Seismic, drilling committed on tracts off Falkland Islands

Phil Richards Nigel Fannin
British Geological Survey
Edinburgh

Phyllis Rendell
Department of Oil
Stanley, Falkland Islands

The seven new production licenses, awarded to groups operated by Amerada, Shell, Lasmo, IPC, and Desire, cover 12,800 sq km of the North Falkland basin (Fig. 1 [82884 bytes] and OGJ, Nov. 4, p. 42). This is an area equivalent to 48 U.K. North Sea blocks. The awards cover virtually all of the deep, sediment filled axial portion of the North Falkland graben.

Eleven companies were successful in the bidding, which closed in July this year. There was strong competition for the blocks, and not all applicants were successful, with some disappointed companies receiving no awards.

Although not everyone received their first preferences of acreage, many did achieve awards either of their first or second priority blocks. Bids were assessed predominantly on the basis of work programs offered for the first two exploration phases (of 5 and 7 years), and the Falkland Islands government welcomed the substantial programs offered.

Program, reserves

Most bidders increased substantially their work program bids between submitting their written offers and making interview presentations to the Falkland Island government's evaluation panel. Many noted that they had been surprised by the number of bidders and level of interest in the area, and that this had caused them to re-evaluate their offers in light of the unanticipated competition.

Several oil companies that participated in early study groups (using reconnaissance seismic data from the area) did not bid, and numerous others evaluated the area from regional data sets and also declined to bid.

However, the success of the bidding round has sparked a new surge of interest amongst many of these nonbidders and will presumably fuel numerous farm-in attempts, particularly in the event of early success in the basin.

Although early success cannot be assured, and some doubts must remain about the risk of source, charge, and reservoir potential in the area, the fast-track approach to exploration adopted by most of the licensees will ensure that the largest potential targets are all drilled within the first 3-4 years.

The successful bidders have committed seven firm exploration wells for the first 5 year period and four more for the second exploration phase. They also offered at least five conditional wells for the first 5 year exploration phase.

They committed more than $150 million for phase one plus an additional minimum $122 million for extra exploration during phase one in the event of success.

Drilling is to start early in 1998 immediately following the interpretation of about 10,000 line km of new 2D seismic data and a substantial amount of exploration 3D data planned for acquisition in early 1997. The presently available data have a spacing of 4.5 km between dip lines in the tightest part of the grid and are therefore not really suited to the siting of exploration wells without further seismic. However, the existing data are good enough to allow the identification of several large, potentially drillable targets. These are ranged throughout the licensed acreage, and several span license boundaries.

Potential reserves estimates should be viewed with extreme caution at this stage, as nothing is known about reservoir type or conditions within identified prospects and it is conceivable that prospect definition will change following the acquisition of new, more closely spaced seismic data. However, operator's estimated recoverable reserves calculations suggest that field sizes may range up to 970 million bbl, with potential reserves of more than 2.5 billion bbl in the seven largest structures identified so far; averaging about 350 million bbl/structure.

This predicted possible reserve figure compares favorably with the approximately 2.2 billion bbl of original reserves estimated for the 14 oil fields in the similar sized area of the south and central Viking graben of the North Sea.

Falklands geology

The Falkland Islands lie at the western end of the Falkland plateau, a displaced terrane that probably lay adjacent to the eastern coast of southern Africa and Mozambique before the start of breakup of Gondwana in the Jurassic and the opening of the South Atlantic in the Early Cretaceous.

The North Falkland basin¹ lies to the north of the islands, beneath 150-1,500 m of water. It consists of a complex system of offset depocenters following two dominant structural trends: a north-south trending graben termed the North Falkland graben, and a set of subsidiary basins to the west of the graben, also controlled by north-south trending extensional faults but constrained by northwest-southeast oriented reactivated Paleozoic thrust sheets (Fig. 1 [82884 bytes]). The North Falkland graben is subdivided, in its northern part, into western and eastern depocenters, separated by a pervasive, north-south trending intra-grabenal high.

The main, north-south oriented structural trend in the North Falkland graben is dominated by apparently normal faults with little or no obvious evidence of strike-slip movement or inversion reactivation on any of the individual fault segments. The graben-bounding faults are not continuous but appear to form a series of slightly offset fault segments, each up to about 25 km long. However, the actual fault plane correlations are difficult to effect accurately with the present spacing of 4.5 km between dip profiles.

Synrift, early post-rift, and late post-rift phases can be identified within the North Falkland graben. The synrift succession may be divisible into two sub-units. The lower sub-unit forms the basal part of the synrift wedge and may also form the uplifted and eroded remnant basins along the western shoulder of the graben, in the area of sub-basins constrained by the northwest-southeast trending fault zone. This succession is older than the main synrift succession in the North Falkland basin, but its age is not known because of the lack of drilling in the area. However, Richards et al.² implied, by analogy with the early infill of the San Jorge basin to the northwest, that it may be Triassic to Lower Jurassic. It may therefore represent an early start to rift-infill in these basins that was terminated, in the northwest-southeast trending sub-basins to the west of the graben, by uplift of the graben shoulder during the main phase of extension from the Mid- to Late Jurassic onwards.

