North Falkland Basin Research Articles

Rotation of the Falklands microplate reassessed after recognition of discrete Jurassic and Cretaceous dyke swarms

ABSTRACT Recently acquired aeromagnetic data for the Falkland Islands have shown that previous interpretations of the dolerite dyke swarms are inadequate. In particular, most of the dykes previously described from West Falkland as forming a ‘north–south’ swarm of Jurassic age are associated with a set of NE–SW linear magnetic anomalies that are entirely separate from another set of truly N–S anomalies. Very few dykes had been previously reported from East Falkland, but the aeromagnetic survey demonstrates clearly that dykes of both the NE–SW and the N–S swarms are present. Ar–Ar age dating of East Falkland dykes has confirmed the Jurassic age of the NE–SW dykes but has established an early Cretaceous age for the N–S dyke swarm. The Jurassic dykes are generally considered a part of the regional Karoo–Ferrar magmatism linked to the initial break-up of Gondwana. We consider the Cretaceous dykes to be associated with the later opening of the North Falklands Basin during the early development of the South Atlantic Ocean. The Jurassic and Cretaceous dykes must respectively pre-date and post-date the microplate rotation envisaged in most models for the Falklands break-out from Gondwana. The shapes of the aeromagnetic anomalies associated with dykes from each of the swarms support the hypothesis that the early Jurassic dykes have experienced a pre-Cretaceous, clockwise microplate rotation of about 120°.

Derisking exploration prospects using integrated seismic and electromagnetic data—a Falkland Islands case study

The British Geological Survey has concluded that the North Falkland Basin contains the second richest documented source rock in the world. They predict that as much as 100 billion barrels of oil have been expelled, much of which could still be trapped beneath the regional seal (Richards and Hillier, 2000). Despite this encouraging hypothesis, during the 50-year exploration history of the area, only six wells have been drilled. These all lie in a small area, and to date there has been no commercial production, although live oil has been recovered to the surface.

EXPLORING FOR FAN AND DELTA SANDSTONES IN THE OFFSHORE FALKLANDS BASINS

Four basins surround the Falkland Islands, but only the North Falkland Basin has been drilled; six wells were drilled there in 1998. Although all six wells encountered good quality sandstones, none of them targeted the basin margins, on what are now thought to be the optimum migration pathways associated with the basin’s thick lacustrine source rocks. Subsequently, a 3D seismic survey acquired in 2004 was designed to identify potential basin-margin -derived sandstones entering the basin along transfer zones. From this survey, a number of basin-margin -attached fans have been identified; these prograded into lacustrine waters of varying depths. These Early Cretaceous alluvial/fan delta/deep-lacustrine fan systems are interpreted to provide excellent potential reservoir facies as they are intimately associated with thick, mature source rocks. They will provide the focus for the next planned phase of exploration in the North Falkland Basin. A phase of drilling is also planned for the basins to the south of the Islands, where large deltaic and fan systems, slightly younger than those imaged in the North Falkland Basin, are seen on seismic to prograde from the same Palaeozoic hinterland that produced the older, North Falkland Basin fans. This paper attempts to show how sedimentary models derived from targeted seismic programmes following initial exploration can be utilised to plan and improve new drilling campaigns in a frontier basin. It presents an analysis of sediment dispersal patterns in basins of marine and lacustrine origin linked to a single hinterland area, and highlights the nature of the relationship between relay ramp/transfer zone development and sediment dispersal patterns in the subsurface.

Controls on the structural architecture and sedimentary character of syn-rift sequences, North Falkland Basin, South Atlantic

The Falkland Islands lie in a critical location for our understanding of Gondwana supercontinent break-up. Subsurface data obtained during recent exploration activity provide important new insights into the structure and stratigraphy of the North Falkland Basin's Late Jurassic syn-rift fill history. Results of seismic interpretation indicate that deposition was largely controlled by prominent planar normal faults, which divide the basin into a series of north-south trending half-grabens and minor grabens. Well data (in the form of exploration wireline logs, cores, petrographic thin sections, major and trace element analyses) indicate that their syn-rift fill was dominated by abundant volcaniclastic sediment supplied through debris flows, hyperconcentrated flows and pyroclastic ash-fall. The geochemical affinity of the volcanic sedimentary components shows a direct link to the widespread, Middle Jurassic to Early Cretaceous, silicic Chon Aike Large Igneous Province, in southern South America and Antarctica. Diagenesis has obscured provenance information and severely reduced porosity within the syn-rift sediments, causing them to have poor reservoir potential. The results enable a radically new interpretation of this key period in the North Falkland Basin's evolution, which helps modify and unify previously disparate views. It is now suggested that basin development was the direct result of the Chon Aike back-arc stretching, influenced in part by the underlying Palaeozoic basement trends. This was followed by Cretaceous-Recent post-rift thermal subsidence of the basin, which was only arrested by local structural inversion related to Atlantic opening. Consequently, it is thought that the extension that created the North Falkland Basin was independent of both Early–Middle Jurassic rotation of the Falkland Islands microplate and the evolution of the South Atlantic Ocean in the Cretaceous and Cenozoic.

