Tectonic Control on Hydrocarbon Accumulation in the Intra-Continental Albertine Graben of the East African Rift System*
Dozith Abeinomugisha1 and Robert Kasande1
Search and Discovery Article #10183 (2009)
Posted April 10, 2009
*Adapted from oral presentation at AAPG International Conference and Exhibition, Cape Town, South Africa, October 26-29, 2008
1Petroleum Exploration and Production Department, Ministry of Energy and Mineral Development, Entebbe, Uganda ([email protected] )
The Albertine Graben is a Tertiary intra-continental rift that developed on the Precambrian orogenic belt of the African Craton. It forms the northern termination of the Western arm of the East African Rift System (EARS) (Figure 1). It stretches from the border between Uganda and Sudan in the north, to Lake Edward in the south. The available geological and geophysical data indicate that rifting could have been initiated during the late Oligocene or Early Miocene.
The tectonic evolution of the Albertine Graben is little understood. It does not fit well with traditional models of rifted continental basins defined by discrete basin bounding faults opposed by a low gradient flexural margin especially over Lake Albert. Similar amounts of displacement on the eastern border faults and western border faults produce nearly a full graben structure that gently dips towards the west, in contrast to half graben structures of the Tanganyika and Malawi rifts.
The available
geological and geophysical data indicate that the Albertine Graben has
gone through extensional and compressional episodes resulting in a variety
of structures. Deformation and prevalence of flower structures in the shallow
sedimentary sections
in some basins in the graben indicate that the neo-tectonic
processes are compressional.
The Albertine Graben has undergone substantial tectonic movement and thick sediments (approximately 6 km) have been deposited in lacustrine and fluvial-deltaic environments (Figure 2). The sedimentary layers dip gently towards the depo-centre on the western margin of the rift. Rapid tectonic subsidence coupled with limited sediment input led to deep stratified lakes with the accompanying deposition of source rocks. The hydrocarbon exploration wells drilled in the Albertine Graben have proven the deposition of source, reservoir and cap rocks.
Both
structural
and stratigraphic traps have been interpreted from geophysical
data acquired in the graben. However, only
structural
traps have been tested
for hydrocarbons by drilling. All the wells that have been drilled in the
Albertine Graben have been on either positive flower (Palm Tree) structures
or on fault blocks. Fault closures against basin bounding faults, or even
intrabasinal faults, have proved prolific for hydrocarbon trapping.
Not
only has tectonics played a crucial role in deposition of source, reservoir
and cap rocks and formation of structural
traps, but also provided migration
pathways for the hydrocarbons. Complex fault patterns, as interpreted from
geophysical data, have provided conduits for hydrocarbon migration, sometimes
long distance migration.
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Abeinomugisha, D. 2003,
Kasande, R. et al., 2004, Lake Albert seismic data interpretation report, Petroleum Exploration and Production Department, Uganda, Unpublished report.
McClay, K.R. 2001,
Morley, C. K., D. K. Ngenoh and J. K. Ego, 1999, Introduction to the East African Rift System, in C. K. Morley ed., Geoscience of Rift Systems – Evolution of East African Rift System: AAPG studies in Geology No. 44, p. 1-8.
Morley, C. K., R. M. Harper and S. T. Wigger, 1999, Tectonic Inversion in East Africa, in C. K. Morley ed., Geoscience of Rift Systems – Evolution of East African Rift System: AAPG studies in Geology No. 44, p. 193-210.
Rose, J. F. and B. Smith, 2002, Heritage Oil and Gas Ltd, Uganda license 1/97, Block 3, Seismic interpretation Report, Unpublished.
Rose, J. F. and B. Smith, 1998, Heritage Oil and Gas Ltd, Uganda license 1/97, Block 3, Seismic interpretation Report, Unpublished.
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