Airborne gravity gradiometer surveying of petroleum systems under Lake Tanganyika, Tanzania*
Doug Roberts 1 5 Priyanka Roy Chowdhury 2 Sharon Jenny Lowe 3 Asbjorn Norlund Christensen 41 Beach Energy, GPO Box 175, Adelaide, SA 5001, Australia.
2 Wintershall Holding GmbH, Friedrich-Ebert-Straße 160, Kassel D-34119, Germany.
3 Southern Geoscience Consultants (SGC), PO Box 694, Belmont, WA 6984, Australia.
4 Nordic Geoscience, 2 Huon Grove, Ashburton, Vic. 3147, Australia.
5 Corresponding author. Email: dcrgeo@tpg.com.au
Exploration Geophysics 47(3) 228-236 https://doi.org/10.1071/EG15075
Submitted: 6 August 2015 Accepted: 2 January 2016 Published: 5 February 2016
Abstract
The Lake Tanganyika South petroleum exploration block covers the southern portion of the Tanzanian side of Lake Tanganyika and is located within the East African Rift System. The rifting process has formed rotated fault blocks which provide numerous play types in the resulting basins. Interpretation of 2D seismic data from 1984 indicated that sufficient sediment thickness is present for hydrocarbon generation. The prospectivity of the lake sediment sequence is enhanced by large oil discoveries further north along the rift system at Lake Albert in Uganda. Airborne gravity gradiometry (AGG) has been used in the Lake Albert region to delineate the structural framework of sedimentary basins.
Based on this analogy, in 2010 Beach Energy commissioned CGG to fly a FALCON AGG and high-resolution airborne magnetic survey over the Lake Tanganyika South block to provide data for mapping the basin architecture and estimating the depth to magnetic basement. A total of nearly 28000 line kilometres of data were acquired.
The subsequent interpretation incorporated the AGG and magnetic data with available 2D seismic data, elevation model data, bathymetry, Landsat and regional geology information.
The integrated data interpretation revealed that the Lake Tanganyika rifting structures occur as half-grabens that were formed through reactivation of Precambrian fault structures. Two major depocentres were identified in the magnetic depth-to-basement map in the north and in the west-central part of the survey area with sediment thicknesses in excess of 4 km and 3 km, respectively. Smaller, shallower depocentres (with less than 3 km of sediment) occur in the south-western region. This information was used to plan a 2100 km 2D marine seismic survey that was recorded in 2012. An interpretation of the results from the seismic survey confirmed a rifting structure similar to that encountered further north at Lake Albert in Uganda. Several targets were identified from the seismic sections for follow-up.
Key words: airborne gravity gradiometer, airborne survey, FALCON, Lake Tanganyika, rift valley.
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