SHELF NORTH OF FALKLANDS MAY BE NEW S. ATLANTIC PETROLEUM PROVINCE

Steve Lawrence
Quad Consulting Ltd.
Bucks, England

Mike Johnson
Spectrum Energy & Information Technology Ltd.
Woking, England

Interpretation of new seismic data shows that all the elements of a favorable petroleum geology exist on the North Falkland shelf and therefore that the area will become an important petroleum province of the future.

Spectrum Energy & Information Technology Ltd. acquired approximately 7,500 line km of speculative seismic data during 1993-94 over the continental shelf north of the Falkland Islands (Fig. 1) (81795 bytes).

These data have been interpreted geologically to determine the petroleum prospectivity of the area prior to the planned offshore Falkland Islands licensing round. In this article we outline this interpretation and show how it provides some important new information on regional geology and the break-up history of southern Gondwana.

PLATE TECTONICS, BASIN EVOLUTION

The North Falkland shelf occupies the north-central part of the Falkland plateau, which forms a 1,200 km long continental promontory of South America (Fig. 1) (81795 bytes). The South Atlantic margin of South America represents a passive margin having formed by perpendicular opening of the South Atlantic during Gondwana break-up. The northern edge of the Falkland plateau is represented by a transform margin, the Agullias-Falkland fracture zone, which operated during the opening of the South Atlantic.

The eastern end of the Falkland plateau is formed by the Maurice Ewing bank, which is separated from the main part of the plateau by the Falkland Plateau basin. In its eastern transect the southern margin of the Falkland plateau is represented by a "pull-apart" basin, the Falkland trough. The western end includes the Burdwood bank, which is probably transpressional in origin and contiguous with the compressional structures of the Cordillera Darwin in Tierra del Fuego.

The Falkland plateau, and adjacent South American mainland, are the product of several deformational phases influenced by both "Andean" tectonics and "Atlantic" opening (linked to southern Gondwana break-up).

Based on evidence provided by the interpretation of the new seismic data, we present arguments against the current popular theory which invokes Falkland Islands rotation during the early break-up of southern Gondwana.1 This new evidence can be summarized as follows:

• Mid-Late Jurassic rift trends conform to the north northwest-south southeast trends seen in southern South America.

• North-south Late Jurassic/Early Cretaceous rift trends align with the Macachin and Rawson rifts of the same generation, and with that of the South Atlantic margin of southern South America, which formed during Early Cretaceous rifting.

• The recognizable effects of Mid Cretaceous compression are consistent with Andean structural history (which includes the closure of the back-arc Rocas Verdes basin).

On this basis we have proposed a new break-up model for southern Gondwana incorporating a rigid Falkland plateau/South America configuration and no internal rotation involving a "Falkland Islands" microplate. Following Permo-Triassic "Gondwanide" arc contraction and orogeny, Mid-Late Jurassic/Early Cretaceous rift basin development over southern Gondwana is attributed to mantle upwelling in a broad back-arc extensional realm. This was followed by Cretaceous and Tertiary flexural and passive margin subsidence in response to South Atlantic opening.

The post-rift basin development stage was punctuated by important compressional events recorded in the Andes and the Andean foreland:

• Mid Cretaceous-caused by accelerated South Atlantic opening with the corresponding change in the rate of convergence at the active margin.

  Early Tertiary-due to a change in Andean active margin configuration and beginnings of Scotia plate "indentation."

BASIN STRATIGRAPHY

Our tectono-stratigraphic analysis shows a consistent history of basin development incorporating:

• Mid-Late Jurassic/Early Cretaceous rifting.

• Early-Late Cretaceous and Tertiary post-rift flexural subsidence punctuated by compressional deformational episodes in the Mid-Cretaceous and Early Tertiary.

The tectono-stratigraphic sequence is defined by four key seismic horizons (Fig. 2) (99536 bytes):

• "Near Base Jurassic," a rift onset or early s rift unconformity and the top of a regional Middle/Upper Jurassic volcanic sequence, ("Purple" horizon).

