Geochemical and biological marker assessment of depositional environments using Brazilian offshore oils*

M. R. Mello t and P. C. Gaglianone Petrobras/Cenpes/Divex, Ilha do Fund~o, CEP: 21910, Rio de Janeiro, R J, Brazil

and S. C. Brassell Department of Geology, School of Earth Sciences, Stanford University, Stanford, California, CA 94305, USA

and J. R. Maxwell Organic Geochemistry Unit, University of Bristol, School of Chemistry, Bristol BS8 ITS, UK

Received 14 August 1987; accepted 23 October 1987

A combined geochemical and molecular characterization of a wide selection of oils from the major Brazilian offshore basins has been undertaken. The elemental (sulphur, nickel and vanadium) and bulk (API and ~13C) properties of each sample have been considered, together with its molecular composition determined using liquid and gas chromatography, and quantitative biological marker investigations using gas chromatography-mass spectrometry for alkanes.

The results reveal significant differences in the chemical features of the various oils which enable them to be divided into five groups. The distinction of the groups appears to reflect differences in the depositional environment of the source rocks of the oils. Each group is correlated tentatively with source rocks laid down in a specific depositional regime, namely lacustrine freshwater, lacustrine saline water, marine evaporitic, marine carbonate or marine deltaic. The diagnostic features that allow this classification are: the relative abundance and carbon number distributions of n-alkanes; pristane/phytane ratios; sulphur, nickel and vanadium contents; carbon isotope data; the absolute concentrations of hopanes and steranes, and their abundance relative to 4-methylsteranes and, also the occurrence and abundance of several specific biological markers, including 18o~(H)-oleanane, gammacerane, 13-carotane, tricyclic terpanes, higher acyclic isoprenoids, 28, 30-bisnorhopane and 25, 28, 30-trisnorhopane. This investigation shows the value of a combined geochemical and molecular approach in the assessment of the palaeoenvironment of deposition of the source rocks which gave rise to the oils.

Keywords: geochemical characterization; molecular characterization; depositional environments; offshore oils; Brazil

Introduction

The assessment and differentiation of the depositional palaeoenvironments of petroleum source rocks using molecular parameters is increasing in importance and application. Both geochemical evidence and biological marker distributions enable the characterization and distinction of ancient marine and non-marine petroleum source rocks (e.g. Mackenzie et al., 1984; Moldowan et al., 1985; McKirdy et al., 1986; Peters et al., 1986). In addition, recent evidence shows that such features provide diagnostic criteria for the distinction of oils derived from source rocks deposited in different environments, such as lacustrine freshwater and hypersaline in China (e.g. Powell, 1986; Brassell et al.,

* Paper presented at the meeting 'Advances in Petroleum Geochemistry' 19 May 1987, Geological Society, London, UK t Present address: Organic Geochemistry Unit, University of Bristol, School of Chemistry, Bristol BS8 1TS, UK

0264-8172/88/030205-19 $03.00 1988 Butterworth & Co. (Publishers) Ltd

1988; Fu Jiamo et al., 1986), marine carbonate in Venezuela, Australia and Florida (Talukdar et al., 1986; McKirdy et al., 1984; Palacas et al., 1984) and lacustrine freshwater in Australia (McKirdy et al., 1986; Philp and Gilbert, 1986). It is evident that the components of a particular sediment extract or oil are a reflection of the precursor compounds in the organisms which contributed organic matter at the time of sediment deposition, and thereby can provide valuable information about the prevailing environmental conditions. Molecular properties for use in palaeoenvironmental assessment should ideally be diagnostic of specific types of organisms with ubiquitous and documented occurrence in recent and ancient well described depositional environments (Brassell and Eglinton, 1986). In addition, such molecular features can be affected by diagenetic processes and may reflect evolutionary changes in the sources of sedimentary organic matter.

Marine and Petroleum Geology, 1988, Vol 5, August 205

Depositional environment characterization." M. R. Mello et al.

I q8 o

Cassipore

Foz Do Amazonas

P 'M ~ ~ ar~/ aranhao "" ~ Ceara

Potiguar

0 o - -

BRAZIL

Sergipe/Alagoas

Bahia Su{

Espirito Santo

. i Campos

Santos

Figure 1 Location map of Brazilian marginal basins

W

UPPER

CRETACEOUS TO

TERTIARY

SLOPE

SYSTEM

ALBIAN- CENOM.

APTIAN

UPPER NEOC.

LOWER NEOC

UPPER JURAS.

BASEMENT

EX~ EvAPo.,TEs r~%-'l .AL,TE

VOLCANIC ROCKS ~ CONGLOMERATE

[Tg%~ ~AS~MENT . o,L ACCU~ULAT,O.S

Figure 2 Idealized geological section with general stratigraphic and structural features for the Brazilian marginal basins

206 Mar ine and Petro leum Geology, 1988, Vol 5, August

E

Depositional environment characterization: M. Ft. Mello et al. In this study we employ a combination of geological

and geochemical data for a number of oil samples in an attempt to characterize and assess the environments of deposition of the source rocks that gave rise to them. A succession of putative source rocks deposited in distinct, well-defined environments exists within the Brazilian marginal basins (Figures I and 2; Mello et al., 1984, 1988). Specifically, the elemental and bulk properties, and biological marker distributions of about fifty oil samples recovered from reservoirs ranging from lower Neocomian to Oligocene within the major Brazilian offshore basins (Figure 1) were investigated. In order to minimize differences due to the effects of maturation, and the effects of biodegradation, water washing, gravity segregation and loss of volatiles, only oils which were comparatively unaffected by such processes and with medium to high API values were selected (as far as possible). Sixteen of the oils (Table 1), chosen to be representative of the total, were selected for a more detailed g.c.-m.s, study to measure the concentrations of specific biological marker compounds by addition of a deuteriated sterane standard.

Geology of Brazilian offshore basins

The Brazilian marginal basins are directly related to the rupture of the African and South American plates. They are classified as components of a typical divergent, rifted continental margin (Ponte and Asmus, 1978; Estrella et al., 1984). In general, they can be linked to a single evolutionary geological history (Figure 2). They were formed during an Early Cretaceous (Neocomian) rifting phase when a thick succession comprising continental, fluvial and lacustrine siliciclastic and carbonate sediments was deposited. The section is mainly composed of fine to coarse clastics and carbonates deposited in freshwater to saline lacustrine environments (Bertani and Carozzi, 1985). In some areas the section overlies, and is intercalated with, volcanic rocks, which are mainly basalts. After rifting, tectonic activity appears to have been restricted to subsidence and basinward tilting, with the development of gravity sliding features (Falkenhein, 1981) and localized reactivations of faults (Ponte and Asmus, 1978).