An overview of the oceanography and meteorology of the Falkland Islands

1.This paper describes an overview of the oceanography and meteorology of the Patagonian Shelf and Falkland Islands, derived from reference material and a 16 month measurement program carried out on behalf of the Falklands Operators Sharing Agreement (FOSA) between June 1997 and October 1998. It was not intended that the referenced information in this presentation be exhaustive, the bias has been placed on describing the measurement survey. 2.Due to commercial confidentiality, presentations of data measured during the survey have been limited to time series and statistics. However, the confidentiality of the data does not necessarily preclude use of the data for future research purposes. 3.The analysis of current, tide, wave, wind and meteorological data is described and the nature of mechanisms driving the current and wave regime are suggested. 4.Offshore oceanographic measurements were carried out at three locations prior to a drilling programme by the semi-submersible drilling rig Borgny Dolphin and on board whilst drilling. Meteorological measurements were performed at two locations on the Islands and on the Borgny Dolphin. Drifting buoys were deployed and conductivity, temperature and depth (CTD) measurements were carried out to determine the position and intra-annual variation of the Falklands Current in the North Falklands Basin. 5.The primary oceanographic current flow features of this area are the Brazil and Falklands Currents, which meet at a confluence off the coasts of Argentina and Uruguay. The position of the front varies between approximately 30 and 40°S, depending on changes in the relative strength of the two current systems. Current flows are stronger on the slope than on the Continental Shelf and are less tidally dominated. Although it is thought that the FOSA mooring locations were influenced by the margins of the Falklands Current, stronger flows have been reported to the north of the measurement location. Typical maximum recorded near-surface currents reached 0.8 ms−1, with near-bed maxima reaching 0.55 ms−1. 6.The wave climate was primarily driven by locally derived storms, resulting in events of short duration. Typical maximum significant wave heights of 8.9 and 9.4 m were recorded to the north and south of the Islands, respectively. Copyright © 2002 John Wiley & Sons, Ltd.

Overview of petroleum geology, oil exploration and associated environmental protection around the Falkland Islands

1. The Falkland Islands offshore exploration area contains several Mesozoic basins with the potential for oil development. Exploration of all of these basins is at a very early stage. 2. The only area to have undergone exploratory drilling so far is the North Falkland Basin, where six wells were drilled in 1998. These wells proved the presence of a rich organic source-rock, which has probably generated significant quantities of oil. However, all six wells targeted a limited number of exploration concepts, and none found commercial quantities of hydrocarbons. Post-drilling analysis of the well data suggests that commercial quantities of oil may be found in other parts of the basin. 3. The exploration to date has been conducted in tandem with an extensive and detailed environmental assessment of the region, funded largely by the oil companies and the Falkland Islands Government. 4. The natural environment of the Falklands is important as a resource as well as providing an unspoilt habitat. Detailed legislation and planning controls were put in place before exploration started in order to protect the environment. Copyright © 2002 NERC

