EVIDENCE OF ACTIVELY MIGRATING HYDROCARBONS FOUND OFF SEYCHELLES

Phillip S. Plummer
Seychelles National Oil Co. Ltd.
Victoria, Mahe, Seychelles

Various exploration data from the offshore Seychelles frontier province indicate the presence of an actively generating oil kitchen. These data include the regular occurrence of tarballs on a number of beaches, migrant hydrocarbon shows in all three exploratory wells, and various geosurvey anomalies, specifically from gas sniffer, ultraviolet fluorescence, and seismic surveys (Fig. 1).

The results of geochemical analyses from numerous tarballs conducted over the past 15 years indicate that actively flowing seepage is occurring locally from at least two mature source rocks.1 These source rocks are likely to be Mesozoic in age with one dominated by marine algal sapropels and the other by landplant derived kerogens with varying degrees of included marine organic matter. One of these source rocks may be a carbonate.

More recent analyses of an extensive occurrence of such tarballs on the northwest coast of Mahe Island have confirmed this result. These tarballs appeared after two days of blustery northwest winds and were very fresh, displaying a high saturates percentage and full alkane suite from C12 to C36.

With phytane/pristane greater than one; no triterpanes diagnostic of evaporitic or terrestrial organic matter; and a mature sterane ratio dominated by C27, Plus low abundances of C28 and rearranged steranes, the tar-balls are concluded to have originated from a mature marine, probably carbonate source rock dominated by algal sapropels of either late Paleozoic or early Mesozoic age.2

Three exploratory wells drilled by Amoco in 1980-81 at the western extremity of the western shelf (Fig. 1) are to date the only wells off Seychelles.

Collectively the wells penetrated a sequence of Tertiary carbonates, Upper Cretaceous/Paleocene volcanics (Deccan equivalents), Middle Jurassic to Lower Cretaceous marine clastics of drift tectonic origin, and late Triassic to early middle Jurassic continental clastics of rift origin (Karroo/Gondwana equivalents).

In all wells, hydrocarbon shows of varying degrees were encountered (table), the most significant being migrant hydrocarbons from Middle Jurassic clastics in 1 Seagull Shoals that display an oil-like n-C15 to n-C30 alkane distribution, and from DST-1 in the late Triassic of 1 Reith Bank in which the presence of 0.7 mg/1 of benzene was interpreted as indicative of a nearby oil or wet gas accumulation.6

In addition, a Middle Jurassic to Lower Cretaceous shale/mudstone sequence 6,520 ft thick, bearing facies of source rock quality was encountered in A-1 Owen Bank (Fig. 2). 3 4 The organics appear to be alternations of Type II/III and Type III (Fig. 2), with overall fair quality for both oil and gas.

Both peak oil and peak gas maturity occur in the lower 4,040 ft of the sequence, and hydrocarbons have been generated and expelled. 5 A shaly source rock facies has also been identified interbedded with sandstones in the late Triassic of 1 Reith Bank 6 7 (table) which contains terrestrial organic matter and is presently at peak gas maturity.

Geochemical correlations between the shows and a tarball from Coetivy Island indicate a strong resemblance between the late Cretaceous/Paleocene show in 1 Reith Bank, the Middle Jurassic show in 1 Seagull Shoals, and the tarball.

When correlated with the various source rocks from the wells, these shows and the tarball relate to the late Triassic landplant dominated source rock in 1 Reith Bank.8 9 10

These hydrocarbons were not, however, correlatable with the thick Middle Jurassic/Lower Cretaceous source rock facies in A-1 Owen Bank, which correlates well with the Middle Jurassic show in that well (Fig. 3).

Thus, while one source rock facies can be correlated over some 375 km from the western shelf to Coetivy Island, other source rock facies are evident in both areas.

PUITS GOUDRON SHOWS

In the north of Coetivy Island (Fig. 1) is a water well identified as Puits Goudron, which translates into English as tar well.

