DIVERSITY OF PLAYS BODES WELL FOR PROMISING SEARCH OFF SEYCHELLES

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

The sedimentary sequence capping the vast Seychelles bank reaches 6-8 km in thickness and contains a variety of play styles suitable for the entrapment of hydrocarbons.

While this sequence extends well beyond the 200 m isobath of the bank, forming a microcontinent, amid this submarine plateau inliers of granitic basement emerge above the Indian Ocean to form the core islands of the Seychelles archipelago (Fig. 1).

The plateau's origin relates to the breakup of Gondwana. Its known history, extending back to the Middle Triassic, involves three phases of rift tectonics. It is also postulated that deposits of possible Permo-Carboniferous age lie in grabens that are as yet unpenetrated by the drill.

TECTONO-STRATIGRAPHIC FRAMEWORK

The island-forming basement inliers comprise various facies of PanAfrican granites that were emplaced 683-713 million years ago.1 Strong northwest-southeast lineaments, plus secondary northeast-southwest and north-south trends, control present-day island morphology and appear to have strongly influenced basin development within the Seychelles microcontinent.

FAILED RIFT DEPOSITION?

Geophysical data indicate the presence of three major depocenters within the Seychelles bank that pre-date the drilled stratigraphy.

Tracing rift development through paleogeographic re- constructions of Gondwana, it becomes evident that these depocenters lie sub-parallel to, and on approximately equal spacing with, the Morondava basin of Madagascar and the Godavari and Mahanadi basins of Peninsula India (Fig. 2a). These latter three basins initiated as failed rifts during the Permo-Carboniferous, and it is postulated that the Seychelles depocenters are coeval and contain similar coal-bearing continental sequences. 2

INITIAL RIFT-DRIFT SEQUENCE

The stratigraphically oldest sediments penetrated offshore Seychelles are Middle Triassic lacustrine deposits encountered in the 1 Reith Bank well.3 These grade upwards through sand/mud cycles of proximal, then distal fluviatile origin, and finally into fluvial-dominated deltaics as the rift developed.

This primarily continental sequence is equivalent to the Karoos of southern Africa and Madagascar and the Upper Gondwanas of Peninsula India. It represents deposition in the rift that initiated the breakup of Gondwana and is best developed along the western margin of the Seychelles microcontinent (Fig. 2a).

As breakup became imminent, Tethys invaded the maturing rift and marine carbonates replaced the fluvio-deltaic deposits. Included within these carbonates is a Middle Jurassic oolitic limestone that is a marker horizon along the East African coast as well as in Madagascar, Pakistan, and the Middle East. With the eventual severing of Gondwana at about 160 million years ago, deposition upon the now passive margin produced a thick succession of fine clastics which continued during the subsequent southeasterly drift until the end of the early Cretaceous (Figs. 2b & 3).

FURTHER RIFTING, DRIFTING

New rifting developed in the early late Cretaceous between Madagascar and Seychelles/India (Fig. 2b). Although short-lived, only some 10-12 million years, a tremendous amount of sediment was generated and depositional rates were in the order of 350 m/million Years, some 12 times that of the earlier rift phase. The tectonics driving this rifting were transform in nature, and not of the more usual thermal extension,

Left-lateral translation across a dislocation of the basement transform created a pull-apart basin in the area of the southern Seychelles margin. Within this basin up to 4 km of sediment accumulated, dominated by thick progradational sequences of fluvial-dominated deltaics (Fig. 3).

No wells have been drilled in this area, however, and it is thus not yet known whether the setting of this pull-apart basin was purely lacustrine, normal marine, or hypersaline. The potential for hydrocarbon generation from the prodelta deposits within this basin, which display the low frequency, high amplitude seismic facies often associated with source rocks (Fig. 6a), will ultimately be dependent upon the Depositional environment.

At 84 million years ago an oceanic spreading ridge replaced the transform fault between Madagascar and Seychelles/India,4 though not so to the north, inducing a counter-clockwise rotation as the latter block drifted northwards (see Fig. 2c). Deposition during this drift phase was of principally calcareous pelagic oozes, as seen from the 1 Seagull Shoals well,5 which lies most proximal to this southern Seychelles margin (Fig. 3),

FINAL RIFT-DRIFT SEQUENCE

As the Seychelles/India block continued its rotational northward drift, a rift developed along the now northeastern Seychelles margin during the latest Cretaceous (Figs. 2c, 3). This rift appears to have resulted from thermal extension tectonics as it culminated in the eruption of the extensive Deccan traps between 66-69 million years ago,6 after which the initiation of the Carlsberg spreading ridge at 65 million years ago 4 ensured the continued northward drift of India.

