petroleum systems

Geological Society, London, Special Publications

doi: 10.1144/GSL.SP.1999.153.01.11p169-179.

1999, v.153;Geological Society, London, Special Publications BrownC. F. Schiefelbein, J. E. Zumberge, N. R. Cameron and S. W. Petroleum systems in the South Atlantic margins

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Petroleum systems in the South Atlantic margins

C. F. S C H I E F E L B E I N 1, J. E. Z U M B E R G E , N. R. C A M E R O N & S. W B R O W N

GeoMark Research, Inc., 9748 Whithorn Drive, Houston, T X 77095, USA 1Present address: Geochemical Solutions International, 2203 Timberloch Place, Suite 235,

Woodlands, Texas 77380, USA

Abstract: Results obtained from the detailed geochemical analysis of more than 150 crude oils were used to : (1) identify a number of different petroleum systems present in the South Atlantic margins; and (2) establish geochemical criteria useful in distinguishing lacustrine- derived oils from marine-derived oils. The multiplicity and type of effective source rocks were determined by using the geochemical characteristics of the crude oils to establish the number of compositionally distinct oil families, and by inferring palaeoenvironmental conditions of source rock deposition and possible age. Using multivariate statistical techniques and bulk geochemical parameters, isotopic compositions and biomarker distributions, a number of general petroleum systems were identified, differing from each other in terms of source palaeoenvironment and age. The oils are separated into two broad groups: lacustrine and marine. Further detailed examination of each broad group, sometimes using more specific geochemical criteria, typically resulted in the establishment of sub-groups of oils according to specific source environment, such as lacustrine fresh v. saline conditions. Areas where oils of mixed provenance occur are also identified. The geochemical criteria used in discriminating between lacustrine and marine oil types were determined based on geochemical differences that exist between known marine-sourced (mainly post-salt) oils from the Kwanza and Gabon (Port Gentil area) Basins and known lacustrine-sourced (pre-salt) oils from the Congo Basin.

In areas where substantial oil production has been established, a regional oil study is an excellent way of identifying, evaluating and comparing the vari- ous petroleum systems in a province by first estab- lishing the number of compositionally distinct oil types or families. A genetic classification of the different oil types is proposed. This is collectively labelled as a 'Petroleum System', being aware that a complete definition of the term implies the knowledge of many other different factors con- trolling the occurrence of oil in nature, such as: maturity of source rock, expulsion, migration, accumulation and retention (Demaison & Huizinga 1991). Geochemical data of oils are interpreted in such a way that oil--oil correlations are made and source rock inferences are proposed. The source inferences are possible since the geochemical characteristics of the oil reveal important information on the source age and the palaeoenvironmental conditions of its deposition. Also, a regional oil study approach is particularly useful because the generated and expelled oil can be con- sidered to be representative of the average of a given source rock section, which can exhibit extreme variations in its geochemical characteristics (Moldowan et al. 1992). Previous investigators have reported the occurrence of both lacustrine (Pre-Salt) and marine (mainly Post-Salt) oil in the salt basins of Brazil and West Africa (Mello et al. 1988a, b; Teisserence & Villemin 1990; Burwood et al. 1995 and refs therein).

Approach

In order to better understand the petroleum systems of the South Atlantic margins, more than 150 oils from the major Brazil and West Africa coastal basins (Ivory Coast, Nigeria, Gabon, Congo and Angola) have been geochemically analysed. Crude oil locations are shown in Fig. 1. The detailed analytical program included measurements of bulk parameters (API gravity, and %S, V and Ni content), whole oil gas chromatography, stable carbon isotope composition of C15+ hydrocarbon fractions, and terpane, sterane and aromatic biomarker distributions using gas chromatography/mass spec- trometry (GC/MS). The analytical data were com- pared employing multivariate statistical techniques such as hierarchical cluster (HCA) and principal component (PCA) analyses using the commercial software program 'Pirouette'. The primary objec- tive of the statistical analyses is to reduce the dimensionality of the information to a few important components that best explain the variation in the data. The geochemical data used in the statistical analyses are primarily source dependent. The 20 variables used were the pristane/phytane ratio, the 13C values of the saturated and aromatic hydrocarbon fractions, and 15 biomarker ratios, includ- ing the distribution of the 1413, 17~-C27 , -Ca8 and -C29 steranes (based on the abundance of the 20R stereoisomers measured from m/z 218), total steranes/total hopanes, C26-tricyclic/C27-

SCHIEFELBEIN, C. F., ZUMBERGE, J. E., CAMERON, N. R. & BROWN, S. W. 1999. Petroleum systems in the South Atlantic margins. In: CAMERON, N. R., BATE, R. H. & CLURE, V. S. (eds) The Oil and Gas Habitats of the South Atlantic. Geological Society, London, Special Publications, 153, 169-179.

