Freetown

Collaborating Authors

National-scale data at prospect-scale resolution: Empowering the next generation of mineral exploration in Sierra Leone

Archer, Tim (Reid Geophysics Limited) | Lebbie, Joseph (National Minerals Agency) | Swaray, Ahmed (Reid Geophysics Limited)

OnePetroSeptember, 2021

National Minerals Agency, NMA Compound, New England Ville, Freetown, Sierra Leone Leveraging the value of mineral wealth is a difficult challenge for all countries, but perhaps particularly for those in the developing world, where infrastructural disorganisation, bureaucratic corruption, and civil unrest have been more-or-less endemic for over a century. The aim of this case study is to showcase an African nation plagued historically by all these various afflictions, but which now appears to be endeavouring to get things right. In 2020, a nationwide airborne geophysical survey of Sierra Leone was flown at 150 m nominal line spacing and 50 m nominal terrain clearance. Contractual deliverables included magnetic, radiometric and supporting data streams. The primary aim of the geophysical survey was to provide a national geoscientific benchmark for resource management, and to encourage foreign investment in the rich mining potential of Sierra Leone.

OnePetro

doi: 10.1190/segam2021-w6-04.1

SEG

SEG-2021-W6-04

SEG/AAPG/SEPM First International Meeting for Applied Geoscience & Energy

applied geoscience, case study, empowering, exploration geophysicist 10, foreign investment, Freetown, government, metals & mining, Mineral Exploration, mining sector, national mineral agency, national-scale data, nationwide airborne geophysical survey, prospect-scale resolution, Reservoir Characterization, Sierra Leone, Upstream Oil & Gas

Country: Africa > Sierra Leone > Western Area > Western Area Urban District > Freetown (0.29)

Geophysics: Geophysics (1.00)

Industry:

Materials > Metals & Mining (0.90)
Government (0.55)
Energy > Oil & Gas > Upstream (0.33)

Oilfield Places: Africa > Sierra Leone (0.95)

SPE Disciplines:

Management (1.00)
Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (0.80)

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A Geophysical Survey Of The Sierra Leone Continental Margin

Jones, E.J.W. (University College London) | Clayton, B.A. (University College London) | Mgbatogu, C.C.S. (U. of Benin) | Stevenson, C.G. (Logicom Computer Services UK Ltd.)

OnePetroMay, 1988

ABSTRACT A geophysical survey off the coast of Sierra Leone was carried out in April 1983 and September 1985 to investigate the structure of the West African continental margin between the Guinea and Sierra Leone fracture zones. A large (>100 mGal) free-air gravity anomaly and an associated negative magnetic anomaly show that the basic igneous complex of the Freetown Peninsula extends northwestwards beneath the continental shelf for approximately 100 km before swinging directly westwards towards the termination of the Guinea Fracture Zone. Reaching almost 20 km into the crust, the intrusion is of lopolithic form and appears to be a mantle-derived feature related to the activation of the Guinea Fracture Zone during the early Jurassic, rather than to melting following a meteorite impact as suggested by Krause in 1963. The intrusion is flanked by sedimentary basins containing at least 2 km of Mesozoic sediments overlain by about 200m of a Cenozoic sequence. Thick sediments occur beneath the continental slope where they are disrupted by faults and gravity slides which are particularly common near the Guinea and Sierra Leone fracture zones. The seismic stratigraphy of the deep-water sediments and the direction of oceanic fracture zones west of Sierra Leone indicate that the continental margin began to develop in the Cretaceous. Seismic reflectors of Eocene and probable mid-Cretaceous age extend under the continental slope from the Sierra Leone Abyssal Plain. The Cretaceous sequence appears to contain a southern extension of the thick black shales of the Gambia Basin, which may form an important petroliferous source rock beneath the shallower waters off Sierra Leone. INTRODUCTION The continental margin of Sierra Leone is one of the least understood parts of offshore Africa. It lies between the eastern terminations of two large oceanic fracture zones in the Equatorial Atlantic, the Guinea near 9°N and the Sierra Leone at 7°N. These structural lineaments have clearly exercised a major influence on the development of the margin, as can be seen in Figure 1(Available in full paper), which shows the shelf narrowing abruptly southwards where the fracture zones intersect the African continental slope. The main part of the Sierra Leone shelf is approximately 120km wide but decreases to only 30 km south of the Sierra Leone Fracture Zone. Sheridan et al (1969) first demonstrated from seismic refraction profiles that the Sierra Leone margin is underlain by thick sediments of probable Mesozoic and Cenozoic age. Several single-channel seismic reflection profiles published subsequently have confirmed the presence of thick sediments beneath the continental slope, but have revealed little of the depositional pattern under the shelf because of interference from strong multiple reflections (McMaster et al, 1975; Emery et al, 1975; Jones and Mgbatogu, 1982). During April 1983 and September 1985 seismic reflection, gravity and magnetic profiles were recorded over the Sierra Leone margin from the vessels R.R.S. SHACKLETON and R.R.S. CHARLES DARWIN. The latter cruise included the acquisition of a multichannel reflection profile along the outer continental shelf using a 32.8 litre (2000 cu.in.) airgun array and a 2.4km, 48-channel streamer.

OnePetro

doi: 10.4043/5713-MS

OTC

OTC-5713-MS

Offshore Technology Conference