Seismic data from the northeastern margin reveals the resultant rift succession to comprise a moderately thick parallel bedded sequence overlain by a thinner progradational unit. This is in turn overlain by drift deposits composed of pelagic oozes in the deep waters, or shelf carbonates in areas such as the Seychelles bank.

BRIEF EXPLORATION HISTORY

Exploration for hydrocarbons began offshore Seychelles in late 1977 when 53% of the 60,000 sq km shallow water acreage was held under three separate licenses.

A total of 6,443 km of seismic, gravity, and magnetic data were acquired before Amoco Seychelles Production Co. drilled three wells on the western shelf in 1980-81 (Fig. 1). Although plugged and abandoned, all three wells recorded shows of migrant hydrocarbons and the presence of reservoirs, seats, and source rock sequences.

There were sound technical reasons for each well being abandoned. A-1 Owen Bank failed to reach the objective due to drilling problems in the then unknown succession, 1 Reith Bank encountered good reservoirs but lacked a capping seal, and 1 Seagull Shoals has since been shown through depth conversion to have been drilled on a structural nose.

Not totally discouraged by these results, Amoco commissioned a 27,900 km aeromagnetic survey and acquired a further 7,100 km of seismic, gravity, and magnetics data over the entire prospective shallow water area in 1982-83. Economic considerations, however, forced it to relinquish the acreage during the industry-wide crisis in 1986.

A second phase of exploration began in 1987 when Enterprise Oil plc took license to some 20,000 sq km. License holdings during this phase peaked in 1990 when 71% of the shallow water acreage was held by three licensees, namely Enterprise, Texaco, and Ultramar. Some 8,500 km of seismic, gravity, and magnetic data have been acquired during this phase.

As of end July 1993, Enterprise and Lasmo (which acquired Ultramar's concession through a takeover) hold license to 15,000 sq km of the Seychelles, Coetivy, and Platte banks (Fig. 1), and Enterprise has entered the second exploration phase of its license which entails the drilling of one well.

HYDROCARBON PLAY MODELS

The sedimentary successions of rift-drift origin that are developed on the various margins of the Seychelles microcontinent comprise a variety of continental and marine plays in which the numerous hydrocarbon indications present in the region 7 suggest that the entrapment of hydrocarbons is very likely.

1. BLOCK FAULTED RIFT PLAY

   This play, predominant on the western shelf and postulated at Farquhar (Figs. 2, 4a), involves the Triassic to Middle Jurassic continental clastics and near-shore marine carbonates associated with the initial Gondwana rift.

   A coeval sequence is also postulated to have developed in a failed rift along the southern shelf (Figs. 2a, 4b). On the northeastern shelf this play is developed in the rift deposits of latest Cretaceous age (Fig. 4c).

   Reservoirs: Sandstones of the fluviatile cycles in the Karoo/Gondwana equivalents on the western shelf displayed 20-22% porosity with DSTs flowing at rates of up to 1,200 b/d, indicative of good permeabilities.8 Similar reservoir characteristics can be expected from the sandstones on the southern and northeastern shelves.

   Other possible reservoirs include sandstones of deltaic topset origin, and oolitic limestones. Porosites of 17% were determined from logs through these limestones on the western shelf.9

   Seals: Regional seals are provided by the thick marine shales and siltstones of transgressive drift origin on the western and northeastern shelves, whilst Cretaceous prodelta muds are postulated to fulfill this role on the southern shelf.

   Shales interbedded within the cyclical sandstone reservoirs could also act as seals to produce stacked reservoir packages,

   Source rocks: Potential source rocks occur in the Middle Triassic lacustrine deposits at 1 Reith Bank. Total organic carbon contents (TOCs) of up to 6.7% were encountered in black mudstones containing up to 70% vitrinite and potential yields of 6,400 ppm hydrocarbon.10 Early Jurassic deltaic lagoonal mudstones encountered in 1 Seagull Shoals also have source potential, with TOCs of 1.96% with 40% vitrinite and potential yields of 1,100 ppm hydrocarbon.11

   Also, the Middle Jurassic carbonates bear resemblance to source rocks, with TOCs of up to 1.32% (averaged from cuttings over 50 ft intervals) comprising predominantly Type III organic matter with potential yields of 2,300 ppm hydrocarbon. 12 This sequence equates to carbonates of the Middle East, which contain the prolific Hanifa source rocks,13 and similar carbonate environments protected from terrestrial input could have developed locally as suggested by the thin carbonate sequence preserved from mid-Cretaceous erosion at 1 Reith Bank.