The first marine incursions into the coastal basins occurred during the Aptian. The combination of Table 1 Selected oil samples for which analytical data are shown in

tectonic quiescence, topographical barriers and arid climate led to a low elastic influx and restrictions appropriate for deposition of mixed carbonate and siliciclastic sediments together with evaporites in coastal, shallow continental to marine environments (Asmus, 1975).

As a consequence of sea floor spreading and the subsidence of the Brazilian continental margin, the proto-South Atlantic Ocean maintained an almost uniform palaeogeographic setting during the Albian. At this time, mainly carbonate sediments were accumulated in a neritic environment in an epicontinental sea (Koutsoukos, 1987). The marine platform and slope carbonates were deposited under conditions of tectonic quiescence, with some adiastrophic tectonism often associated with detached listric faults soling out on the Aptian salt. The late Cretaceous to Holocene marine shelf-slope succession, characterized by carbonate and mainly siliciclastic sediments, represents the open marine stage in the Brazilian marginal basins, with basin subsidence, seaward tilting, large adiastrophic growth-faulting structures and local volcanic events. Important in this context is the deposition of widespread sapropelic sediments, such as calcareous mudstones and black shales with high organic carbon contents, in almost all the offshore basins, during late Cenomanian to Santonian times. This pattern of sedimentation in the Brazilian basins is consistent with a global oceanic anoxic event (e.g. Schlanger and Jenkyns, 1976). Oxygenated conditions have prevailed in most of the Brazilian marginal basins since the Campanian, with deposition of mixed elastic and carbonate sediments.

Experimental and analytical procedures

All the oil samples were submitted to bulk and elemental analyis according to procedures described previously (Mello et al., 1984). Samples were fractionated into saturated, aromatic and polar fractions using thin layer chromatography (silica gel G; hexane developer). The aliphatic hydrocarbons were analysed by gas chromatography (g.c.) employing a Carlo Erba Mega chromatograph, equipped with on-column injector and a 50m OV-1 column. Hydrogen was employed as carrier gas with a programme of 50-310C at 5C min -1. Peak areas were used for comparative quantitation. G.c.-m.s. analyses were Table 3

Oil sample API Group Depth (m)

Reservoir

Age Lithology

1 39 I 1696 2 30 I 2780 3 30 I 2340 4 36 I 1841 5 29 II 3166 6 32 II 2857 7 30 II 2815 8 30 II 1524 9 19 III 1953

10 20 III 1475 11 29 III 1998 12 30 III 2764 13 22 IV 1107 14 30 IV 2889 15 42 V 4289 16 44 V 4288

Aptian Sandstone Neocomian Sandstone Neocomian Sandstone Neocomian Sandstone AIbian Carbonate Neocomian Coquina Neocomian Coquina Palaeocene Sandstone Albian Sandstone Aptian Sandstone Aptian Sandstone Aptian Sandstone Eocene Carbonate Maastrichtian Sandstone Eocene Carbonate Eocene Carbonate

Mar ine and Pet ro leum Geo logy , 1988, Vol 5, August 207

Depositional environment characterization: M. R. Mello et al. Table 2 Bulk and elemental properties of Brazilian offshore oils

Property Group I Group II Group III Group IV Group V

h~3C %o ~ 0.5 3 Low < 0.07, medium ca. 0.2, high /> 0.3

carried out using a Finnigan 4000 spectrometer coupled to a Carlo Erba 5160 gas chromatograph equipped with on-column injector, and fitted with a 60m DB-1701 column. Helium was employed as carrier gas and a programme of 50-310C at 4C rain -] was used. Data were acquired and processed using an Incos 2300 data system. Quantitative data on biological marker concentrations were obtained from selected oil samples (Table 1) by adding a fixed amount of internal standard (2,2,4,4-d4, 5o~(H), 14o~(H), 17o~(H)-cholestane) to each alkane fraction; steranes were quantitated by comparing peak areas in m/z 217 chromatograms with the peak area of the standard in m/z 221 chromatograms. Other components were quantitated by comparison of peak areas with that of the standard in the Reconstituted Ion Chromatogram (RIC) traces. To confirm the order of concentrations of specific biological markers, quantitation was also carried out using mass chromatograms (e.g. by comparison of m/z 221 for the standard with m/z 191 to obtain relative concentrations of C~ 0~f3 hopane). To ensure comparable results, all the analyses were performed sequentially, under similar conditions. Peak identities were established by mass spectral examination, g.c. retention time and, in a number of cases coinjection of standards. The presence and absence of C3o regular steranes was checked by metastable linked scan techniques using a VG-70E instrument at Norsk Hydro Research Centre, Norway (of. Moidowan et al., 1985). Stable isotope analyses for carbon on whole oils and their fractions were undertaken using a vacuum combustion line linked to a high resolution Varian MAT-230 instrument, at Petrobras research centre, Brazil. The data are presented in delta-notation

Table 3 Biological marker concentration properties of Brazilian

4 Low < 50%, medium ca. 55%, high > 60% 5 Low < 1, high > 1 6 Low< 1, high > 1

relative to Pee Dee Belemnite (PDB).

Results

The results reveal significant differences in the geochemical characteristics of the oils, best described in terms of a classification into five groups (I to V). Table 2 lists the range of bulk, elemental and alkane properties for each group of oils. The n-alkane distributions and pristane/phytane ratios were determined from g.c. analysis of the saturate fractions. Tables 3 and 4 similarly show molecular properties, based on the distributions and abundances of acyclic isoprenoids (C25 and C3,), [~-carotane, sterane and terpane families, determined by g.c.-m.s, analysis. In the following sections, each group of oils is considered in turn.

Group I oils These oils are confined to the Ceara, Potiguar, Sergipe/Alagoas and Bahia Sul basins (Figure 1). They are pooled mainly in sandstone reservoirs belonging to lacustrine freshwater facies with ages ranging from lower Neocomian to Aptian (Table 1). The bulk and elemental data reveal a set of common characteristics, such as low sulphur concentrations (~60%), n-alkane distributions with odd and high molecular weight predominance, pristane dominant over phytane (values >1.2), low n-Cly/n-C3l (

Depos i t iona l env i ronment character i za t ion : 114. R. Me l lo et al. Table 4 Biological marker ratios* for Brazilian offshore oils

Ratio** Group I Group II Group III Group IV Group V

8Diasterane Index N O OOe 94-Me sterane Index 000 O0 (10 lHopane/steranes ~ 09)0 11Tricyclic Index 4) ~ O910 12Tetracyclic Index O 00 ~ 13Bisnorhopane Index ca. 0 000 ~ ca. 0 14Gammacerane Index 000 ca. 0 1sC34/C3s hopanes INN} ~ ~6Ts/Tm > 1 < 1 < 1 < 1 > 1 Inferred depositional Lacustrine/ Lacustrine/ Marine Marine Marine environment freshwater saline water evaporitic carbonate deltaic

*For range of values, see Table 5 Low ** For details of measurements, see Table 5 N Medium

High

The specific biological markers addressed in this study are mainly represented by the long chain acyclic isoprenoids, sterane and terpane families (Tables 3 and 4). Representative g.c. and g.c.-m.s, traces of a typical example of this consistent oil group are shown in Figure 4. The presence of C25 and C3o acyclic isoprenoids was confirmed by mass chromatography using m/z 183,239 and 253, since they were present in low abundance. Thus, the C25 (2,6,10,14,18 and/or 2,6,10,15,19 pentamethyleicosanes) and C3o (squalane) components were identified in the RIC traces (cf. Figure 4) and quantified (Table 3).