at University of Leeds on January 8, 2014http://sp.lyellcollection.org/Downloaded from



170 C. F. S C H I E F E L B E I N E T A L .

Ceara ~ ~ (~_.~. NIGEI~/~

~ ~ f ~ Benguela ~ CONGO

Fig. 1. Crude oil location map.

pentacyclic terpanes, gammacerane/C30-hopane , C31-hopanes/C30 hopane, C29-hopane/C30-hopane and oleanane/C30. Ratios based on the distribution of tricyclic and tetracyclic terpanes were also used: C19-Tri/Cz3-Tri, C23-Tri]Cz4-Tri, C26-Tri/C25-Yri and C24-Tetra/C26-Tri. Finally, the distribution of the triaromatic dinosteroids and triaromatic 3- and 4- methylsteroids measured from m/z 245 (Moldowan et al. 1995) were used and the abundance of an unknown doublet relative to a C27 diasterane (measured from m/z 259) was also included.

With this approach, a number of general petroleum systems were identified, differing from each other in terms of age and type of depositional environment of the source rocks. The oils are separated into two broad groups: lacustrine and marine. Areas where oils of mixed provenance occur are also identified. Further detailed examination of each petroleum system, sometimes using more specific geochemical criteria, typically resulted in the establishment of sub- groups of oils presumably derived from specific source environments, such as fresh v. saline lacustrine depositional conditions.

Results and discussion

Selected results of the analyses of the oils are shown in Figs 2-8. The cross-plot of the saturated/aromatic (sat/aro) hydrocarbon ratio v. sulphur content (Fig. 2) indicates that, in general, many of the oils from the South Atlantic margin have low sulphur contents and are enriched in saturated hydrocarbons (sat/aro > 2.0), consis-

tent with an origin from organic matter deposited in a lacustrine environment. Exceptions are oils from the Kwanza and Gabon (Port Gentil area) Basins that have increased concentrations of sulphur and lower sat/aro ratios (sat/aro < 2.0). Although the majority of these high sulphur oils originated from marine source rocks (Teisserence & Villemin 1990; Burwood et al. 1990), these parameters may also be affected by post-genera- tive alteration processes, such as bacterial alteration and water washing.

Figure 3 is a cross-plot of the pristane/phytane isoprenoid ratio v. the stable carbon isotopic composition of the C15+ aromatic hydrocarbons. The areas indicated for oils originating from source rocks deposited in marine or lacustrine environments are arbitrarily defined based on relationships observed for the South Atlantic margin oils. The isoprenoid ratios for the lacustrine-derived oils from the Congo Basin are > 1.5, while the majority of the marine oils from the Kwanza and Gabon Basins have isoprenoid ratios < 1.5. Many Lower Congo Coastal Basin oils from Post-Salt reservoirs have intermediate isoprenoid ratios, possibly indicative of lacustrine and marine oil mixing in the reservoir. The majority of the samples from Nigeria have elevated isoprenoid ratios associated with an origin from marine organic matter deposited in a deltaic environment. The isoprenoid ratios for most of the Brazilian oils from the Campos, Santos and Reconcavo Basins are elevated, consistent with a lacustrine origin. The isotopic compositions of the South Atlantic margin oils varies consider- ably, from c. -31 to -21 permil.




P E T R O L E U M SYSTEMS I N T H E S O U T H A T L A N T I C M A R G I N S 171

Fig. 2. Saturate/aromatic ratio v. API gravity for the South Atlantic margin oils.

Fig. 3. Pristane/phytane isoprenoid ratio v. the stable carbon isotopic composition of the C15+ aromatic hydrocarbons for the South Atlantic margin oils.




172 C. E SCHIEFELBEINETAL.

Marine Oil; m/z 191

29H

23T Tet Ts Tm I 24T \ 28 21T __- L t 25T \26T T 29T] [ 28H]

, i i i i i i

30H

GA

31E 32H 35H 33H 34H

i i

Lacustrine Oil; m/z 191 30H

29H GA

23T Tet ~ 1H ' 28T 29T~ 32H

~.~ 35H

c27 Diast Marine Oil; m/z 259

/ ~ A Unknown Doublet

Lacustrine Oil; m/z 259 Unknown Doublet

C27 Diast A

Marine Oil; m/z 245 7,8 1'6, ~ 3 6 i 3 ~ 6 ) 3

Lacustrine Oil; m/z 245 3 3 3

3 3

4,6 3 / 6 8 6

Fig. 4. Representative terpane (mlz 191), rearranged sterane (mlz 259) and triaromatic dinosterane (mlz 245) mass chromatograms for marine and lacustrine oils.




