GEOLOGY/GEOPHYSICS Surveys suggest strong hydrocarbon potential at Caribbean/Atlantic overlap

Synthetic section across the eastern French Guiana margin (Southern Monocline).

Some segments of Atlantic continental margins have yet to receive significant attention from oil companies. Two of them, partly under French jurisdiction, represent quite different geological settings.

The first (West Indies) is an active convergent margin, while the second (Guiana) is a composite passive margin. Neither is part of a significant oil and gas province, but both are located only a few hundred kilometers from a major hydrocarbon producing area, Trinidad and eastern Venezuela.

French West Indies

The West Indies are a group of islands located along the eastern edge of the Caribbean. They represent the volcanic arc of the Lesser Antilles active margin beneath which the Atlantic oceanic crust is presently being subducted at a velocity of about two cm/year.

This arc forms quite a regular curve about 850 km long, from Grenada island to the south to the tiny island of Sombrero to the north, linking the Venezuelan borderland against which the arc collides with the eastern tip of the Greater Antilles (Puerto Rico and the Virgin Islands). The Greater and Lesser Antilles are separated by the Anegada Passage, an extensional complex of Neogene age including small pull-apart basins.

The Lesser Antilles active margin developed as early as late Cretaceous times when the Caribbean plate started its eastward movement from the Pacific to the Atlantic between North and South America, overriding an old Jurassic-early Cretaceous ocean now almost completely subducted (Pindell et al.,1988).

Main structural units of these margins are (from east to west) the Atlantic abyssal plain, the Barbados Ridge accretionary prism and the Tobago Trough forearc basin, the Lesser Antilles Tertiary volcanic arc, the Grenada Basin backarc basin and the Aves Ridge remnant (late Cretaceous) volcanic arc.

A thick succession of interbedded volcanics, volcanoclastics and shallow marine carbonates are exposed on the islands of Antigua, St Barthelemy, St Martin and Anguilla with intervening intrusive plutons. The other islands are recently uplifted Neogene carbonates banks and Miocene to recent volcanoes, some of them still active. Seismic surveys shot by the French oil industry from 1976 to 1982 have shown that sedimentary basins, up to 2.5 sec. thick and of presumed Tertiary age, were present around the islands at water depths in the range 10-2,000 meters (Bouysse et al. 1994).

There are no wells in the French islands and waters. The nearest wells (dry) were drilled on the Saba Bank (SB 1, Marathon, 1977; SB 2, Petrofina, 1982) and on Puerto Rico (Toa Baja1, P.R. Electric Power Authority, 1989). These wells were located in sedimentary basins similar in many respects to those that can reasonably be expected to be present around the French islands. However, the much higher heat flows presently prevailing there are believed to have considerably enhanced the petroleum potential of these basins.

Nearest oil and gas production and discoveries are located on Barbados and on the northern shelf of Trinidad and Venezuela, i.e. at the eastern and southern edge of the Tobago forearc basin. The same basin extends north up to Martinique and Guadeloupe. Similar source-rocks, reservoirs and seals could thus hopefully be present in the vicinity of these islands.

A petroleum assessment of these French waters has recently been conducted by the Institut FranYais du Palphatrole, including the reprocessing of some seismic data (Mascle and Vieban, 1994). The best prospective plays appear to be shallow water carbonates of Paleocene to Miocene age deposited on the top of, or lateral to, Paleogene volcanoes.

Such carbonates of Eocene age have been found below the Saba Bank with a few good porous intervals. They can also be observed on the St Barthelemy and Antigua islands, but here reservoir qualities are poorly preserved. Other potential plays include deeper water carbonates (lowstand fans or wedges, basinal turbidites) and volcanoclastics sandstones, as encountered below the Saba Bank in the Oligocene interval.

Distribution and qualities of potential source rocks are poorly constrained. The only positive indications we can get come from the close islands of Puerto Rico and Barbados. On the first, which in Eocene times was the northernmost segment of the Lesser Antilles Arc, continental (lignites) and marine source rock intervals with fair to good potential can be sampled in Oligocene-Miocene strata. On Barbados, the source rock at the origin of the Woodburn oilfield is generally believed to be Eocene black shales with a mixture or marine and continental organic matter.

It may reasonably be assumed that source-rocks similar to those of Puerto Rico and Barbados could be present in the offshore basins on the Lesser Antilles arc and/or in the Tobago basin. Because of the present high heat flows encountered across the volcanic arc (average 105 mW/sq m offshore Guadeloupe), the top of the oil window could be reached at depths of only 1,700 meters.