An overview of the oceanography and meteorology of the Falkland Islands

1. This paper describes an overview of the oceanography and meteorology of the Patagonian Shelf and Falkland Islands, derived from reference material and a 16 month measurement program carried out on behalf of the Falklands Operators Sharing Agreement (FOSA) between June 1997 and October 1998. It was not intended that the referenced information in this presentation be exhaustive, the bias has been placed on describing the measurement survey. 2. Due to commercial confidentiality, presentations of data measured during the survey have been limited to time series and statistics. However, the confidentiality of the data does not necessarily preclude use of the data for future research purposes. 3. The analysis of current, tide, wave, wind and meteorological data is described and the nature of mechanisms driving the current and wave regime are suggested. 4. Offshore oceanographic measurements were carried out at three locations prior to a drilling programme by the semi-submersible drilling rig Borgny Dolphin and on board whilst drilling. Meteorological measurements were performed at two locations on the Islands and on the Borgny Dolphin. Drifting buoys were deployed and conductivity, temperature and depth (CTD) measurements were carried out to determine the position and intra-annual variation of the Falklands Current in the North Falklands Basin. 5. The primary oceanographic current flow features of this area are the Brazil and Falklands Currents, which meet at a confluence off the coasts of Argentina and Uruguay. The position of the front varies between approximately 30 and 40°S, depending on changes in the relative strength of the two current systems. Current flows are stronger on the slope than on the Continental Shelf and are less tidally dominated. Although it is thought that the FOSA mooring locations were influenced by the margins of the Falklands Current, stronger flows have been reported to the north of the measurement location. Typical maximum recorded near-surface currents reached 0.8 ms−1, with near-bed maxima reaching 0.55 ms−1. 6. The wave climate was primarily driven by locally derived storms, resulting in events of short duration. Typical maximum significant wave heights of 8.9 and 9.4 m were recorded to the north and south of the Islands, respectively. Copyright © 2002 John Wiley & Sons, Ltd.

Modelling the South West Atlantic

1. This brief review paper of modelling developments in the SW Atlantic, presents material generated by a barotropic model of the whole continental shelf and shelf edge, which constitute a distinct biogeochemical province defined by its fundamental oceanographic processes and their implications to the regional wildlife. 2. The basic circulation components solved by the model include the Patagonian shelf tidal regime, the Falkland–Malvinas Current and wind-driven currents and surges. 3. Output from the model has already been used as the hydrodynamic base of an oil spill model, which was part of the Falkland Islands contingency plans in the North Falkland Basin during a recent oil exploration campaign. As part of the continuing evolution of the model, ongoing developments aiming at simulating observed baroclinic processes by assimilating measurements are outlined. Copyright © 2002 John Wiley & Sons, Ltd.

ABSTRACT: Controls on Conjugate Passive Margin Evolution : Insights from Seismic Stratigraphic Studies in the North Falklands Basin and South African Basins

ABSTRACT: Controls on Conjugate Passive Margin Evolution : Insights from Seismic Stratigraphic Studies in the North Falklands Basin and South African Basins

ABSTRACT: Controls on the Structural and Stratigraphic Architecture of Syn-Rift Sequences, North Falkland Basin, South Atlantic

ABSTRACT: Controls on the Structural and Stratigraphic Architecture of Syn-Rift Sequences, North Falkland Basin, South Atlantic

POST‐DRILLING ANALYSIS OF THE NORTH FALKLAND BASIN—PART 2: PETROLEUM SYSTEM AND FUTURE PROSPECTS

Six wells were drilled in the North Falkland Basin in 1998. Five of these wells recorded oil shows, and up to 32% gas was also recorded in mud returns to the rig floor. However, none of the wells encountered commercially viable petroleum accumulations.The syn‐rift and early post‐rift intervals contain thick, lacustrine claystones with oil source potential as indicated by TOC values up to 7.5% and Rock‐Eva1 S2 values of up to 102 kg HC per tonne of rock. These source rocks were immature or only marginally mature in five of the wells but had attained maturity in one of them. Modelling suggests that the main source interval may well be within the peak oil generation window in deeper, undrilled parts of the basin. Calculations of the amount of oil expelled range up to 60 billion barrels.Most of the wells tested a closely‐related set of plays in large structures asSociated with a sandstone interval near the top of the late syn‐rifi to early post‐rij? source‐rock succession. Post‐drilling geological modelling of the basin suggests that oil is unlikely to have migrated into this sandstone play at the localities tested, and that the wells consequently failed largely due to a lack of charge. Howevel; the play maintains exploration potential elsewhere. Other plays, particularly those stratigraphically asSociated with the base rather than the top of the source rock, may have a higher chance of exploration success.