The origin of the name appears to be lost to history, but analysis of the water has revealed a gas/oil distribution to be crude oil-like which, being effectively non-biodegraded, confirmed the well to be evidence of active, natural oil seep-age. 11 12

This result has not, however, proven to be repeatable, and an analysis of carbonate sand from the well has since revealed non-oil-like organic matter produced from algal growth.10

GEO-SURVEYS

Various geochemical/geophysical surveys have revealed evidence of the presence of migrated hydrocarbons in the Seychelles offshore.

1. Gas sniffer survey.

   In 1983 a waterborne gas sniffer survey was conducted over the entire prospective portion of the main Seychelles Bank, the adjacent banks, and Farquhar some 750 km southwest of Mahe (Fig. 1).

   The sensor, towed behind a seismic vessel some 20 m above the sea floor, or at about 200 m depth in deep water, was designed to detect the presence of various low molecular weight aliphatic hydrocarbon compounds, such as methane, ethylene, ethane, propane, iso- and normal-butane, in the water column.

   Butane and total hydrocarbon anomalies two to three times the background level were encountered on the northern shelf of the Seychelles Bank and over Farquhar. Butane anomalies are generally indicative of oil accumulations being present in the vicinity.13

   Significant anomalies were also detected over Coetivy Bank. Recordings of the ethane/ethylene ratio up to 10 times that of ambient seawater and iso-butane in concentrations of up to three times background have been interpreted to indicate the presence of microseepage.13

   This seepage is likely to have originated from light gases associated with an oil accumulation rather than wet gas, dry gas, or biogenic gas. 12

2. UV seepfinder survey.

   In 1991 an airborne UV seepfinder survey was conducted over a significant portion of offshore Seychelles. This passive solar fluorosensor detects the strong UV fluorescence from polycyclic aromatic hydrocarbons in thin oil films on the sea surface that originate from seepage offshore. Recording is achieved on separate channels specifically filtered to receive wavelengths coinciding with the spectral signatures of light, medium, and heavy hydrocarbons.14

   Although much of this survey remains confidential, a very significant result was obtained in open acreage over the western shelf in the vicinity of the three exploration wells. The resultant anomalies in this area (Fig. 4) were most pronounced on channels designed to detect light to medium hydrocarbons and confirmed the good migrant oil show reported a decade previously from 1 Seagull Shoals, and lesser shows from 1 Reith Bank and A-1 Owen Bank.

3. Seismic data.

   In the deepwater just south of the Seychelles Bank some 40 km southeast of 1 Seagull Shoals (Fig. 1), two possible anomalous seismic events occur on line TCO-111.

These events may represent direct hydrocarbon indications (DHIs) as both display flat, anomalous amplitudes (especially of the negative, white loop) towards the top of separate anticlinal structures (Fig. 5). Also, a phase reversal of the strong seismic event capping the southwestern structure occurs where it is intersected by the flat spot.

As these anomalies lie not too distant from the live oil shows in 1 Seagull Shoals and the UV seepfinder anomalies associated with that show, as well as those of the other wells, they are likely to indicate the presence of migrated hydrocarbon accumulations.

PALEOGEOGPAPHY

Although today Seychelles lies isolated amid the western Indian Ocean, it relates geologically, to the onetime Gondwana supercontinent.

It is interesting to note the various occurrences of hydrocarbons in its then adjacent neighbors, specifically Madagascar, Somalia, and India (Fig. 6). The presence in these countries of discoveries in deposits ranging in age from Triassic to Lower Tertiary suggests that the entire known geological sequence in Seychelles must be considered as having the potential for the discovery of hydrocarbons.

Also of great significance is the paleogeographic distribution of potential source rock facies, although they are perhaps the least understood component of the hydrocarbon system of the region.

Of particular relevance is the presence of Middle Jurassic oolitic limestones in Pakistan, India, and Seychelles as well as Madagascar, Tanzania, and Kenya 15 in what was a gulf of Tethys extending into the East/West Gondwana rift from the Middle East carbonate basin.

It is source rocks associated with these oolitic limestones in the Middle East basin that are primarily responsible for much of the oil of that region, 16 and that thus boost the potential for oil prone source rocks offshore Seychelles.