Seismic section south of St Barthelemy island.

French Guiana

The French Guiana passive margin is located between four and six degrees north on the western side of the Equatorial Atlantic Ocean, and between Surinam and Brazil to the west, south, and east. The continental shelf is about 100 km wide. The continental slope is composite: on the eastern segment, water depths increase progressively down to the Atlantic abyssal plain (Southern Monocline), while to the west the Demerara Plateau, with water depths ranging from 500-1,500 meters, forms a prominent submarine high before the abyssal plain is reached.

The geological history of the margin can be divided into two main stages (Gouyet et al. 1994). In Jurassic and early Cretaceous times, the Demerara Plateau was the southern extremity of the Central Atlantic rift and ocean. At this time, sedimentation on the Demerara Rise was in a lacustrine and inner shelf environment, with significant siliciclastic influxes and perhaps some evaporite intervals.

As early as middle Albian times, shallow marine to open marine environments of deposition prevailed in response to the opening of the E-W trending Equatorial Atlantic Ocean along a dextral shear zone between South America and Africa. This opening is well marked on the Demerara Rise by a strong regional unconformity sealing transtenssive (tilted blocks) or transpressive (en echelon folds) structures.

From Cenomanian times the French Guiana margin is a typical passive margin with the seaward propagation of detrital prograding wedges. The total sediment thickness (Jurassic to Pleistocene) may reach 10 km below the Demerara Plateau, while this sediment thickness (early Cretaceous-Pleistocene) below the shelf and upper slope of the eastern segment of the margin does not exceed 5,00 meters.

Two wells have been drilled in French Guiana waters (only Precambrian rocks are exposed onshore). Both wells were located on the Demerara Plateau, near the Surinam boundary. Sinna Mary 1 (1975, EAP) encountered an early Cretaceous water-bearing sandy reservoir just above the Precambrian basement, and FG 2-1 (Exxon) bottomed in early Cretaceous basalts.

In Surinam, about 20 wells were drilled offshore, without success. However, a small oilfield, Tambaredjo, was discovered onshore, near the coast, and is now producing about 4,500 b/d of low sulfur and naphthenic heavy oil at shallow depths from coastal marine Paleocene sands.

On the Brazilian side of the French Guiana border, a single undeveloped offshore gasfield, Pirapema, was discovered under 135 meters of water: the gas would be of biogenic origin. We should also mention further west the Takutu graben (a Jurassic-early Cretaceous rift contemporaneous from the deeper sequences of the Demerara Plateau) which straddles the Guiana-Brazilian border and where a subcommercial accumulation of oil was discovered in 1982 from Jurassic fractured basalts.

A synthesis of all available geological and geochemical data has recently been conducted by the Institut FranYais du Palphatrole (Vially and Mascle, 1996). Geochemical analysis of potential source rocks produced in Surinam and Trinidad confirm the presence of excellent upper Cretaceous marine rocks.

Through analogy with the conjugate African margin, and exploration wells in Surinam and Guiana, potential source rock (lacustrine and continental) can also be expected in the Jurassic to lower Cretaceous interval. Accordingly, potential plays on the French Guiana offshore include:

1) On the Demerara Plateau: late Jurassic-early Cretaceous clastics or carbonates in pre-Albian tilted blocks or folds (well imaged by seismic)

2) On the eastern segment, late Cretaceous-early Tertiary clastics within prograding wedges. An additional play would be biogenic gas in the Tertiary sequences.

These two Atlantic continental margins can be considered as still being little explored. The related geological and geophysical data have recently been gathered, and proven and/or potential petroleum systems and plays have now been proposed. These data are now available from the Institut FranYais du Palphatrole. Application for licences in these two areas may be filed with the French administration at any time for offshore seismic surveys, with the authorization being delivered in less than six months.'

References:

Bouysse and Mascle (1994) Sedimentary Basins and Petroleum Plays around the French Antilles, EAPG publication.

S.Gouyet, P.Unternehr and A.Mascle (1994) The French Guiana Margin and the Demerara Plateau: Geological History and Petroleum Plays, EAPG publication.

A.Mascle and F.Vieban (1994) Les Antilles FranYaises, IFP non-exclusive report.

J.Pindell, S.Cande, W.Pitman III, D.Rowley, J.Labrecque, W.Haxby (1988) A Plate Kinematic Framework for the Models of Caribbean Evolution, Technophysics vol.155.

R.Vially, A.Maslce (1996), French Guiana, IFP non-exclusive report.

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