P E T R O L E U M SYSTEMS I N T H E S O U T H A T L A N T I C M A R G I N S 173

Fig. 5. Extended hopanes (C31+)/C30 hopane v. C26/C25 tricyclic terpane ratios for the South Atlantic margin oils.

Fig. 6. Sterane/hopane ratio v. the proportion of C27 diasterane relative to the unknown doublet present in m/z 259 for the South Atlantic margin oils.




174 C. E SCHIEFELBEINETAL.

Fig. 7. r Sterane compositions (m/z 218) for the South Atlantic margin oils.

Fig. 8. Triaromatic dinosterane compositions (m/z 245) for the South Atlantic margin oils.




PETROLEUM SYSTEMS IN THE SOUTH ATLANTIC MARGINS 175

Terpane (m/z 191), rearranged sterane (m/z 259) and triaromatic dinosterane (m/z 245) mass chromatograms representing marine and lacustrine oil from the South Atlantic margins are shown in Fig. 4.

Figure 5 is a cross-plot of the proportion of extended hopanes (C31+) relative to C30 hopane v. the proportion of C26 relative to C25 tricyclic terpanes. In general, lacustrine-derived oils have very low proportions of extended hopanes and C26/C25 tricyclic terpane ratios > 1.2. Marine-derived oils have higher proportions of extended hopanes and C26/C25 tricyclic ratios < 1 (see m/z 191, Fig. 4).

Figure 6 is a cross-plot of the proportion of steranes relative to hopanes v. the proportion of a C27 diasterane relative to an unknown doublet (from m/z 259; Fig. 4). In general, lacustrine- derived oils have very low proportions of steranes and contain high concentrations of the unknown doublet relative to the C27 diasterane. Marine- derived oils have higher proportions of steranes and C27 diasteranes.

Figure 7 is a ternary diagram depicting the relative proportion of ctl~ steranes (m/z 218). Lacustrine-derived oils from the Congo Basin appear to be enriched in C27 steranes, while the marine-derived oils from the Kwanza and Gabon Basins have more diverse oq3[~ sterane compositions.

Figure 8 is a ternary diagram depicting the relative proportion of triaromatic dinosteranes (m/z 245; Moldowan et al. 1995). Lacustrine- derived oils from the Congo Basin appear to be enriched in triaromatic dinosteranes (labelled as 3 in Fig. 4), while the marine-derived oils from the Kwanza and Gabon Basins have more diverse compositions and contain higher relative abundances of 3-methyl (labelled as either 4 or 6 in Fig. 4) and 4-methyl (labelled as either 7 or 9 in Fig. 4) triaromatic steranes.

With the help of multivariate statistical analyses (HCA and PCA), oils from the South Atlantic margin can be separated into two broad groups, either marine or lacustrine. The marine group in the top portion of the cluster analysis dendrogram shown in Fig. 9 contains several sub-clus- ters representing either pure marine oils, mixed lacustrine-marine otis, marine transitional oils, or oils originating from Tertiary source rocks deposited in a deltaic environment. The lacustrine oils in the bottom cluster of the dendrogram are separated into three sub-groups, primarily reflecting differences in water chemistry (e.g. fresh v. saline). These separations are possible because the geochemical characteristics of oil reveal important information on the source age and the paleoenvironmental conditions of its deposition (Didyk et al. 1978; Seifert &

Fig. 9. Cluster analysis dendorgram for the South Atlantic margin oils.




176 C. E S C H I E F E L B E I N E T A L .

Moldowan 1978, 1981; Moldowan et al. 1985; Peters et al. 1986; Powell 1986; Zumberge 1987; ten Haven et al. 1988; Mello et al. 1988a, b; Volkman 1988; Pu et al. 1991). For example, con- straints on the age of the source rock can be made by the presence or absence of oleanane, a specific biomarker derived from angiosperms and associated with a Tertiary-aged source due to the evolution of higher land plant organic matter (Ekweozor & Udo 1987). When an oil contains significant concentrations of oleanane (Ol/C30- hopane > 0.2), a Cenozoic source age can be con- fidently inferred (Moldowan et al. 1994).

The South Atlantic margin oils originating from source rocks deposited in a marine environment (marine oils) are characterized by variable to high sulphur content, generally lower sat/aro and pristane/phytane ratios, high proportions of steranes relative to hopanes, high proportions of extended hopanes, and variable o~[3f3 sterane and triaromatic dinosterane compositions. Marine oils have low C26/C25 tricyclic terpane ratios and abundant diasteranes relative to the unknown doublet (from m / z 259). Marine oils can also be identified based on the presence of the 24-n- propylsteranes, a highly characteristic biomarker for marine algae as detected by MRM analyses (Moldowan et al. 1990).