   Finally, the thick late Jurassic/early Cretaceous mudstones encountered in A-1 Owen Bank display TOCs of up to 1.7% (again cuttings samples average) comprising Type III or II-III organic matter.12

   Structures: Tilted fault blocks provide the major trapping mechanism, whilst horst blocks and possible rollovers into slump-induced reverse faults are locally developed.

   On the western shelf, A-1 Owen Bank was drilled downflank and failed to reach the objective cyclic continental sequence. With hydrocarbon shows and mature source rocks having been encountered in this downflank well, Owen Bank Updip (Fig. 5a) becomes an attractive prospect of moderate size (40 sq km in area with 700 in vertical closure capable of holding some 380 million bbl of oil in place).

   On the southern shelf a faulted elongate horst block, mapped on a 5 km parallel seismic grid as having 200 sq km areal and 700 msec vertical closure, lies with its crest at about 2,000 in depth (Fig. 5b). From geohistory analysis the source rocks in this area lie in the oil window. 14

2. CLASTIC PINCHOUT PLAY

   The late Cretaceous progradational sequence developed on the southern shelf of the Seychelles bank wedges out between the underlying basement and overlying transgressive marine unit of drift origin (Figs. 4b, 5b). This play has not been drilled and is thus defined solely from seismo-stratigraphic interpretation.

   Reservoirs: Sandstones developed upon the deltaic topset could be preserved as channels, or as strandlines along the topset/foreset boundary.

   Seals: Top-seal is provided by shales of the transgressive marine unit that onlaps basement, while seat-seal is provided by granitic basement.

   Source rocks: Shaly prodelta deposits are frequently organically rich, containing a mixture of marine and terrestrial organic matter. Such a source rock has been interpreted to exist locally from the analysis of numerous tarballs that are regularly stranded on a number of the Seychelles islands.15

   Structures: A lack of post Cretaceous deformation leaves the topset pinchout between the basement and transgressive marine sequence as the trapping mechanism, which would require embayments in the faulted basement to form viable leads.

3. WRENCH FAULT PLAY

   Within the late Cretaceous succession in the deep water between the Seychelles and Platte banks wrench faults are identified which originated during left-lateral translation of the basement effecting the rift between Madagascar and Seychelles/India. Again seismo-stratigraphic interpretation defines this play.

   Reservoirs: Sandstones developed upon the 500 m thick topset facies of the progradational fluvial deltaic succession could provide a substantial sequence of stacked reservoirs.

   Seals: Fine clastics of the overlying transgressive marine sequence provide a topseal on the upthrown side of the fault, while pelagic oozes form the top-seal on the downthrown side of the fault (Fig. 6a). Interbedded fluvio-deltaic shales could provide individual seals between reservoirs on the topset.

   Source rock: As with the pinchout play, organic rich prodelta muds are postulated to source the reservoirs.

   Structure: Elongate anticlines are typically arranged en echelon in the cover sequence above wrench faulted basement. Lack of seismic coverage precludes the definition of prospects, but Fig. 6a indicates such a dome having some 500 m vertical closure across an 18 km width.

4. CARBONATE REEF, ONLAP/DRAPE PLAYS

   Within the Tertiary succession over the shallow banks, reefs are locally developed, frequently coinciding with minor fault displacements and present-day bank edges. Also, reef complexes developed on onetime basement highs are locally present in deep waters. On the northern shelf, highs of probable volcanic origin are locally developed onto which the Tertiary carbonates onlap and overstep.

   Reservoirs: Vuggy reef or shelf limestones. Vuggy limestones penetrated on the western shelf frequently led to drilling fluid losses, suggesting good porosites and permeabilities.

   Seals: Overlying and laterally equivalent micritic carbonates and shales.

   Source rocks: Organic rich shales of the rift/drift sequences from the underlying fault blocks, or from the prodelta sequences could source the reef or shelf limestones along faults. Source rock quality shales (TOC up to 7.82%) at the base of the Tertiary carbonates may be marginally mature. 16

   Structures: Stratigraphic trapping within vuggy reef limestones by fades changes into tight lagoonal lime- stones/calcareous shales. A potentially large reef play occurs in the deep water north of the western shelf (Figs. 1, 6b).