POST‐DRILLING ANALYSIS OF THE NORTH FALKLAND BASIN— PART 1: TECTONO‐STRATIGRAPHIC FRAMEWORK

Six wells were drilled in the extensional North Falkland Basin in 1998. The wells encountered a Devonian to Cenozoic stratigraphy dominated by thick Mesozoic syn‐ and post‐rift successions. Although most previously published models predicted that the succession would most likely be of marine origin, it is in fact predominantly terrestrial; marine conditions did not become established in the basin until the Late Cretaceous. The oldest rocks recorded are Devonian and these were penetrated in only one well. The overlying succession comprises: a fluvio‐lacustrine, early syn‐rft interval of ?mid‐Jurassic to Tithonian age; a late syn‐rift fluvio‐lacustrine interval of Tithonian to Berriasian age; a rift‐sag transitional unit of Berriasian to Valanginian age; an early post‐rift lacustrine unit of Valanginian to early Aptian age; a middle post‐rift, transgressive unit of Aptian to Albian age; a late post‐rift, terrestrial to marine unit of Albian to early Palaeocene age; and a post‐up lift thermal subsidence unit of Palaeocene to Recent age.Much of the sediment appears to have been derived from volcanic and/or metamorphic terranes, probably located to the north or NW of the basin. As well as the volcanic material which occurs in the ground mass and as lithoclasts in many of the units, some volcaniclastic rocks and minor amounts of ashfall tufls are observed, particularly within the late syn‐rift succession.

Frontier Exploration in the South Atlantic: Structural Prospectivity in the North Falkland Basin

Seismic stratigraphic analysis of the continental shelf north of the Falkland Islands confirms that it is dissected by a significant extensional fault system that broadly defines the North Falkland Basin. The basin comprises a series of extensional subbasins that developed during two distinct rift episodes. Analysis of structural styles shows that while extension occurred predominantly on a series of planar normal faults with changes in rift polarity in northern areas, the southern portion of the basin has listric faults that sole out along preexisting thrusts. After rifting, the basin largely underwent postrift thermal subsidence with the passive infill of remnant topography; however, evidence exists for at least two uplift events affecting the postrift sediments. The earlier period of uplift tilted the basin to the south, resulting in the formation of a southerly prograding forced regressive wedge; the later episode of uplift tilted the basin northward, resulting in truncation of earlier strata and minor compressional reactivation and inversion in southernmost parts of the basin. The later tilting allows for the possibility that the original rift system once extended across the Falkland Islands. Although the lack of well control does not permit a comprehensive assessment of hydrocarbon plays, the seismic data do at least demonstrate the trapping potential of the basin. Interpreted structural and depositional styles found within the prerift, synrift, and postrift sequences are similar to those found in proven hydrocarbon provinces such as the North Sea and the Gulf of Suez. The success or failure of the frontier hydrocarbon province thus is likely to depend upon factors other than structural ones.

Evidence for multi-phase rifting in the North Falklands Basin

Abstract The post-Gondwanide basin development of the North Falklands Shelf has been evaluated using seismic sequence stratigraphy to determine the structural and stratigraphic evolution of the region. Two areally distinct post-Gondwanide basin systems (megasequences) are identified on the North Falklands Shelf, each with an independent structural and stratigraphic history. This contrasts with published models which interpret all basin areas to have been initiated in the early-mid Jurassic. Megasequence I is developed in a series of northwest and subsidiary north and east trending half-graben, and typically shows sedimentary growth into faults indicating an active rifting environment. Pervasive small-scale synthetic faulting is largely non-rotational, suggesting that it represents later deformation. No post-rift sediments associated with this phase of extension are identified. Comparisons with seismic geometries of other basins in the region, based on a restored plate model, indicate that Megasequence I represents part of a widespread mid-late Jurassic rifting event, ultimately leading to the opening of the Rocas Verdes (oceanic) Basin. Megasequence I is cross-cut by a north trending half-graben system which represents a failed arm of the South Atlantic Rift. The sedimentary fill of this basin, Megasequence II, can be subdivided into two units. The lower unit locally demonstrates sedimentary growth into the main eastern bounding fault of the half-graben and is thought to represent the synrift phase of Atlantic extension. Within the upper unit of Megasequence II there is no evidence of fault-controlled sediment growth into the main easterly bounding fault and the overall geometry is indicative of passive fill of pre-existing topography, supporting a post-rift origin for this section. Similar seismic geometries are observed in parts of the North Atlantic Rift System and the northern North Sea. In particular, the North Shetland Basin has a Permo-Triassic to late Jurassic rift and post-rift section. Renewed extension in the early Cretaceous resulted in break-up and domino faulting, with the generation of high relief fault blocks. The intervening lows were sites of deposition for onlapping deepwater sediments.