The South Atlantic margin oils originating from source rocks composed mainly of algal kerogen (Type I) deposited in a lacustrine environment (lacustrine oils) are distinguished by low sulphur contents, C26-tricyclic/Czftricyclic ratios >> 1, low proportions of steranes relative to hopanes, and low abundances of extended (C31+) hopanes. Lacustrine oils are enriched in C27 c~13~ steranes and C29 triaromatic dinosteranes. Another important lacustrine classifier is the abundance of an unknown compound detected in the m / z 259 mass chromatogram (with a m / z 414 parent) relative to regular steranes. This compound is present in significant concentrations in lacustrine oils from West Africa, offshore Brazil, Utah and the Far East, but it is only a minor constituent in marine and terrestrial oils (relative to steranes). In general, lacustrine oils from the South Atlantic margins have pristane/phytane ratios > 1.5 and c. < 3.0. Classification based on stable carbon isotopes is difficult because the isotopic composition of lacustrine oils can vary between -20 to -30 permil (Burwood et al. 1995).

The South Atlantic margin oils classified as 'Tertiary deltaic' originated from mainly terrige- nous kerogen (Type III) deposited in fluvial deltaic or paralic environments. Tertiary deltaic oils are characterized by the absence of 24-n-propylsteranes, high abundances of low molecular weight (C~9 and C20 ) tricyclic terpanes and C24

tetracyclic terpanes, a predominance of C29 (z~ steranes, elevated pristane/phytane ratios, positive canonical variables and abundant oleanane.

Conclusions

The PCA loadings and scores plots shown in Fig. 10a and b, respectively, can be used to separate the oils according to source type. The distribution of oil types is shown in Fig. 11.

The South Atlantic margin oils originating from source rocks deposited in marine or transitional marine environments (positive PC1 and variable PC2 values; Fig. 10b) are primarily from the Ivory Coast and the Gabon (Port Gentil area), Kwanza and Benguela Basins in West Africa and the Santos, Espirito and Sergipe Basins offshore Brazil. Certain oils from offshore Ivory Coast and the Lower Congo Coastal and Kwanza Basins displayed intermediate geochemical characteristics (mixed oils), possibly suggesting commingling of marine-derived and lacustrine-derived oils (Connan et al. 1987; ten Haven 1996) and/or the presence of a mixed kerogen assemblage (Burwood et al. 1995).

The South Atlantic margin oils derived from Tertiary source rocks deposited in a deltaic environment (positive PC1 and negative PC2 values; Fig. 10b) are from Nigeria, Benin, Equatorial Guinea, Cameroon and the Foz do Amazonas Basin of northern Brazil. Tertiary oils may also be present offshore the Ivory Coast and have been reported offshore northern Angola (Connan et al. 1987).

Lacustrine oils are widespread in West Africa and offshore Brazil and are thought to originate from organic-rich Pre-Salt (Neocomian- Barremian) source rocks (Mello et al. 1988a, b; Burwood et al. 1995). Lacustrine oils have negative PC1 and variable PC2 values (Fig. 10b). Lacustrine oils from West Africa are present in the Congo, Lower Congo Coastal and Benguela Basins. Brazilian oils with lacustrine affinities are present in the Reconcavo, Campos, Potiguar, Ceara and Bahia Sul Basins.

Certain geochemical characteristics of lacustrine oils can often be used to separate the oils according to more specific source environments. For example, the relative abundance of gammacerane may reflect conditions of increasing palaeosalinity (ten Haven et al. 1988). Carbon isotope values for the lacustrine oils are variable and may reflect differences in algal speciation, water chemistries and/or carbon budget (Sofer 1984; Katz & Mertani 1989). The pristane/phytane isoprenoid ration may also reflect environ- mental conditions (ten Haven et al. 1988).




P E T R O L E U M SYSTEMS IN THE SOUTH ATLANTIC MARGINS 177

Fig. 10. Principal component (a) loadings and plot (b) scores for the South Atlantic margin oils.




178 C. E S C H I E F E L B E I N E T A L .

Fig. 11. Oil family location map for the South Atlantic margin oils.

Alternatively, the differences in oil chemistries may be related to facies variations within a single, long-term lake system. Lacustrine oils from the Campos Basin offshore southern Brazil are geochemically similar to lacustrine oils from offshore central and southern Angola (Lower Congo and Benguela Basins), implying an origin from algal source rocks deposited in similar saline lacustrine environments. In addition, oils from the Reconcavo and Lower Congo Basins are geochemically similar, implying an origin from algal source rocks deposited in similar brackish/saline lacustrine environments. None of the Brazilian oils included in this study could be correlated to the West African oils from the Congo Basin, thought to originate from algal source rocks deposited in fresh, deepwater lacustrine environments.

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