   Stratigraphic trapping within vuggy shelf limestones by onlap onto volcanics in embayment settings. Compactional drape over volcanic edifices or basement highs can produce broad anticlinal structures (Figs. 5c, 6b) comparable to giant Bombay High oil field of western offshore India.

SOURCE ROCK MATURITY

Although no hydrocarbon accumulations have yet been discovered offshore Seychelles, there are numerous indications that hydrocarbons have been and/or are being generated from mature source rocks.

Such indications have been previously reported, including numerous tarball strandings,15 shows within all three wells, plus anomalies from remote UV Seepfinder and gas sniffer surveys, DHIs on seismic data, and inferences from paleogeographic reconstructions.7 A compilation of the source rock data from the three wells has also been made, indicating the presence of at least five distinct horizons of varying quality and maturity.16 Other horizons of potential source rocks have yet to be sampled by drilling.

CONCLUSIONS

The Seychelles region, therefore, comprises a Mesozoic/Cenozoic succession that developed during three phases of rift/drift tectonics.

Within this succession a variety of reservoirs, seals, and source rocks are present which combine to form five structural plays. Within these plays the Accumulation of hydrocarbons is strongly suggested by the presence of numerous indications of migrated hydrocarbons.

The offshore Seychelles, both in the extensive shallow water areas and the deeper waters, thus contain significant potential to warrant the area being considered a highly prospective frontier region.

REFERENCES

1. Yanagi, T., Wakizaka, Y., and Suwa, K., Rb-Sr whole rock ages of granite rocks from the Seychelles Island, in Suwa, K. (ed.), 8th Prelim. Rep. Afr. Stud., Nagoya Univ., Vol. 8, 1983, pp. 23-26.

2. Plummer, P.S., and Belle, E.R., Mesozoic tectono-stratigraphic evolution of the Seychelles region in the context of western Indian Ocean development, Sed. Geol., in review.

3. Croxton, C.A., Hitchings, V.H. and Marshall, P.R., The biostratigraphy and palaeoenvironments of the interval 1,400-12,790 ft, with petrography, X-ray diffraction and potassium/argon age dating of selected samples, from the Amoco Seychelles Petroleum Co. Reith Bank-1 well, offshore Seychelles, Robertson Research International Report 2636P/F, 1981 (unpublished).

4. Schlich, R., The Indian Ocean: Aseismic ridges, spreading centres and basins, in Nairn, A.E. and Stehli, F.G. (eds.), The Ocean Basins and Margins, Vol. 6: The Indian Ocean, Plenum: New York, 1982, pp. 51-147.

5. Croxton, C.A., Hepworth, B., Hitchings, V.H., and Marshall, P.R., The biostratigraphy and paleoenvironments of the interval 1,181-8,995 ft, with petrography and potassium/argon age dating of selected samples, from the Amoco Seychelles Production Co. Seagull Shoals-1 well, offshore Seychelles, Robertson Research International Report 2647P/F, 1981 (unpublished).

6. Biswas, S.K. and Thomas, J., The Deccan traps and Indian Ocean volcanism, in Plummer, P.S., (ed.), First Indian Ocean Petroleum Seminar, Proc. U.N. Sem., Seychelles, 1992, pp. 187-209.

7. Plummer, P.S., Evidence of migrating hydrocarbons found off Seychelles, OGJ, Aug. 2, 1993, pp. 83-87.

8. Amoco Seychelles Petroleum Co., Reith Bank-1 completion report, 1981 (unpublished).

9. Amoco Seychelles Petroleum Co., Seagull Shoals-1 completion report, 1981 (unpublished).

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

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

12. Torkelson, B.E., Source rock analysis, Amoco Seychelles A-1 Owen Bank well, offshore Seychelles, Amoco Research Center, Technical Survey 805348CH, 1980 (unpublished).

13. Murris, R.J., Middle East: Stratigraphic evolution and oil habitat, AAPG Bull., Vol. 64, 1980, pp. 597-618.

14. Brennan, P.A., Hook, S.C., and Pasta, D., Geohistory model of western Seychelles Bank, Texaco, Geohistory Report 16, 1990 (unpublished).

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

16. Plummer, P.S., Geology and Geochemistry of Mesozoic source rock sequences and the hydrocarbon potential of the Seychelles offshore, J. Petrol. Geol., Vol. 17, No. 2, April 1994.

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