The steranes were assigned by mass chromatography, using m/z 217, 231 and 259 for steranes including C21 and C22 components (peaks 1 to 3 and 5), 4-methylsteranes and diasteranes, respectively. In general, the oils have very low concentrations of steranes, with 4-methyl steranes (40c, 20 R and S, mainly C3o not shown) present in concentrations similar to those of steranes (Tables 3, 4 and Figure 4). Diasteranes, and the low molecular

weight steranes are in low or trace abundance. The sterane distributions, represented mainly by C27, C28 and C29 homologues present as (50c(H), 14o~(H), 170fill) and 5offH), 1413(H), 17~(H), 20 R and S isomers), show the C27 components always higher than C29 (Table 3 and Figure 4). Remarkable is the absence of the C3o components in all the oils of this group (checked using metastable g.c.-m.s.; Table 3; Moldwan et al., 1985). The extents of isomerization of the C29 steranes at C-20(S/S+R), and at C-14 and C-17 (ocf313/o~[3~+o~ao 0 tend to be low for oils (

Depositional environment characterization: M. R. Mello et al.

ti

n23

I

Saturates : 65% Pr/Ph : 18

Sulphur: 0.09% V/Ni: 0.05

Carbon isotope -28%o

m/z 217 10

16

33

35 m/z 191

28 39 30 41

19 20 21 22 2324 25~ 26~ .~L

Figure 4 Gas chromatogram of total alkanes, bulk and elemental parameters, and partial m/z 217 and m/z 191 chromatograms for oil (n 1) typical of group I; (for peak assignments see appendix I)

43 . . . . 44 45

i

present: the C27 hopanes Ts and Tm(18o~(H) 22,29,30-trisnorneohopane and 17(x(H) 22,29,30- trisnorhopane, peaks 28 and 30), are present with Ts/Tm ratios always >1 Table 4 and Figure 4).

A peculiar feature is the presence, albeit in low abundance, of gammacerane (peak 40). The use of a standard and an efficient g.c. column (DB-1701,60 m), allowed its identification, with complete separation from the hopanes. Another feature in some of the oils is the presence, also in very low concentrations, of a series of C27 3~ 8,14-secohopanes (assigned using m/z 123 chromatograms, see structures).

Group II oils The oils belonging to group II are confined to the eastern and southern areas of the margin, in Espirito Santo and Campos basins, respectively (Figure 1). They occur in reservoir rocks deposited in non-marine and marine facies which range from Neocomian to Miocene in age (Table 1). Their geochemical features (Tables 2, 3. 4 and Figure 5) include medium sulphur contents (0.25-0.32%), higher V/Ni ratios (0.3-0.5), and 6 13C(~)/N~) values spanning a narrow range of relatively heavy values of -23.0 to -25.6 for the whole oil, -23.7 to -25.8 for the saturate fraction and between -22.3 and -24.4 for the aromatics (Table 2; Figure 3). The compositional data show a small reduction in saturates content (around 60%) relative to the group I oils, as well as a slight dominance of aromatics over NSO compounds. The n-alkane carbon number

predominance, around Ct7 to C2~, increases the n-Cr/n-C31 ratios (around 3.5). For all the oils the pristante/phytane ratios are high (around 1.5) and are accompanied by an odd over even preference in the n-alkanes.

These oils have higher relative abundances and concentrations of the long chain acyclic C25 and C30 isoprenoids than the oils in group I (Table 3; Figures 4 and 5). That the i-C25 component is mainly 2,6,10,14,18- pentamethyleicosane was confirmed using m/z 253 chromatograms (as well as m/z 239 and 183). [3-carotane occurs, although in low concentration, in most of the samples from this group (Table 3 and Figure 5). Its identity was confirmed by the m/z 125 chromatograms. Steranes are present in slightly higher concentrations than in the group I oils (Table 3), with steranes (o~0~c~ and 0~[313, 2(1 R and S) and 4-methylsteranes (4e, mainly C30; not shown) occurring in similar relative abundance, with the former predominant in most of the samples.

Diasteranes, mainly 1313(H), 17o~(H) 20 S and R isomers, are also present, in smaller amounts than steranes but with a similar carbon number distribution dominated by the C27 components. As for the group I oils no C3o steranes were detected (Table 3). Low molecular weight C2j and C22 steranes and 4-methylsteranes (peaks 1-5), with a dominance of the latter, are present in significant concentrations relative to the steranes (Table 3, Figure 5). Once again, the extent of C29 sterane isomerisation at C-20 (S/S+R)

210 Marine and Petroleum Geology, 1988, Vol 5, August

Depositional environment characterization: M. R. Mello et al. and C-14 and C-17 (0~1313/0~1313+0~o0), tends to show low values for oils (Figure 5).

The m/z 191 mass chromatograms show that the oils contain high relative abundances of tricyclic terpanes extending from C20 to C35, with the exception of C22, C27 and C32 members (Figure 5). Moderate abundances of the C24 tetracyclic terpane (peak 24; Figure 5) were detected in all the oils of this group (Table 4). The hopanes occur in higher concentrations than in the group I oils (Table 3). The Ts/Tm ratio is always < 1 (Table 4 and Figure 5). The presence, albeit in low amounts, of 170(H),2113(H)-28,30-bisnorhopane (peak 32) and 25,28,30-trisnorhopane is noteworthy, especially as they are not present in the group 1 oils. The identity of the latter compound was confirmed by mass chromatography using m/z 177 and m/z 370, since it gives no response in the m/z 191 chromatogram. Gammacerane (peak 40; Figure 5) is also present in all the samples in relative abundances comparable to those observed in the group I oils (Table 4 and Figure 5).