When did the Falklands rotate?

Similarities in the styles and relative timings of tectonic events in the Outeniqua Basin, South Africa and the North Falkland Basin suggest that basin formation in both regions may have preceded rotation of the Falklands microplate. Contrary to previous models for the break-up of Gondwana, which suggest Jurassic rotation, the data implies Valanginian rotation, contemporaneous with the first recorded motion on the Agulhas Falkland Fracture Zone and South Atlantic rifting. The data also suggests that the formation of the Falkland Plateau Basin may also be a Cretaceous event as opposed to the previously assumed Jurassic age. Such a model is consistent with new offshore seismic evidence while the inconclusive nature of the supportive evidence for Jurassic rotation does not exclude later rotation as a possibility.

GEOLOGY OF THE NORTH FALKLAND BASIN

The North Falkland Basin comprises two main structural elements: a north‐south trending graben, termed the North Falkland Graben; and a set of subsidiary basins to the west of this graben, also controlled by north‐south trending extensional faults, but which have been constrained by NW‐SE oriented, reactivated Palaeozoic thrust faults. The North Falkland Graben can be divided, in its northern part, into western and eastern depocentres, which are separated by a north‐south trending intra‐graben high.Early syn‐rift, late syn‐rift, early post‐rift and late post‐rift successions have been identified within the North Falkland Graben. The subsidiary grabens to the west probably contain only early syn‐rift deposits overlain by late post‐rift sediments: these basins were possibly uplifted along the shoulder of the main graben during its mainphase of extension in the Late Jurassic to Early Cretaceous.Although predictions of sediment‐infill type have only been made by regional correlations and seismic facies analysis in these undrilled basins, two major deltaic units of early post‐rift age are confidently inferred to be present in the North Falkland Graben, and to pass laterally into possible anaerobic mudstones of source‐rock quality.This petroleum system will soon be tested by a number of oil companies sharing production licences for the entire axial area of the North Falkland Graben.

PETROLEUM POTENTIAL OF THE FALKLAND ISLANDS OFFSHORE AREA

Rocks of Precambrian, Palaeozoic and Early Mesozoic ages are present on the Falkland Islands. The islands are surrounded by extensive Mesozoic‐Cenozoic basins that developed as failed rift systems during the initial stages of Gondwana separation. Large tracts of acreage in three of these basins (the Falkland Plateau Basin, South Falkland Basin and North Falkland Basin) were opened for licensing for the first time in October, 1995.The area has undergone little exploration other than the acquisition of regional seismic data, and interpretations of stratigraphy and basin history are therefore somewhat conjectural. The basins are believed to contain Jurassic to earliest Cretaceous syn‐rift sediments. Probable marine and lacustrine source rocks, and reservoir rocks eroded from clean Palaeozoic quartzites, are predicted to occur within this syn‐rift succession. Thick argillaceous intervals probably accumulated during a post‐Valanginian thermal sag phase. This sag phase was interrupted by regional uplift, particularly during the Early Cenozoic.A variety of “play” concepts can be established within each basin, and potentially large structural targets can be defined on the reconnaissance seismic data available from the area.The region is considered to be one of the World's last true “frontier” exploration areas, where large, deep, Mesozoic failed‐rift basins remain to be drilled and explored.

The geological evolution of the Falkland Islands continental shelf

The Falkland Islands are surrounded by four major sedimentary basins: the Falkland Plateau Basin to the east, the South Falkland Basin to the south, the Malvinas Basin to the west, and the North Falkland Basin to the north. The four main basins appear to have formed initially as Triassic through earliest Cretaceous extensional rifts associated with the break-up of Gondwana. A ?Valanginian end to rifting was followed by thermal sag. There is evidence of Cenozoic uplift in at least the North Falkland Basin, possibly coincident with Andean compression and the development of overthrusting along the plate boundary to the south of the islands resulting from opening of the Scotia Sea. There is no evidence from offshore seismic and gravity-magnetic data to support interpretations that the Falkland Islands have rotated clockwise through up to 180° during Gondwana separation. With the exception of the South Falkland Basin all the major basins probably underwent initially, more or less east-west extension, and had a similar orientation to adjacent South American and western southern African basins. The Falkland basins probably shared a similar geological history with the offshore southern African and South American basins.