• "Intra Lower Cretaceous unconformity," a regional Valanginian "end-rift" unconformity ("Blue" unconformity).

• "Middle Cretaceous unconformity," the culmination of an important phase of compressional deformation, ("Green" unconformity).

• "Lower Tertiary unconformity," corresponding to an Early Tertiary phase of Andean deformation ("Red" unconformity).

We have rationalized this stratigraphy with that of the Falkland Islands;1 2 the eastern Falkland plateau, (including the Maurice Ewing bank and Falkland Plateau basin), represented by DSDP boreholes; and the adjacent South American basins. Here we briefly describe how we fit the geology of the Falkland Islands into a South American/North Falkland shelf stratigraphic scheme.

We correlate the Devonian Gran Malvinas group meta-sediments of the Falkland Islands with metamorphosed sediments in the "Gondwanide" forearc province.3 The Lower Lafonian group is correlated with the Upper Carboniferous to Lower Permian Tepuel and Rio Genoa groups of Patagonia.4

The Pampa de Tepuel formation of the Tepuel group is composed of diamictites which we correlate with the Lafonian diamictites of the Falkland Islands.

The Upper Lafonian group is poorly exposed on the Falkland Islands and correspondingly poorly known. We equate these sediments partly with the Permian Panguipulli formation of Chile4 and partly with inferred Lower Jurassic graben fill characterized by Mid-Late Jurassic volcanic cover as developed in mainland South America and postulated in the North Falkland shelf.

BASIN STRUCTURE

In our structural analysis we recognize three important structural provinces which reflect a structural history consistent with the regional South American-Falkland plateau structural setting (Fig. 3) (138541 bytes):

• North Falkland graben formed by Late Jurassic/Early Cretaceous rifting with Mid-Cretaceous fault-block reactivation.

• Western rift complex initiated by Mid-Late Jurassic rifting and affected by Mid-Cretaceous compression.

• Southern half graben formed by Late Jurassic/Early Cretaceous listric rifting with Early Tertiary transfer-related strike-slip tectonics.

The structure of the Falkland Islands is also shown to be consistent in history and style with that of the North Falkland shelf. Devonian meta-sediments in the western part of West Falkland and the northern part of East Falkland are deformed in a series of open folds with northwest-southeast axial trends.2 The eastern edge of West Falkland has a northeasterly structural grain manifest as the Hornby Mountain anticline and a coastal monocline with near vertical dips into the Falkland Sound. Across a northwest-southeast monocline in central East Falkland, dips shallow out in the Upper Lafonian group sediments which occupy the southern part of the island.

The structural evidence from the immediate offshore is of northwest-southeast listric faults approaching the coast, and conforming to the morphology of the northern coastline of both East and West Falkland, as well as Lower Lafonian group outliers mapped in northern West Falkland. It is suggested that this listric rift faulting (which on seismic data is seen "growing" during the Early Cretaceous) can be extrapolated across both Falkland Islands. In our model, Upper Lafonian group sediments in the southern half of East Falkland are preserved in a series of half-graben with listric northwest-southeast controlling rift faults. This also explains the shape of the arcuate coastal inlets of East Falkland.

The offshore half-graben have been subject to strike- slip tectonics in the Early Tertiary along the line of a possible rift transfer zone. A similar transpressional reactivation of an original transfer zone would explain the structural effects seen along the Falkland Sound monocline, as well as the apparent geological differences between the two islands.

PETROLEUM GEOLOGY

The petroleum geology of the Falkland plateau has recently been addressed in two other works.5 6

In this account we focus on the North Falkland shelf where petroleum source rocks and reservoirs have been assessed from tectono-stratigraphic setting and by comparison with equivalent sequences in the South Atlantic region (Fig. 4) (64195 bytes).