Group III oils Oils in this group occur in Bahia sul, Sergipe/Alagoas, Potiguar and Ceara basins, being pooled in reservoirs ranging from Aptian to Palaeocene (Table I and Figure 1). Their geochemical characteristics include high sulphur contents (0.5-1.5%), medium V/Ni ratios (--0.2), and 6 13C(O~x)) values within the narrow range of -25.4 to -26.6 for whole oil, -26.4 to -27.3 for saturates and -25.4 to -26.4 for the aromatic fraction (Table 2; Figure 3). The amounts of saturates range

from 35% to 55% with NSO components higher than aromatics. A close examination of the relative abundances of normal and branched alkanes shows a dominance of low molecular weight n-alkanes (around C15-C17), a slight even/odd n-alkane dominance, low pristane/phytane ratios (

Depositional environment characterization: M. R. Mello et al.

Saturates: 50% Pr/Ph : 0-5

Sutphuc: 0.7% V/Ni: 0.2

Carbon isotope : -26%0

m/z 217

8

10

16

13

15

35 4o m/z 191

33

39

j] i11 18 19 20 21 22 2324 22 26 L__ i . A ~ L t I [~'k * Figure 6 Gas chromatogram of total alkanes, bulk and elemental parameters, and partial m/z 2 17 and m/z 191 chromatograms for oil (n 10) typical of group III; (for peak assignments see appendix I)

(e.g. peak 10) relative to 20 S (e.g. peak 8) plus low margin (Figure l). They are accumulated in reservoirs abundances of the 0~[313 components (e.g. peaks 9; ranging from Maastrichtian to Tertiary age (Table 1). Figure 6). Also notable is the occurrence, although only The bulk geochemical properties of these oils are, in in low concentrations, of C~o steranes which were not most respects, similar to those of the group III oils detected in samples of groups I and II (Table 3). In (Table 2) with high sulphur contents and V/Ni ratios addition, the oils contain relatively high concentrations (0.5-0.76% and 0.3 to 0.5 respectively). Their saturate of low molecular weight steranes and 4-methylsteranes, fractions range from 46% to 54%, with aromatics although in low abundance relative to the C27-C2,) around 20% and NSO components around 30% (Table steranes (cf. Table 3 and Figure 6). 2). The 6 13C (0/~)) values of the whole oils range from

Like the steranes, the terpanes are present in high -26.8 to -27.6, around -27.5 for the saturate and concentrations. The distributions of the tricyclic -26.7 to -27.1 for the aromatic fraction (Table2; and terpanes differ from groups II and IV, since they are Figure 3). These oils, like those of group III, show a present in lower relative abundance and contain no predominance of low molecular weight n-alkanes homologues higher than C2,). Overall, the around Cls-2o, with a slight predominance of even characteristics of the m/z 191 mass fragmentograms for numbered homoiogues and a dominance of phytane these oils include: a high concentration of over pristane (Table 2 and Figure 7). The distribution gammacerane (sometimes the major peak; peak 40), of the long chain C25 and C30 (squalane) isoprenoids C3~ ~> C34 hopanes (peaks 45 and 44, respectively), the and carotanes ([3 and ) is also similar to that of the presence of significant amounts of 28,30-bisnorhopane group III oils, where high concentrations of both (peak 32) and high abundances of C2,) to C~ compound types are found (Table 3; cf. Figures 6 and 17o~(H),2113(H) hopanes relative to their 1713(H), 7). 21cx(H) counterparts (Table 3 and Figure 6). Like group Steranes are present in high abundance, second only lI, the Ts/Tm ratios (peaks 28 and 30) are always < 1, in to their concentrations in the group III oils (Table 3). A contrast to group I (Table 4 and Figures 4, 5 and 6). In significant feature of their distributions is the high addition, high amounts of another C~7 hopane relative abundance of low molecular weight (25,28,30-trisnorhopane, see structures)are observed components and the low relative abundance of (m/z 177 and m/z 370 mass chromatograms), diasteranes, mainly represented by C27 homologues

(Table 3," Figure 7). The steranes show a slight Group IVoils dominance of C2v, 50~,14o~(H),170~(H) 20 R These oils are found only in Cassipor6 and Maranhfio components over their C29 counterparts (Figure 7). basins in the extreme northern part of the continental High concentrations of C3o steranes were recognized

212 Marine and Petroleum Geology, 1988, Vol 5, August

Depositional environment characterization: 114. R. Mello et al. using metastable reaction monitoring (Moldowan et al., 1985; Table 3).

The terpane distributions in the m/z 191 mass chromatograms resemble those of the group II oils (cf. Figures 5 and 7). In particular, they show a marked similarity both in concentration and carbon number range of C19 to C35 tricyclics (Table 4 and Figures 5 and 7). Other features common to these two oil groups are the presence of gammacerane, 28,30-bisnorhopane and 25,28,30-trisnorhopane (not shown in m/z 191), with the last two components tending to occur in higher relative abundance in the group IV oils (Table 4). Other hopanes are also present in high concentrations with 17et(H),210~(H) components (mainly C30) dominant and with minor amounts of 17[~(H), 21ct(H) hopanes. As for the group III oils, Ts/Tm is always 60%), compared with the aromatic and NSO fractions (Table 2).

The 8 13C(%0) values are -24.4 and -25.1 for the whole oils, -25.1 and -26.2 for the saturates, and -23.6 and -24.3 for the aromatic fraction (Table 2 and Figure 3). The saturate fraction of these oils shows a

predominance of n-alkanes around C18_22" with a slight even/odd preference. Their pristane/phytane ratios are close to/or 1 (Table 4 and Figure 8).

Discussion

The application of biological marker compounds to the assessment of depositional environment should be made with caution. It is important to stress the need to

, ..L_ ji

i

Saturates : 50%

Pr/Ph : 07

Sutphur : 05%

V/Ni : 0.3

Carbon isotope : -2?%0

m/z 217 10

9 8 n

12 -1

13 15 16

20

33

35 rn/z 191

21

22 23 r~ r-~ 29 f I N 31 38 1 41 43

18 24 27 34 / 40

. . . . . . . ~ . . . . . . . .~L , . . . . . . .. L ,~L, a L. ~ : , J , I J~ . . . . . m . . . . L_ . ,~L . . A . .~ i i i ,

F igure 7 Gas chromatogram of total alkanes, bulk and e lementa l parameters , and partial m/z 217 and m/z 191 chromatograms for oil (n 14) typical of g roup IV; (for peak ass ignments see append ix I)

Marine and Petroleum Geology, 1988, Vol 5, August 213

Depositional environment characterization: M. R. Mello et al. n23

Saturates : 60% ' Pr/Ph 0-7

. . . . Sulphur : 0.35%

,i= V/Ni : 1-0 I~- Carbon

isotope -25~

m/z 217

3

,k..,,J..

