07b. an overview of the petroleum provinces and

07B.
AN OVERVIEW OF THE PETROLEUM
PROVINCES AND PROSPECTIVE
BASINS OF EAST AFRICA.
By Ernest N. T. Rubondo
Assistant Commissioner
Petroleum Exploration and Production Department
Ministry of Energy and Mineral Development
Uganda.
TABLE OF CONTENTS
TABLE OF CONTENTS ............................................................................................................................................II
LIST OF FIGURES ...................................................................................................................................................III
LIST OF TABLES.....................................................................................................................................................III
INTRODUCTION .......................................................................................................................................................1
1. GEOGRAPHICAL LOCATION..............................................................................................................................3
2. TECTONIC SETTING ............................................................................................................................................5
3.0 THE SEDIMENTARY BASINS OF EAST AFRICA.............................................................................................9
3.1 PASSIVE MARGIN BASINS ................................................................................................................................11
3.2 CONTINENTAL RIFT BASINS ..............................................................................................................................11
3.2.2.THE EAST AFRICAN RIFT SYSTEM .................................................................................................................12
3.3 INTRACRATONIC BASINS ..................................................................................................................................15
4.0 THE PETROLEUM POTENTIAL OF THE SEDIMENTARY BASINS IN EAST AFRICA .................................15
4.1.0 SURFACE SEEPAGES AND PETROLEUM SHOWS IN WELLS................................................................................15
4.1.2 SOURCE ROCKS ...........................................................................................................................................20
4.1.3 RESERVOIR ROCKS ......................................................................................................................................21
4.1.4 STRUCTURE .................................................................................................................................................24
5. THE DEVELOPMENT OF PETROLEUM EXPLORATION IN EAST AFRICA. .................................................26
5.1 EXPLORATION HISTORYy .....................................................................................................................26
5.2 CURRENT STATUS.......................................................................................................................................29
6. ANALOGY ...........................................................................................................................................................30
6.1 PASSIVE MARGIN BASINS. ...............................................................................................................................30
6.2 CONTINENTAL RIFT BASINS ..............................................................................................................................31
8.0 CONCLUSION...................................................................................................................................................32
ii
LIST OF FIGURES
Figure 1: Location map of East Africa ........................................................................................ 3
Figure 2: Breezes Beach, Zanzibar along the coast of East Africa ............................................ 4
Figure 3: The breakup of Gondwanaland and tectonic development of East Africa. .................. 5
Figure 4: Map Showing African basin types ............................................................................... 7
Figure 5: Map showing the tectonosratigraphic setting
of the sedimentary basins of East Africa .......................................................................... 8
Figure 6: A map showing sedimentary basins of East Africa...................................................... 9
Figure 7: The relationship between the Anza, Muglad, and Melut basins ................................ 12
Figure 8: The Paraa Oil seepage on Victoria Nile (top) and the Tundaua oil seepage on
Pemba island (bottom) in the Albertine Graben and Coastal basins respectively............ 16
Figure 9: Outcrop of the Kisegi sandstones in the Albertine Graben and the
Jurassic limestones in the Lamu basin (bottom).............................................................. 23
Figure 10: Structural Cross Section through the North Lokichar
and North Kerio basins, Kenya Rift................................................................................. 24
Figure 11: East West structural section across the Albertine Graben ...................................... 25
Figure 12: Structural cross section acrosss the Seleous and Mandawa basins ....................... 25
LIST OF TABLES
Table 1: Summary of the sedimentary basins of East Africa .................................................... 10
Table 2: Geochemical data of the oil seepages of the sedimentary basins in East Africa........ 16
Table 3: Oil and Gas Shows encountered in deep Wells drilled in the basins of East Africa.... 19
iii
Introduction
East Africa is located on the eastern part of the African continent and covers a total
area of over 1.76 million square kilometers. It forms part of the African craton and is
underlain by some of the worlds oldest rocks.
Tectonic movements, which commenced during the Carboniferous and continued
intermittently until recent times have played a major role in the geological
development of East Africa. This development commenced with the break-up of the
super continent of Gondwanaland, and the subsequent drifting of its components,
resulting in the formation of the passive margins and failed rifts along the East
African coastline. Later continental rifting and local sagging led to the formation of
features like the Anza Graben, the East African Rift System, and some intracratonic
basins in the region.
