Phillip S. Plummer
Seychelles National Oil Co.
Victoria, Mahe, Seychelles
The regular occurrence of tarballs on beaches in the Seychelles archipelago attests to the presence of a mature and generating hydrocarbon kitchen in the Mesozoic succession on and/or adjacent to the Seychelles bank.
Since 1978 many tarballs have been collected and analyzed from Coetivy Island (Fig. 1) while, despite a seemingly annual occurrence of tarballs on the main island of Mahe and nearby Silhouette some 300-350 km to the north northwest, to date only one such sample has been analyzed from each island.
The various geochemical analyses of the tarballs have established that they represent remnants or residues of natural, often high wax crude oils.1 2 3 4 5 6 7 They are, however, not correlatable to known crudes from the region, such as those from the Suez or Arabian gulfs or India.2 8 9
Nor do they resemble other high wax crudes,1 or the bitumen deposits of Madagascar.2 Thus the tarballs are concluded to have originated from local, naturally occurring seepage.2 5 7 8
GEOCHEMICAL ANALYSES
GAS CHROMATOGRAPHY
Gas chromatographs for the tarballs are often depleted in alkanes in the C17 to C30 range (Fig. 2), suggestive of excessive exposure to evaporation and/or biodegradation depleting the lighter hydrocarbon fractions.1 2 3 5 9
However, several samples still retain a significant presence of the lighter C17 to C30 alkanes (Fig. 3), indicative of less exposure.3 5 7 9 These results suggest that the tarballs originate through regular replenishment from local seeps having a relatively active flow.2 7
One almost fresh sample collected from Mahe Island displays a normal paraffin distribution concluded to be consistent with an oil generated from a mature marine source rock.7
BIOMARKERS
Oil-to-oil and oil-to-source rock correlation can be achieved using biomarkers, typically steranes and terpanes.
The dominance of C29 over C27 steranes in samples3 suggests a landplant derived kerogen in the source rock.10 Some samples, however, show a dominance of C27 steranes, indicative of a marine source, and those also having a significant C30 sterane peak imply a source rock containing marine algal sapropels.3 10
Meanwhile, isomers in the C30 range and the total sterane fragmentograms suggest thermal equilibrium indicative of a source rock that has attained the peak oil generative stage.5 7 8
In some samples phytane is predominant over pristane,3 4 6 7 9 supporting a marine origin,10 although many imply a mixture of marine and landplant debris with Pr:Ph close to unity,5 8 while others indicate landplant dominance2 (see table).
Meanwhile, the triterpane components indicate that the marine dominant source rock is approaching the main stage of oil generation, while the source rock dominated by landplant kerogens is more mature, though still at peak oil level.3
While low diasterane values and a predominance of C35 over C34 homohopane in some samples suggest a possible carbonate marine source rock,6 7 alkyl benzene fragmentograms confirm the likelihood of two source rock types.5
The presence of prominent C27 cholestanes implies that these source rocks are younger than early Paleozoic,5 supporting an early Jurassic or early Cretaceous age concluded from previous analyses.3 4
SARA ANALYSIS
Analyses for saturates and aromatics (HCs) versus resins and asphaltenes (non-HCs) reveal samples that are depleted in HCs, and hence indicative of excessive biodegradation and water-washing,4 6 9 as well as samples dominated by HCs and hence less biodegraded5 9 (table).
This supports the thesis of regular replenishment of the tarballs from local, active natural seeps.
When asphaltenes were analyzed for C, H, O, N, and S the groupings revealed a strong geographical bias as well as being influenced by the degree of biodegradation.3 This bias may be a reflection of two source facies (Fig. 4).
C13 ISOTOPES
C13 isotope analyses lend support to interpretations of source rock origin, with landplant kerogens typically giving dC13 values more negative than -30%, marine algal sopropels 27 to -25% and lacustrine/lagoonal organics more positive than -24%.3
Many of the samples analyzed support a source rock of mixed marine algal and landplant kerogens with dC13 values of between -29 and -27%,2 3 4 5 6 7 while occasional samples reveal a more purely marine origin of the source rock3 5 (table).
Again, an early peak oil stage of maturation for the source rock was concluded from the C13 analyses,8 and confirmed by VR/E values of between 0.8 and 1.0%.5
API GRAVITY
Although measured API gravity of the samples was often not obtainable,6 7 where measured9 it lies between 16.4 and 17.6. It is believed, however, that biodegradation will have lowered these values and the crude oils in the active seeps can be expected to be of significantly higher API gravity.9
CONCLUSION
The tarballs from Seychelles appear to be derived from local, naturally occurring seeps of relatively active flow.
They appear to herald from at least two source rock facies seemingly of Mesozoic age, one dominated by marine algal sapropels and the other dominated by landplant derived kerogens with varying levels of marine organics; one of these source rocks may be a carbonate.
Although both source rocks appear to lie in the peak oil stage of generation, the landplant dominated source rock is perhaps the more mature.
The tarballs' analytical similarity and regular occurrence on islands some 300 km apart suggest a potential kitchen area of at least 30,000 sq km on and adjacent to the Seychelles Bank which must, therefore, be ranked as having good prospectivity for the discovery of liquid hydrocarbons.
REFERENCES
- nvironmental Analysis Ltd., untitled report on geochemical fingerprinting of two oil residues from Seychelles for Burmah Oil Co., 1978.
- Torkelson, B.E., Beach tar analysis: Beach tar from Coetivy Island, Seychelles. Amoco Research Report 81910 CH, 1991.
- Cooper, B.S., Collins, A.G., Thompson, S. and Croxton, C., A geochemical evaluation of 10 tarball samples and two coal fragments from Coetivy and Silhouette islands, Seychelles, submitted by Amoco Seychelles Petroleum Co., Robertson Research Report 4687P/D, 1982.
- Kumar, R. and Das, S.P., Genetic correlation of tars of Seychelles with potential source rocks of Cretaceous age, ONGC, Kdmipe. Project GC.0101.103, 1985.
- Dungworth, G., Petroleum geochemical analyses for tar samples and a water analysis from Coetivy Island, Seychelles, Paleoservices Report G1660, 1986.
- Elrod, L.W., Seychelles beach tar analysis, Texaco technical memo 88-179, 1988.
- Elrod, L.W., Geochemical evaluation of Seychelles oil, Texaco technical memo 92-xx (Prelim.), 1992.
- Reid, J.P. and Pinch, J.J., Organic Chemistry, in Seychelles Basin Exploration Summary, Amoco Report, 1982, pp. 56-65 attachment B.
- Kumar, R., Das, S.P. and Rao, S., Identification of beach tar samples from Coetivy Island, Seychelles, ONGC KDMIPE Report RDO/201/Samp./82, 1982.
- Isaksen, G.H., Molecular geochemistry assists exploration, OGJ, Mar. 18, 1991, pp. 127-131.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