10 15

35 m/z 191

33

39 -1

2830. i 411 ..... .... l j ~ ~ 27 34

Figure 8 Gas chromatogram of total alkanes, bulk and elemental parameters, and partial m/z 217 and m/z 191 chromatograms for oil (n 15) typical of group V; (for peak assignments see appendix I)

understand and disentangle the effects of source, maturity and biodegradation on bulk, elemental and, principally, biological marker properties. It is well recognized that variations in several molecular parameters occur with an increase in maturity, with the relative abundances and concentrations of specific compounds increasing or decreasing. In this study we only consider molecular properties which although, to some extent are maturity dependent, are principally source related. This approach was confirmed by biological marker data obtained from immature and mature samples of the source rocks that gave rise to the oils (Mello et al., 1988). In the following section each oil group defined above is discussed separately.

Group 1 oils

The high wax content and odd/even n-alkane predominance plus the bias towards high molecular weight n-alkanes (>C~3) in the oils of this group indicate major contributions of long chain lipids from higher plants and freshwater algae (Lijmbach, 1975; Didyk et al., 1978; McKirdy et al., 1986) to the depositional environment of their source rocks. The high pristane/phytane ratios (Table 2 and Figure 4) for these oils, probably reflect the relationship between their precursors and the chemistry of the environment (cf. ten Haven et al., 1987), e.g. low salinity, rather than simply the anoxic/oxic condition of sedimentation (Didyk et al., 1978). It is accepted that pristane originates from phytol (Didyk et al., 1978) and/or

tocopherols (Goossens et al., 1984) of photosynthetic organisms, whereas phytane may arise in part from phytol or more likely from archaebacteria lipids in organisms such as methanogens and halophiles (Kaplan and Baedecker, 1970; Risatti et al., 1984; ten Haven et al., 1985). In a freshwater environment, photosynthetic organisms containing phytol and tocopherols would be expected to be abundant. With an increase in salinity (higher Eh), however, the archaebacterial population might be expected to increase in abundance, Thus, the more saline the environment, the greater the potential for an increase in the concentration of phytane precursors. This may help explain the high predominance of pristane in freshwater environments compared with dominance of phytane in hypersaline environments (see below: ('.t~ ten Haven et al., 1985 and 1987).

The low values of sulphur and V/Ni ratios (Table 2) in the group I oils, are in agreement with their freshwater origin, since such properties are primarily a function of the Eh-pH conditions and sulphide activity of the depositional environment in sediments (Tissot and Welte, 1984; Orr, 1986: Lewan, 1984).

The isotopically light values of 6 '3C (always

Depositional environment characterization: M. R. Mello et al. pristane/phytane ratios to be ineffective for distinguishing non-marine and marine environments on a global basis. Taken together, the carbon isotope values (6 ~3C) and pristane/phytane ratios in the present study do, however, discriminate between these distinct groups (Table 2), and can be considered a useful geochemical measure for the differentiation of non-marine and marine related oils found in the Brazilian offshore basins.

The biological marker distributions (Tables 3 and 4) also show characteristic features, based in the occurrence or absence of specific compounds, that suggest a lacustrine freshwater origin for the group I oils. The comparatively low relative abundances of C25 and C30 isoprenoids are consistent with a freshwater origin, since their biological sources are held to be archaebacteria, which might be expected to be relatively more abundant in marine or more saline waters (Waples et al., 1974; Albaiges, 1980; Brassell and Eglinton, 1986). In general, the group I oils have a low concentration of steranes, and an absence of C30 steranes held to be diagnostic of marine environments (Moldowan et al., 1985). Moldowan et al. (1985) and McKirdy et al. (1986), also reported a paucity of steranes in lacustrine freshwater oils from Brazil, China, Sudan and Australia. This characteristic may be due to the organisms living in such a habitat, using lipids other than sterols as rigidifiers and protectors of cell wall materials. A possible explanation for such a phenomenon could be that terrestrial and freshwater plants live under higher oxygen conditions than their saline counterparts and therefore require greater protection for their cells. The dominance of C27 steranes (Figure 4), contrasts with previous reports (Huang and Meinschein, 1979; Mackenzie et al., 1984; Hoffmann et al., 1984 and Moldowan et al., 1985) of a predominance of C29 steranes in non-marine environments. Such environments may be expected to receive major contributions of higher plant material, whose precursor sterols are mainly C29. None of the oils of group I show such a feature. Hence, interpretation of a predominance of C29 steranes as an indication of higher plant input, or as characteristic of a non-marine environment must be made with caution.

The apparently diverse, but bacterial, precursors for the hopanes and tricyclic terpanes is well documented (Ourisson et al., 1979 and 1982).

The main significance of these widespread bacterial markers lies in their abundance, rather than distribution pattern. The lower concentrations of these compounds in the group I oils relative to the groups II and IV indicate a lower bacterial input (Tables 3 and 4), perhaps related to salinity differences.

A feature of all the group I samples is the presence of gammacerane, but in low concentration (Table 4; Figure 4). This compound was first identified in the Green River shale (Hills et al., 1966), and was initially considered a diagnostic marker of lacustrine environments (Seifert and Moldowan, 1981). Many authors (Rohrback, 1983; Mello et al., 1984; McKirdy et al., 1986 and Fu Jiamo et al., 1986), have since reported it in marine carbonate and hypersaline environments. The only biologically-occurring compound with a gammacerane-type structure is tetrahymanol, a constituent of protozoa, although their precursor/product relationship has not been verified from sediment data (Brassell and Eglinton, 1986).

Recently, Moldowan et al. (1985) have stated that gammacerane cannot be used to distinguish between marine and non-marine samples, since it occurs in several different environments. Such evidence suggests the possibility of a bacterial origin for gammacerane, given its widespread occurrence in time and space. Hence, our results (Table 4 and Figures 4-8) suggest that the value of gammacerane as an environmental indicator lies in its relative abundance (and concentration), rather than its mere presence (see group III below).

Integration of the data given in Tables 2, 3 and 4 and illustrated in Figure 4 shows a set of bulk, elemental and molecular data for this group that suggests an origin from source rocks deposited in a lacustrine freshwater environment. Similar data have been reported for oils sourced by such sediments in China, Sudan, Chad and Australia (Wang Tieguan et al., 1988; Moldowan et al., 1985; Powell, 1986 and McKirdy et al., 1986).

Group H oils The higher sulphur contents and V/Ni ratios relative to the group I oils (Table 2), may reflect the more saline character (high Eh) of the depositional environment of the source rocks of the group II oils. Enhanced salinity might also explain why the oils from this group are isotopically heavy (613C values around -25%0). Plants from saline environments can preferentially utilize carbonate complexes as their carbon source for photosynthesis. These are richer in a3C than atmospheric carbon dioxide, which is enhanced in 12C (Tissot and Welte, 1984).