The areas with the potential for generation and accumulation of petroleum in East
Africa can be classified into four major tectonostratigraphic settings namely Passive
Margins, Central African Rift System, East African Rift System and Intracratonic
basins.
There are over fifteen major sedimentary basins in these four settings, and these
basins collectively cover an area of over 500,000 square kilometers which is
approximately over 30% of the area of East Africa.
These basins hold a variety of opportunities for oil and gas exploration and
development, and these opportunities are not only displayed by the presence of
1
economic reserves of gas in two of the basins, but also the existence of thick
sequences of sedimentary formations in all of them. Seven oil seepages are known
to exist in 6 of the basins, while oil and gas shows have been encountered in over 30
of the 60 exploration wells drilled in the region.
In an effort to establish the petroleum potential of the region, geophysical surveys,
geological studies and exploration drilling have been intermittently undertaken in the
different basins over the last 70 years. These include the acquisition of over 90,000
line kilometers of seismic data together with the drilling of over 60 exploration wells
in the region. These efforts are very minimal indeed when compared to the total
area covered by these basins and the petroleum potential exhibited therein.
While large amounts of oil and gas have been, and continue to be, discovered in
tectonostratigraphic settings in other parts of the world which are similar to those in
East Africa, insufficient exploration efforts continue to delay the evaluation of the
excellent opportunities existing in the region.
2
1. Geographical Location
East Africa derives its name from its location on the eastern side of the African
Continent and extends between latitudes 5.30°North and 11° South. It is located
between the Great Lakes in the western arm of the East African Rift Valley System
of the central part of the continent to the west, and the Indian Ocean to the east
(Figure 1). It shares common boundaries with Sudan, Somalia and Ethiopia to the
north, Democratic Republic of Congo, Rwanda and Burundi to the West, Zambia,
Malawi and Mozambique to the South and Somalia to the east.
Figure 1: Location map of East Africa
3
Figure 2: Breezes Beach, Zanzibar along the coast of East Africa
East Africa displays a variety of physical features. From over 1200 kilometers of a
long and narrow coastal line with sweeping sandy beaches to the east, figure 2, it
rises westwards to an extensive plateau covered by Savannah and bush at an
elevation of over 1,000 metres. The plateau is fringed by narrow belts of rain forests
like the Maramagambo forest and major highlands, which include Mt. Kenya
(5200m), Mt. Meru, Mt. Rwenzori (5110m) and Mt. Kilimanjaro, the highest in Africa
at 5879m. Over 10% of East Africa is covered by freshwater lakes, which include
lakes Victoria, Tanganyika, Turkana, Albert and Malawi. Among these, Lake Victoria
is the second largest lake in the world while Lake Tanganyika is the second deepest
lake in the world with an average depth of 1500 meters.
4
Climate across East Africa varies from the arid to semi arid experienced in the north
eastern part of the region to the heavy rainfall experienced around the Lake Victoria
Basin, the Rwenzori mountains and the southern parts of Tanzania.
2. Tectonic Setting
East Africa forms part of the African Craton which hosts some of the oldest rocks in
the world. Some of these rocks are known to be as old as 4000 million years. The
area was affected by rifting which commenced during the Carboniferous and
continued through the Mesozoic, leading to the breakup of the super continent of
Gondwanaland. This breakup resulted in the formation of plates like the Indian sub
continent, Antarctica and Australia together with the separation of the Gulf of Aden
from Africa (Figure 3).
Figure 3: The breakup of Gondwanaland and tectonic development of East
Africa.
5
The break up of Gondwanaland was comprised of two phases of rifting and drifting.
Significant sediment accumulation occurred during the process of rifting, with
continental sediments being deposited at relatively high sedimentation rates in the
rift basins formed along the East African coast. The continental margins which
underwent thermal subsidence and subsequent drifting became repositories of
marine sediments and developed into passive margins, while those which never
reached the drifting state remained as failed rifts. Examples of the former include
the Rufiji, Mandawa, Lamu and Ruvuma basins while those of the latter include the
Selous and Ruvu basins.
The major rifting phase which lead to the break up of Gondwanaland was
accompanied by smaller rifting events which resulted in the formation of continental
rift basins. These later events lead to the formation of the Anza Graben and the East
African Rift System which commenced during the Cretaceous and Tertiary
respectively. The Anza Graben is part of the Central African Rift System as shown in
Figure 4.