CONCLUSIONS

Evidence exists to support that at least two hydrocarbon source rocks are not only present in the Seychelles offshore but also mature and actively generating hydrocarbons.

One of these source rocks appears to contain principally terrestrial organic matter, and is of likely Triassic age and of rift tectonic origin. A second source rock appears to be a Middle Jurassic/early Cretaceous marine shale/mudstone of drift tectonic origin and, although bearing mixed organic matter, is dominated by landplant kerogens.

Additionally, evidence indicates the presence of a marine carbonate source rock comprising algal sapropels little diluted by terrestrial organic matter that may be of Middle Jurassic age and deposited in an arm of the prolific Middle East carbonate basin.

That these various source rocks are mature and generating hydrocarbons, which are actively migrating, is evidenced by the persistent presence of tarballs on some beaches of Seychelles, by live shows in the three exploration wells, and the presence of anomalies on various geosurveys.

Indeed, some of these indications have been interpreted as indicative of hydrocarbon accumulations being present in the offshore of the western shelf6 and Coetivy land.12 Such results indicate the Seychelles offshore to be a very attractive, albeit frontier oil exploration province.

REFERENCES

1. Plummer, Phillip S., Geochemical analysis may indicate oil kitchen near Seychelles Bank, OGJ, Aug. 31, 1992, pp. 52-54.

2. The Geochem Group, Analytical data for three tarballs collected from a beach on the northwest coast of the island of Mahe, Seychelles, Report No. 6354, 1992.

3. Amoco Seychelles Petroleum Co., Owen Bank A-1 completion report, 1981.

4. Cooper, B.S. and Collins, A.G., A geochemical evaluation of selected intervals in the Amoco Seychelles Owen Bank A-1, Reith Bank-1 and Seagull Shoals-1 welts, offshore Seychelles, Robertson Research report 4694P/D, 1982.

5. Torkelson, B.E., Source rock analysis, Amoco Seychelles No. A-1 Owen Bank well, offshore Seychelles, Amoco Production Co. research center, Tech. Serv. 805348CH, unpublished, 1980.

6. Amoco Seychelles Petroleum Co., Reith Bank-1 completion report, 1981.

7. Cooper, B.S. and Collins, A.G., A geochemical evaluation of the interval 450-12,790 ft (T.D.) in the Amoco Seychelles Reith Bank-1 well, offshore Seychelles, Robertson Research Report 4604P/D, 1981.

8. Amoco Seychelles Petroleum Co., Seagull Shoals-1 completion report, 1981.

9. Cooper, B.S. and Collins, A.G., A geochemical evaluation of the interval 4,500-8,995 ft (T.D.) in the Amoco Seychelles Seagull Shoals-1 well, offshore Seychelles, Robertson Research Report 4637P/D, 1981.

10. Collins, A.G., Hitchings, V.H., Laing, J.F. and Marshall, P.R., The petrography, biostratigraphy and petroleum geochemistry of six field samples from Coetivy Island, Seychelles, submitted by Amoco Seychelles Petroleum Co., Robertson Research Report 2617P/F, 1981.

11. Dungworth, G., Petroleum geochemical analyses for tar samples and a water analysis from Coetivy Island, Seychelles, Paleoservices Report G1660, 1986.

12. Pierce, W., Bush, M. and Saad, R., Geology and petroleum potential of the Seychelles, unpublished Amoco report, 1984.

13. InterOcean Systems Inc., Amoco Seychelles sniffer geochemical survey, unpublished Amoco report, 1983.

14. Prospex Ltd.; Seepfinder: Identification of natural crude oil seepage offshore, company brochure, 1992.

15. Coffin, M.F. and Rabinowitz, Ph. D., Evolution of the conjugate East African-Madagascar margins and the western Somali Basin, GSA Special Paper 226, 1988.

16. Murris, R.J., Middle East: Biostratigraphic evolution and oil habitat, AAPG Memoir 35, 1984, pp. 353-372.

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