The predominance of pristane over phytane and the odd/even n-alkane preference is somewhat unexpected, since oils derived from saline lakes generally show opposite characteristics (ten Haven et al., 1988; Moldowan et al., 1985 and Fu Jiamo et al., 1986). Such differences may reflect the establishment of a lake with moderate salinity rather than hypersaline conditions. A more saline environment might increase the abundance of phytane precursors (archaebacteria) over pristane precursors (Table 2 and Figure 5). Alternatively, the differences could simply reflect relative input from higher plants, since the wax content and bimodal n-alkane distribution (maxima at C~6 and Czs; Figure 5) of the group II oils clearly indicates the importance of such contributions.

The group II oils show higher concentrations and greater relative abundances of the regular C25 isoprenoid and i-C30 (squalane) than the oils of group I (Table 3, Figures 4 and 5). This may reflect an increase in the salinity of the depositional environment (Brassell et al., 1988; see also group III below). A further characteristic of the saline nature of group II oils is the occurrence, albeit in small concentrations, of [3-carotane. This compound was first identified in saline sediments of the Green River formation (Murphy et al., 1967), and more recently, Hall and Douglas (1983), suggested that its presence might be related to a lacustrine saline environment. Moldowan et al. (1985) regarded [3-carotane as a terrestrial marker because it had not been reported from sources of marine origin. Its abundance in samples from lacustrine saline and hypersaline environments (Shi Ji-Yang et al., 1982; Jiang and Fowler, 1986) supported by the evidence from this study suggests that salinity is the controlling

Marine and Petroleum Geology, 1988, Vol 5, August 215

Depositional environment characterization: M. R. Mello et al. characteristic of [3-carotane concentrations.

The group II oils, like group I, show low concentrations of steranes and an absence of C30 steranes (Table 3). These features have been considered characteristic of non-marine oils from Australia, Sudan, Chad, China, Brazil and U.S.A. (McKirdy et al., 1986; Moldowan et al., 1985). A significant feature is the presence of higher concentrations of low molecular weight C2~ and C22 steranes, and 4-methylsteranes (Figure 5; Table 3) than in the group I oils. These compounds appear to be associated with carbonate and hypersaline environments (ten Haven et al., 1985, 1988; Connan et al., 1986; Fu Jiamo et al., 1986). The predominance of C27 steranes over Cz9 steranes (Figure 5) in all the samples again demonstrates that a C29 sterane predominance is not always diagnostic of non-marine environments. The concentrations of the bacterially-derived hopanes in these oils are extremely high (Table 3), perhaps reflecting the importance of bacterial lipids in saline lakes. In addition, the prominence of tricyclic components ranging from C2(~.35 (Table 4; Figure 5) may be a result of the specific saline conditions of such lakes. Similar features occur in oils derived from a non-marine environment in Angola (Connan et al., 1988). Tricyclic terpanes have been recognised in oils from various origins (e.g. Aquino Neto et al., 1982 and 1983) and it has become evident that they arise from bacterial precursors, perhaps specific membrane lipids (Ourisson et al., 1982). The low Ts/Trn ratios, typically 100

9 Sum of peak areas of all C3o 4-methyl steranes in m/z 231 chromatogram (recognized using mass spectra and m/z 414 chromatogram) over sum of peak areas of C27 2OR, 20S 5(~,14~,17~(H)-cholestane (8+ 10)x100. Low < 60, Medium 60-80, High > 80

10 Peak area of C3o 17(~,2113(H)-hopane (35) in m/z 191 chromatogram over sum of peak areas of C2720R and 20S 5(~,14~,17~(H) - cholestane (8+10) in m/z 217 chromatogram. Low < 4, Medium 4-7, High > 7

11 Sum of peak areas of C19 to C29 (excluding Cz2, C27) tricyclic terpanes (18-23, 25, 26) in m/z 191 chromatogram over peak area of C3o 17e, 21~(H)-hopane (35) x 100. Low < 50, Medium 50-100, High > 100

12 Peak area of C24 tetracyclic terpane (24) in m/z 191 chromatogram over peak area of C3o 17(~, 2113(H)-hopane (35) x 100. Low < 5, Medium 5-10, High > 10

13 Peak area of C2e 28,30-bisnorhopane (32) in m/z 191 chromatogram over peak area of 17~,2113(H)-hopane (35) x 100. Low < 10, Medium 10-50, High > 50

14 Peak area of gammacerane (40) in m/z 191 chromatogram over peak area of 17,2113(H)-hopane (35) x 100. Low < 50, Medium 50-60, High > 60

15 Peak areas of C34 22R and 22S 17,2113(H)-hopanes (44) in m/z 191 chromatogram over peak areas of C3s counterparts (45). Low < 1, High > 1

16 Peak areas of 18~(H)-22,29,30-trisnorhopane (Ts) over 17(x(H)-22,29,30-trisnorhopane (Tm) in m/z 191 chromatograms

*See Figures 4-8 and Appendices.

216 Marine and Petroleum Geology, 1988, Vol 5, August

Depositional environment characterization: 114. R. Mello et al. linked with a low pristane/phytane ratio. This ratio is directly related to the salinity of the environment of deposition. Low ratios (

Depositional environment characterization: M. R. Meflo et al. oils generated from the La Luna and Querencual formations (Venezuela), Sunniland oils (South Florida), Aquitaine basin oils (France), and oils from the Magdalena Valley (Colombia) (Palacas et al., 1984: Moldowan et al., 1985; Cassani, 1986; Talkudar et al., 1986; Connan et al., 1983). The 6 ~-~C values for the whole oils, and their saturate and aromatic fractions (Figure 3) are typical of oils of marine carbonate origin, being significantly heavier than the values of the group IlI oils (Figure 3," Sofer, 1984: Tissot and Welte, 1984: Palacas et al., 19841.

The distributions of the C2s and Cso isoprenoid components, and [~-carotane in these oils (Table 3 and Figure 7), are consistent with the enhanced salinities in the environment during carbonate deposition. The presence of [3-carotane is further evidence of its value as a diagnostic indicator of salinity in both marine (this work) and lacustrine (Hall and Douglas, 1984) environments, rather than a terrestrial marker (of. Moldowan et al., 1985).

The sterane concentrations are high, only lower than those of the group II1 oils, suggesting that salinity ma> also influence the biological marker distributions in marine carbonate sediments. Similarly, high concentrations of low molecular weight steranes are present (Table 3 and Figure 7), as in other carbonate oils (ten Haven et al., 1985: Talukdar et al., 19861. As for the group III oils, diasteranes are present in low abundance, a feature commonly observed for oils derived from carbonate sediments in Australia, Venezuela, Tunisia and France (McKirdy et al., 1984: Cassani, 1986; Connan el al., 19831. As for the other marine oils, C~o steranes are present, and in higher relative abundance than in the oils of group Ili (Table 3). Perhaps this increase reflects the establishment of wholly marine conditions.

