Southern Basins Geology

Introduction — Structure & Stratigraphy — Source Rock — Reservoirs — Hydrocarbon Indications — Play Types — Data

Three connected basins surround the southern Falklands:

Although all connected, each of the three basins exhibits its own unique structural style.

These three basins are isolated from the North Falkland Basin, and have a completely different infill.

All three of the Southern Basins probably share a similar stratigraphic infill, which is more marine than the predominantly continental North Falkland Basin.

None of the three southern basins has been drilled in Falklands waters.

Seventeen or so wells have been drilled in the Argentine part of the Malvinas Basin, with some limited success.

Boreholes drilled by the DSDP on the Maurice Ewing Bank at the easternmost extremity of the Falkland Plateau Basin tested rich marine source rocks that probably extend throughout much of the Southern Basin area, probably pinching out just into Argentine waters.

The Southern Basins are at the very earliest stage of exploration.

Only regional reconnaissance seismic data covers the area.

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Structure

Three separate basins with distinct structural styles are recognised in the southern basins area:

• The Falkland Plateau Basin is an essentially passive margin basin, with Jurassic and Cretaceous marine rocks both gently onlapping, and prograding off, a Palaeozoic plateau
• The South Falkland Basin is a narrow basin zone of ?Jurassic extensional fault blocks with an overthrust sequence of Cenozoic rocks developed along the boundary between the Scotia and South American plates: this basin forms a connection between the Falkland Plateau Basin and the Malvinas Basin
• The Malvinas Basin is a ?Jurassic failed rift that deepens towards the south, where it is bounded by the transform plate margin of the Scotia and South American Plates: the basin runs eastwards into the South Falkland Basin.

This basin lies beneath 200 m to 2500 m of water to the east of the Falkland Islands.

It is bounded to the west by the Falklands Platform, to the north by a steeply sloping feature termed the Falkland Escarpment, to the east by the Maurice Ewing Bank and to the south by the Scotia/South American plate boundary, which has a sinistral and northwards over-thrust movement: this plate boundary produces a topographic feature known as the Scotia Ridge, which includes shallow water areas such as the Burdwood Bank (immediately south of the Falklands), as well as the islands of South Georgia and the South Sandwich Islands.

To the south-west, along the deep water trough known as the Falkland Chasm, the basin merges with the South Falkland Basin.

The central parts of the Falkland Plateau Basin may be underlain by thinned continental crust (about 16 km thick) or by oceanic crust, although the northern margin of the basin (the Falkland Escarpment) is underlain by a prolongation of probably gneissic continental crust that joins the Falklands Plateau with the Maurice Ewing Bank. To the west of the basin the Falklands Plateau itself is underlain by continental crust about 30 km thick.

The western boundary of the basin is in relatively shallow water (c.750–1000 m).

It is defined by a north-east – south-west oriented series of normal, down to the east faults in a linear trend approximately 650 km long.

This fault zone appears to comprise numerous shears offset along transfer zones, rather than a single fault.

The amount of throw along the fault zone varies from a few hundred metres to perhaps a kilometre or so.

There is a zone of high magnetic and gravity anomalies oriented along and (spread across) the western basin margin fault zone.

This gravity high is located over an area of apparent isostatic equilibrium, and can be explained partially as an edge effect due to crustal thinning beneath the sedimentary basin. However, the steep western slope of the observed anomaly and its sharp apex preclude a wholly deep crustal explanation; one explanation for these other features of the anomaly are pre- and syn-rift basaltic rocks, possibly sheet basalts, together with a deep-seated, dense magnetic body near the basin margin fault.

Along the western margin significant faulting is observed at possible Permo-Triassic to Jurassic level.

The apparent gentle westwards onlap of Cretaceous and Cenozoic reflectors near the basin margin suggests that the main phase of extension was in the Permo-Triassic or Jurassic.

Gentle onlap of the basin margin replaced active wedging some time in the Late Jurassic or Early Cretaceous.

The early extension was probably followed by post-rift sag through the Cretaceous and Cenozoic.

This basin lies beneath 500 m to 2500 m of water to the immediate south of the Falkland Islands.

The basin can be defined bathymetrically, since it corresponds, more or less, with the deeper water zone of the Falklands Chasm, the foredeep that runs adjacent to the northern edge of the Scotia plate.

It is bounded to the south by the Scotia/South American plate boundary, which is a combined sinistral strike-slip and north-east-directed thrust fault.

It is contiguous with the Malvinas Basin to the west and the Falkland Plateau Basin to the east, but has a somewhat different structural style.

