GC-MS, which was indispensable in this study, was supported by the National Environment Research Committee (NERC) of Britain . The authors are also grateful to Pei Cunmin and Geng Ansong for their assistance in preparing the manuscript.

References Boon, J . J . , Rijpstra, W. I . C . , de Lange, F.~ de Leeuw, J . W. , Yoshioka, M. and Shimizu,

Y(1979) Black Sea a-sterol molecular fossil for dinoflageUate blooms. Nature, 277, 125-127. Brassell, S. C . , Wardroper, A . M . K . , Thomson, I . D . , Maxwell, J . R. and Eglinton, G . (1981)

Specific acyclic isoprenoids as biological markers of rnethanogenic bacteria in marine sediments. Nature, 290, 693 - 696.

Fu Jiamo and Sheng Guoying (1986) Organic geochemical characteristics of major-type sedimentary forma- tions of oil and gas-bearing basins in China. In : Advances in Science of China-earth Sciences, 1 (¢d. Tu Guangchi ) , John Wiley, 251-286.

12 C H I N E S E J O U R N A L O F G E O C H E M I S T R Y Vol.11

Fu Jiamo, Sheng Guoying, Peng Ping'an, Brassell, S . C . , Eglinton, G . and Jiang Jigang (1986) Peculi-

arities of salt lake sediments as potential source rocks in China. In : Advances in Organic Geochemistry

(1985) (eds. Leythaeuser, D . and Rullkotter, J . ) . Org. Geochem., (1 0 ) , 119-126,

Fu Jiamo, Sheng Guoying and Liu Dehan (1988) Organic geochemical characteristics of major types of ter-

restrial petroleum source rocks in China. In: Lacustrine Source Rocks (eds. Kelts, K . , Fleet, A . J .

and Talbot, M . ) . Blackwells, Oxford, 279-290.

Jiang Jigang and Zhang Qian (1982) Off genesis and evolution in the salt lake sediments of the Qianjiang

Group in the Jianghan Basin. Oil and Natural Gas Geology, 3 (1) (in Chinese).

Mackenzie, A. S . , Brassell, S . C . , Eglinton, G . and Maxwell, J . R. (1982) Chemical fossils - - - - the

geological fate of steroids. Science, 217, 491-504.

Mckirdy, D . M . , Cox, R . E . , Volkman, J . K . and Howell, V. J . (1986) Botryococcane in a new

class of non-marine crude oils. Nature, 320, 5 7 - 5 9 .

Sheng Guoying, Fan Shanfa, Liu Dehan, Su Nengxian and Zhou Hongming (1980) The geochemistry of

n-alkanes with an even odd predominance in Tertiary Shaheji¢ formation of Northern China. In: Ad-

vances in Organic Geochemistry (1979) ( ed . A . G . Douglas and J . R . Maxwell), Pergamon,

115-121 .

Ten Haven, H . L . , de Leeuw, J . W. and Scheuck, P . A. (1985) Organic geochemical studies of a

Messinian evaporitic basin, northern Apennines ( I t a ly ) , I-hydrocarbon biological markers for a

hypersaline environment. Geochirn. Cosrrochem. Acta, 49, 2181-2191 .

Ten Haven, H . L . , de Leeuw, L . W . , Sinninghe Damste, J . S . , Schenck, P . A . , Palmer, S . E .

and Zumberge, J . E . (1988) Application of biological markers in the recognition of palaeohypersaline

environments. In Lactistrine Petroleum Source Rocks (eds. K . Kelts, A. Fleet and Talbot, M . ) .

Blackwell, Oxford, 123-130.

Tissot, B. P . and Welte, D . H . (1984) Petroleum Formation and Occurrence. 2nd Edn. Springer,

Berlin.

Tissot, B. P . , Pelet, R. , Roucache, J . and Combaz, A. (i977) Alkanes as geochemical fossil indicators

of geological environments. In : Advances in Organic Geochemistry (1975) (eds. Campos, R. and Goni,

J . ) . Enadimsa, Madrid., 117-154.

Yang Wanli et al. (1985) Genesis, Migration and Accumulation of Terrestrial Oil and Gas in the Songfiao

Basin. Heilong~iang Science and Technology Press, Harbin (in Chinese).

biomarker compounds as indicators of paleoenvironments

Documents