In contrast to the steranes, the relative abundances and distributions of the tricyclic terpanes differ from the group Ill oils and show a similar pattern to those of the group II oils (Table 4 and Figures 5 and 7), suggesting indirectly that their precursors are suppressed by hypersalinity (q'~ group lll). Examples of well-documented carbonate oils with comparable features occur in Venezuela (Cassani, 1986; Talukdar et al., 1986), South Florida (Palacas et al.. 1984), and Colombia (Zumberge, 19841. The presence of 28,30-bisnorhopane, 25,28,30-trisnorhopanc and gammacerane in significant arnounts, like the group II oils (Table 3 and Figures 5 and 7) presumably reflects the similar salinity and chemical characteristics of the two environments (Table 4).

The assignment of a marine carbonate environment for the group IV oils is based on the similarities between their specific chemical features and those of well defined carbonate oils (see above). In summary, these marine carbonate oils share some characteristics with evaporitic oils, and others with lacustrine saline oils.

Group V oils The predominance of medium to high molecular weight n-alkanes suggest that the group V oils are derived, in part, from a terrestrial input of higher plants (Figure 8). On the other hand, the high sulphur contents and V/Ni ratios of these oils, together with pristane/phytane ratios close to/or < 1, linked with slight even over odd n-alkane preference (Table 2 and Figure 8), suggest a

marine carbonate influence on the depositional environment of their source rocks (el. group IV). Further features which point to such an influence are the dominance of C~5 over the Cs4 hopanes, and the presence of high relative abundances of low molecular weight steranes (~f. group IV, Figures 7 and 8).

The heavy 6 ISC values for the whole oil and the other fractions (Table 2," Figure 3), are typical of highly mature oils derived from marine environments (Tissot and Welte, 1984; Fuex, 1988).

Diasteranes are present m the highest relative abundances of all the oil groups ( Table 4 and Figure 8), perhaps reflecting high maturity or the high amounts of clay minerals in their source rocks, consistent with high terrigenous input (q/~ above). The presence of (;,~o steranes indicates a marine depositional environment (Table 3," Moldowan el al., 1985).

The main feature which distinguishes the group V oils lies in their terpane distributions, which include biological markers diagnostic of higher plant inputs, notably high abundances of 1804H)-oleanane (peak X; Figure 8): this compound was first identified (Hills and Whitehead, 19661 in an oil from the Niger delta. Further work by several authors has suggested an origin from precursors in higher plants of the angiosperm family (Ekweozor el al., 1979a and 1979b). More recently, it has been reported to occur in an increasing number of oil samples, but always appears to be linked to terrestrial inputs in predominantly Tertiary basins, mainly of deltaic nature (Taranaki delta, New Zealand: Beaut'oft-Mackenzie delta. Canada: Po Basin, Italy: Niger delta, Nigeria; Grantham et al., 1983; Philp and Gilbert, 1986: Riva et al., 1986: Brooks, 1986). The occurrence of high abundances of the Ce4-tetracyclic terpanc relative to the tricyclic terpanes (Figure 8) appears to be an indicator of a significant input of higher plant material (Trendel el al., 19821. This compound has also been reported in high relative abundance in coals and oils derived predominantly from terrigenous source material, an observation that suggests its use as a marker of higher plant input (Philp and Gilbert, 1986; Abdullah et al., 1988). Hence, the presence of high abundances of 1804H)-oleanane and the Ce4 tetracyclic terpane in the group V oils, with an abundance of high molecular weight n-alkanes indicates their deltaic origin. In addition, these oils possess other geochemical characteristics (Tables 2-4; Figure 8) consistent with an origin from source rocks deposited during the development of a marine deltaic system over a carbonate platform. Well-documented examples of oils derived from deltaic environments include those from the Tertiary Niger delta (Hills and Whitehead, 1966; Ekweozor et al., 1979a), the Mahakam delta in Indonesia (Schoell et al., 1983: Grantham et al., 1983; Hoffmann et al., 1984), and the Beaufort-Mackenzie delta in Canada (Brooks, 1986).

Conclusions

This investigation confirms the value of a combined geological, geochemical and biological marker approach, using oil samples, in the assessment and differentiation of depositional environments of petroleum source rocks. The results (Tables 2-4," Figures 4-8) reveal signifcant differences within the oils from Brazilian offshore basins enabling their classification into five distinct groups. These groups

218 Marine and Petroleum Geology, 1988, Vol 5, August

Deposit ional env i ronment characterization: M. R. Mel lo et al. correlate with source rocks laid down in five different depositional regimes; namely, I-lacustrine fresh water; II-lacustrine saline water; III-marine evaporitic; IV-marine carbonate, and V-marine deltaic.

A quantitative approach using the concentrations of biological marker compounds has been shown to be valid and useful. For example, the paucity of steranes and absence of C3o steranes in the oils from non-marine environments (groups I and II), versus their markedly higher concentration in the marine-related ones (groups III and IV; see discussion for group V) is important (cf. Rullk6tter et al., 1984; Moldowan et al., 1985). Also, the abundance of the bacterially-derived hopanoids appears to be salinity dependent and reaches a maximum in the oils derived from group II (lacustrine saline) and group III (marine evaporitic). The data in Tables 2 - 4 show clearly that no single geochemical property is sufficient to suitably characterize and assess a specific environment of deposition for the source rocks that gave rise to the oils. However, consideration of the various properties in a multiparameter approach, does provide diagnostic criteria for differentiation and assessment of specific depositional environments. depositional environments.

As observed in Tables 2 and 3, the association of a high wax content, a dominance of high molecular weight n-alkanes, low sulphur and V/Ni values, light 6 13C values, high pristane/phytane ratios, an absence of C30 steranes and a paucity of the other steranes discriminates the lacustrine freshwater environment (group I). The oils from a lacustrine saline water environment (group II) show a similar set of characteristics diagnostic of non-marine oils, but differ in respect of elemental, isotopic and molecular features arising from enhanced salinity, for example, higher values of sulphur and V/Ni ratios, heavier 6 13C values, and the presence of the C25 regular isoprenoid, [3-carotane, low molecular weight steranes (C21-22), 28,30-bisnorhopane and abundant tricyclic terpanes up to C35.