In terms of preserved structural style the basin is most similar to the southern part of the Malvinas Basin.

It exhibits numerous major normal faults downthrowing to the north that display reactivation into recent times, and a number of thrust faults of probable Cenozoic age, some of which splay off the southern bounding fault/plate boundary.

Most faults within the basin trend generally east to west and may have a major strike-slip component but look superficially like true normal faults.

They are orientated parallel to the boundary between the South American and Scotia plates, and are probably reactivated older, possibly basement-controlled faults that have been rejuvenated by the continued north-east- directed thrusting and sinistral strike-slip motion along the plate boundary.

Wedging of reflectors above the Permo-Triassic section (Fig. 6) suggests that much of the normal faulting in the basin was probably of Jurassic through Early Cretaceous age, contemporaneous with the deposition of sediments in the actively rifting trough.

Syn-depositional movement related to active extension may have ceased in the Valanginian or Hauterivian, as in other South American and offshore Southern African basins.

However, some wedging of reflectors is observed above this level, possibly as high as top Lower Cretaceous.

Normal fault movement may have continued throughout the Cretaceous as the basin sagged in a thermal relaxation phase following earlier extension, but most faults die out upwards before the top Cretaceous level.

Many of the faults have apparently been reactivated in the late Cenozoic, and some penetrate to the sea bed.

Much of this fault reactivation, probably with a strike-slip component, may have occurred in the Oligocene through mid Miocene as a result of uplift and lateral displacement of the Scotia arc along the Falkland Thrust as the West Scotia Basin to the south opened.

The plate boundary fault appears to have a more or less normal fault geometry in many places, with deep basins to the north and shallower basins or basement (Burdwood Bank) to the south.

However, it is complicated in places by a number of Cenozoic thrusts.

These thrusts affect only the higher stratigraphic levels in a zone within about 5–20 km of the basin margin.

The onset of thrusting marks a change from transtension to transpression along the plate boundary and has a significant effect on stratal geometries within the South Falkland Basin.

The lower part of the succession transected by thrust sheets appears, if the effects of thrusting are palinspastically reconstructed, to thicken southwards into the plate boundary. Thickening of the succession northwards, away from the basin margin as a result of syndepositional thrusting occurs near the mid-Cenozoic interval, possibly some time in the late Paleogene or Neogene. The thrusting may be genetically related to the Oligocene through Miocene opening of the Scotia Sea to the south of the Falkland Islands.

The South Falkland Basin plunges eastwards.

This basin lies beneath 150 m to 250 m of water to the west of the Falkland Islands.

The basin extends westwards to the Rio Chico High in Argentine waters, and then further westwards to the onshore area of Tierra Del Fuego and southern Argentina, where it is termed the Austral or Magallanes Basin.

To the east it terminates by onlap onto Palaeozoic rocks of the Falklands Platform.

To the south it is constrained by the South American–Scotia plate boundary.

To the south-east it passes laterally into the South Falkland Basin, which essentially underlies the deeper water area adjacent to Burdwood Bank.

There are several distinct structural provinces in the basin, each possibly related to different elements in the formation and post-extensional modification of the basin:

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No wells have been drilled in South Falklands waters, and therefore the presence and maturity of source rocks in the Falklands area is somewhat conjectural.

However, good quality source rocks have been drilled on the Maurice Ewing Bank to the east, and some source rock systems are also present in the Argentine parts of the Malvinas Basin to the west.

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Permo-Triassic, Mesozoic and Cenozoic sediments occur beneath or within all three of the Southern Basins, and each interval could contain potential reservoir rocks.

Hydrocarbon indicators for the Falkland Plateau Basin are confined to data from the only borehole penetrations of the basin – the DSDP holes on the Maurice Ewing Bank some 750 km to the east of the Islands.

No hydrocarbon shows have been established for the South Falkland Basin due to the lack of drilling.

However, there are some high amplitude events on seismic sections, especially above tilted fault blocks, that might be considered as direct hydrocarbon indicators.

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Note that play types and prospects/leads developed for the Southern Basins so far are made on the basis of widely spaced reconnaissance seismic data only. Parts of this dataset were reprocessed in 2003 by Falkland Oil and Gas Ltd (FOGL) , but the data were acquired in 1993.

Potential 'play' types for these basins include:

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Seismic data

There are three sets of 2D seismic data over the Southern Basins:

All released data are supplied free of charge to bona-fide exploration companies and supplied on the following basis: Data are provided by the British Geological Survey on behalf of the Falkland Islands Government, and should not be distributed to third parties without the consent of the Falkland Islands Government.

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