Several potential source rock intervals are predicted:

• Lower Permian marine claystones of the pre-rift section (equivalent to the Black Rock member of the Port Sussex formation of the Falkland Islands.6

• Late Jurassic/Early Cretaceous anoxic marine claystones of the synrift sequence (SR-1) and equivalent to oil-prone potential source rocks penetrated in DSDP boreholes in the eastern Falkland plateau.7

• Anoxic marine claystones developed in the Barremian-Aptian (SR-2) and Cenomanian-Turonian (SR-3) intervals of the post-rift sequence. Equivalent to proven oil-prone source rocks in South America and the South Atlantic.

A number of potential reservoir intervals are predicted:

• Continental alluvial fan and fluviatile sands of the Upper Jurassic syn-rift sequence.

• Late rift/early post-rift deltaic and transgressive marine paralic sands equivalent to the Springhill formation reservoirs of the Magallanes basin.

• Post-rift Middle/Upper Cretaceous deltaic/delta-slope and basin floor sands.

SOURCE ROCK MATURITY,
HYDROCARBON GENERATION

We have analyzed the maturity history of prospective source rocks by basin modeling. The present day maturity condition of prospective source rocks shows that:

• In the North Falkland graben SR-1 source rocks are in the "oil window" over large areas and wet-gas generating in deeper parts. SR-2 source rocks are in the "oil window" in eastern basin kitchen areas.

• In the Southern half graben SR-1 source rocks are mostly oil or wet-gas mature, but SR-2 source rocks are marginally mature.

• In the Western rift complex SR-1 source rocks are potentially in the oil window depending on amount of missing section built into modeling below Mid-Cretaceous erosion.

Basin modeling has shown that most hydrocarbon generation was after the critical structural phases of Early Cretaceous (rifting) and Mid Cretaceous (compression). In the North Falkland graben structures formed during Early Tertiary inversion can also be charged by hydrocarbons expelled from deep source rocks during Mid-Late Tertiary burial.

PETROLEUM PROSPECTIVITY

Several hydrocarbon play-types have been identified (Fig. 5) (148683 bytes):

• Rift fault-blocks of Jurassic/Early Cretaceous generation in the North Falkland graben, Western rift complex, and Southern half graben.

• Inversion and fault-block reactivation structures formed during Mid-Cretaceous compression. Extensive fault-block reactivation in the Western rift complex and more localized inversion in the North Falkland graben.

• Inversion/extension (strike-slip) structures formed during Early Tertiary transfer reactivation in the Southern half graben and subtle fault-block reactivation in the North Falkland graben.

Our seismic horizon mapping has shown that potentially large structural prospects occur in relation to these play-types in all structural provinces. Their delineation, however, awaits the award of first round licenses and further exploration.

REFERENCES

1. Marshall, J.E.A., The Falkland Islands: a key element in Gondwana palaeogeography, Tectonics, Vol. 13, 1994, pp. 499-514.

2. Greenway, M.E., Geology of the Falkland Islands, British Antarctic Survey Report 76, 1972, 42 p.

3. Dalziel, I.W.D., and Forsythe, R.D. 1985, Andean evolution and the terrane concept., in Howell, D.G. ed., Tectonostratigraphic terranes of the Circum-Pacific region, Circum-Pacific Council for Energy & Mineral Resources, Earth Science Series No. 1, 1985, pp. 565-581.

4. Uliana, M.A., and Biddle, K.T., Permian to Late Cenozoic evolution of northern Patagonia: Main tectonic events, magmatic activity and depositional trends, in McKenzie, G.D. ed., Gondwana six-structure, tectonics and geophysics, Geophys, Monograph 40, 1987, pp. 271-286.

5. Richards, P.C., and Fannin, N., Falkland Islands offshore offers high risks-costs, good potential, OGJ, Jan. 17, 1994, pp. 67-70.

6. Marshall, J.E.A., The Falkland Islands and the early fragmentation of Gondwana: implications for hydrocarbon exploration in the Falklands plateau, Marine & Petrol. Geol., Vol. 11, 1994, pp. 631-636.

7. Deroo, G. et al., Organic geochemistry of Upper Jurassic-Cretaceous sediments from Site 511, Leg 71, Western South Atlantic, initial report Deep Sea Drilling Project 71, 1983, pp. 1,001-13.

THE AUTHORS