Richards et al.^2-3 considered the upper sub-unit of the synrift succession to be of Mid-Late Jurassic to Valanginian age. These authors noted that the cessation of active rifting in the other offshore Falklands basins and in many of the surrounding South American and offshore southern African basins also occurred in the Valanginian.

The "steers-head" shaped early post-rift succession developed immediately above the Valanginian end-rift unconformity equates stratigraphically with the "D-129" sag-sequence of the nearby San Jorge basin.² The early post-rift unit is observed to be a sequence of relatively high amplitude, more or less continuous, bright reflectors that may partly represent marine shales of Valanginian to Aptian age, deposited in an anoxic setting: such shales were encountered in boreholes drilled on the Maurice Ewing Bank, several hundred kilometers to the east, on the eastern margin of the Falkland Plateau basin.^2-3

The "steers-head" is confined to the grabenal axis areas and its immediately adjacent shoulder regions and may therefore represent the aerially restricted limits of the basin in which anoxic sediments were able to accumulate.

An additional seismic facies identified within the early post-rift unit has considerable reservoir potential. This succession comprises two separate developments of apparently prograding reflectors that probably represent deltaic deposition into the restricted basin, perhaps depositing reservoir quality sandstones against source-rock quality shales.

By analogy with surrounding basins it is predicted that the remainder of the Cretaceous section of the late post-rift succession will comprise probably aerobic marine claystones and nannofossil rich ooze similar to that found in the Falkland Plateau basin.⁴ However, fluvial sediments similar to those in the San Jorge basin to the northwest may also be present.

The late post-rift unit is subdivided into two by a prominent, undulating reflector that can be mapped regionally across this and the other offshore Falklands basins. This unconformity, possibly spanning the Maastrichtian to Paleocene interval as in the Falkland Plateau basin,³ may reflect a period of regional uplift, resulting from Andean mountain building to the west, coupled with opening of the Scotia Sea and associated northwards indentation of the Scotian plate into the South American plate to the south of the islands. The uppermost part of the late post-rift unit may be composed of prograding deltaic sediments deposited into a shallow marine environment.

The subsidiary grabens to the west probably contain only early synrift deposits overlain by late post-rift sediments: these basins were possibly uplifted along the shoulder of the main graben during its main phase of extension in the Late Jurassic to Early Cretaceous.

Although predictions of sediment-infill type can only be made through regional correlation and seismic facies analysis in the undrilled North Falkland basin, two major deltaic units of early post-rift stage are confidently inferred to partly infill the North Falkland graben, passing laterally into possible anaerobic mudstones of source rock quality. Since large tracts of the graben have recently been licensed to oil companies for new exploration, the relatively intensive drilling program to be conducted within the next 5 years will provide data to test the area's hydrocarbon potential.

Acknowledgment

This article is submitted by permission of the director, British Geological Survey (Natural Environment Research Council).

References

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

2. Richards, P.C., Gatliff, R.W., Quinn, M.F., and Fannin, N.G.T., Petroleum potential of the Falkland Islands offshore area, Journal of Petroleum Geology, Vol. 19, 1996, pp. 161-182.

3. Richards, P.C., Gatliff, R.W., Quinn, M.F., Williamson, J.P., and Fannin, N.G.T., The geological evolution of the Falkland Islands continental shelf, in Storey, B.C., King, E.C., and Livermore, R.A., eds., Weddell Sea tectonics and Gondwana break-up, Geol. Soc. London Spec. Pub. 108, 1996, pp. 105-128.

4. Jacquin, T., and Graciansky, P.Ch., De., Cyclic fluctuations of anoxia during Cretaceous time in the South Atlantic Ocean, Marine and Petroleum Geology, Vol. 5, 1988, pp. 359-369.

The Authors

Phil Richards has worked since 1980 as a consultant and team leader for the British Geological Survey's Petroleum Group, providing exploration consulting services to governments and oil companies. He is a BGS principal geologist, currently advising the Falkland Islands government on the scientific and technical aspects of exploration in the area. He was graduated in geology from Aberystwyth University in 1980 and with a PhD in applied geology from Strathclyde University for part-time research between 1986-89.

Nigel Fannin spent 21 years with the British Geological Survey's Marine Geology Group and as coordinator of Europe-wide offshore mapping projects. He has run the BGS Petroleum Group in Edinburgh since 1992 and is responsible for formulation of exploration planning advice to the Falkland Islands government. He was graduated in geology from Queen's University in Belfast and with a PhD in sedimentology from Reading University in 1970.

Phyllis Rendell is comparatively new to the oil industry, having established a successful career in education and educational administration, latterly as director of education in the Falkland Islands. As a senior member of the Falkland Islands government she has been involved in exploration issues around the islands since 1992 and has been a participant in all bilateral oil meetings between the Falkland Islands and Argentina since that time. She was recently appointed as the Falkland Islands government's first director of oil.

Copyright 1996 Oil & Gas Journal. All Rights Reserved.