The distinction between the non-marine oils (groups I and II) and those related to a dominant input of marine organic matter (groups III, IV), is based on a variety of parameters (Tables 2-4). The most useful are the high wax content and the abundance of high molecular weight n-alkanes in the non-marine oils and the presence of C30 steranes, and the abundance of steranes, 28-30 bisnorhopane and 25,28,30- trisnorhopane in the marine oils. Distinction between the marine evaporitic (group III) and marine carbonate (group IV) oils is made using compounds such as gammacerane, 13-carotane, low molecular weight steranes and tricyclic terpanes. In the evaporitic oils gammacerane and [3-carotane occur in very high abundance. In the carbonate oils, there is a high relative abundance of tricyclic terpanes up to C35 and of C21 and C22 steranes. Several features shared by these two environments distinguish them from all the others, namely a dominance of phytane over pristane linked with an even over odd n-alkane preference, high sulphur contents, high sterane concentrations, low relative abundance of diasteranes, a tendency for a dominance of C35 hopanes over their C34 homologues, and high amounts of long chain regular C25 and C30 (squalane) isoprenoids. The marine deltaic oils (group V) can be differentiated from all the other groups of oils using diagnostic markers for specific higher plant

contributions, namely 18c~(H)-oleanane and high relative abundance of the C24 tetracyclic terpane. They also show some of the features of the carbonate-derived oils, such as low pristane/phytane ratios, even/odd n-alkane dominance, high V/Ni ratios, dominance of C35 hopanes over their C34 counterparts, and high relative abundances of low molecular weight steranes. These features demonstrate the value of biological markers in the assessment of depositional environments using oils. From the results of this work we propose, as an extension of previous studies (Waples et al., 1974; Hall and Douglas, 1983; Brassell and Eglinton, 1986), that pristane/phytane ratios, even/odd n-alkane preference and abundances of specific acyclic isoprenoids (2,6,10,14,18- pentamethyleicosane and squalane), [3-carotane and gammacerane may be considered useful salinity indicators related to the water column in the depositional environment of source rocks.

Acknowledgements

The authors would like to thank the Geochemistry section of Petrobr~is research centre for all the elemental and bulk analyses, Birger Dahl and Mr L. W. Mohriak for helpful comments, and Mrs A. P. Gowar and Miss L. Dyas for advice during analytical work. The authors are also grateful to Nils Telnaes for the metastable g.c.-m.s, results and NERC for g.c.-m.s. facilities (GR3/2951 and GR3/3758) and to Petrobrds for permission to publish.

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Appendix I

Compound assignment Pr- 2,6,10,14-tetramethylpentadecane (pristane) Ph- 2,6,10,14-tetramethylhexadecane (phytane) i - C25 2,6,10,14,18-pentamethyleicosane (regular),

and/or 2,6,10,15,19-pentamethyleicosane (irregular)

i - C3o squalane [3--Carotane

1- 1313(H), 17ot(H)-diapregnane (C20 2- 5offH),14[3(H),17oc(H)-pregnane (C21) +

(13~(H),17~(H)-diapregnane (C20?) 3- 5o(H),1415(H),1713(H)- +

5o(H), 14o~(H),17ot(H)-pregnane (C21) 4- 4o~-methyl-5ot(H),14[3(H),17[3(H)- +

4o~-methyl-5ot(H),14o~(H),17offH)-pregnane (C22) 5- 5o~(H),1413(H),1713(H) - +

5offH),14oc(H),17offH)-homopregnane (C22) + 131~(H), 170~(H)-bishomodiapregnane (C23)

6- 1313(H),17oc(H)-diacholestane, 20S diasterane 7- 1313(H),17oc(H)-diacholestane, 20R diasterane 8- 5ec(H),14o~(H),17oc(H), 20S cholestane 9- 5offH),1413(H),1713(H), 20R + 20S cholestanes 10- 5o~(H),14ot(H),17oc(H),20Rcholestane 11- 5oc(H),14oc(H),17o~(H),20S24-methylcholestane 12- 5o~(H),1413(H),1713(H), 20R + 20S

24-methylcholestanes 13- 5~(H),14oc(H),17oc(H),20R24-methylcholestane 14- 5o~(H),14oc(H),17o~(H),20S24-ethylcholestane 15- 5ec(H),1415(H),1713(H), 20R + 20S

24-ethylcholestanes 16- 5offH),14oc(H),17offH),20R24-ethylcholestane 18- C2o tricyclic terpane

Mar ine and Pet ro leum Geo logy , 1988, Vo l 5, August 221

Depositional environment characterization: 114. R. Me/Io et al. 19- C2~ tricyclic terpane 20- C23 tricyclic terpane 21- C24tricyclicterpane 22- C25tricyclicterpane 23- C26 tricyclic terpanes 24- C24 tetracyclic terpane 25- C2stricyclicterpanes 26- C29 tricyclic terpanes 27- C26tetracyclic terpane 28- C2v 18o~(H)-trisnorneohopane(Ts) 2% C~otricyclicterpanes 30- C27 17o~(H)-trisnorhopane(Tm) 31- C3~ tricyclicterpanes 32- 17o~(H), 18o~(H) ,21 [3(H)-28,30-bisnorhopane

(C2~)

33- C29 17o~(H),21[3(H)-norhopane 34- C29 1713(H),21o~(H)-norhopane 35- C3o 17o~(H),2113(H)-hopane 36- C33tricyclicterpanes 37- C3~ 17[3(H),21o~(H)-hopane 38- (734tricyclicterpanes 39- C.~l 17o~(H),21f3(H)-homohopane (22S + 22R) 40- C3o gammacerane 41- Ca2 17o~(H),2113(H)-bishomohopane (22S + 22R) 42- C35 tricyclic terpanes 43- C33 17o~(H),21[3(H)-trishomohopane (22S + 22R) 44- C34 17c~(H),2113(H)-tetrakishomohopane (22S +

22R) 45- C3~ 17o~(H),21[3(H)-pentakishomohopane (22S +

22R)

222 Marine and Petroleum Geology, 1988, Vol 5, August

Depositional environment characterization: M. R. Mello et al.

APPENDIX 2

C23 n- ALKANE PRISTANE PHYTANE

i-C25 I l l _ i-C25 (REGULAR) ~ (IRREGULAR)

SQUALANE i-C3o

PREGNANES AND HOMOPREGNANES

R=H,Me

METHYLPREGNANES DIASTERANES R

- CAROTANE

ococo~ STERANES R

R:H,Me,Et

oC p ~ STERANES R

R:H, Me,Et

4-METHYL STERANES

R

i

25, 28, :30 TRISNORHOPANE

TRICYCLIC TETRACYCLIC TERPANES TERPANES

R1 R2

R: H,CH3,C2Hs, i-C 3H7 _ R1 : H, R2:CH3 RI=R2 = CH3

28, :30 BISNORHOPANE

17ec (H) TRISNORHOPANE

o~ 19 HOPANES

R: H,CH3~.C6H13

18 ec (H) TRISNORNEOHOPANE

I~ o~ HOPANES

R = H ,CH3....C6H13

HOP-13 (18)-ENE GAMMACERANE 18 O~ (H) OLEANANE 8,14 - SECOHOPANES

R= H,CzH5, i-C3H7

Marine and Petroleum Geology, 1988, Vol 5, August 223