In addition, localized sagging arising mainly from tectonic movements led to the
development of intracratonic basins in some parts of the region. These intracratonic
basins include the Hoima and Lake Kyoga basins. The sedimentary basins in East
Africa can therefore be classified into the following tectonostratigraphic settings
which are shown in Figure 5.
a)
Rifted Passive Margins
b)
Mesozoic Continental Rifts
c)
Tertiary Continental Rifts, and
d)
Intracratonic Basins
6
Figure 4: Map Showing African Basin types
7
Figure 5: A Map Showing the Tectonostartigraphic Setting of the sedimentary
basins of East Africa
8
3.0 The Sedimentary Basins of East Africa
Over 30% of East Africa is underlain by sedimentary basins. There are over fifteen
major basins, which cover a total area of over 500,000 square kilometers. Figure 6
shows the distribution of these basins and Table 1 their summary.
Figure 6: A map showing Sedimentary Basins of East Africa
9
Table 1: Summary of the sedimentary basins of East Africa
No
Basin
Area
(Km2)
Seismic
(Km)
Sediment
Thickness
Sub-basins
Hydrocarbon
Occurrence
1,800
No. of
deep
Wells
2
1
Ruvuma
16,000
Over 5 km
-
-Mnazi Bay
Gas Field
-Msimbati Gas Seepage
2
Mandawa
15,000
3,500
3
4.5 km
-
3
Rufiji
16,000
2,200
2
10 km
-
-Songo Songo
Gas Field
-Wingayongo Oil Seepage
4
Selous
50,000
4,700
1
10 km
-
-
5
Ruvu
15,000
3,000
2
Over 8 km
-
-
6
Dar es Salaam
Platform
Coastal
18,000
5,000
5
5 km
-
-
12,000
9,000
13
9 km
Pemba,
Zanzibar &
Mafia
Tundaua Oil seepage
Permo-Triassic to
Tertiary
8
Lamu
Embayment
132,770
16
12 km
Gas shows in wells
Permo-Triassic to
Tertiary
9
10
Mandera
Anza Basin
51,920
94,220
15,991
(onshore)
14,071
(offshore)
1,287
14,362
2
11
10 km
10 km
Tarbaj Hill Oil seep
Oil and Gas shows in wells
11
Kenya Tertiary
Rift Basins
38,904
7,652
-
4 km
Oil and Gas shows
Karoo and Post Karoo
Late
Jurassic/Cretaceous to
Tertiary
Tertiary
12
Albertine
Graben
22,000
398
2
Over 5km
5 Oil seepages
Tertiary
13
Modern Rift
Basins
79,000
6,000
2
Over 4km
14
Pemba
Zanzibar
Deep Sea
Mafia Deep
Offshore
45,000
4,300
0
40,000
6,700
0
7
15
-Chalbi
-Yamicha
-Kaisut
-Suguta
-Magadi
-Turkana
-North &
South
Lokichar
-Nyanza
-North &
South Kerio
Age
Permo-Triassic to
Tertiary
Permo-Triassic
through
Jurassic and
Cretaceous to
Tertiary
Permo/Triassic-LowerMid
Jurassic in the North
Permo-Triassic to
Neogene
Permo-Triassic Karoo
-Oil seepage on L.
Tanganyika
-Sublacustrine flow of
asphalt
Tertiary
10km
-L. Edward –
George
-Semliki
-N. Lake
Albert
-Pakwach
-Rhino
Camp
-Rukwa
-Ruhuhu
-Ruaha
-L.
Tanganyika
-L. Nyanza
-
-
Jurassic to Tertiary
10km
-
-
Jurassic to Tertiary
10
3.1 Passive Margin Basins
The basins under this setting are distributed along the East African Coast and are a
result of the break-up (rifting and drifting) of Gondwanaland. They are distributed
both onshore and offshore and include, from South to North, the Ruvuma Basin, the
Mafia deep offshore, the Mandawa Basin, the Coastal Basins, the Dar-es-Salaam
Platform, the Pemba Zanzibar Deep-Sea and the Lamu Basin. All together the
basins in the passive margin cover a total are of over 162,000 square kilometers.
The Selous, Ruvu, part of Lamu, and Mandera basins are failed rift basins which
predate the-break up of Gondwanaland they cover a total area of 249,690 square
kilometers in East Africa.
3.2 Continental Rift Basins
The basins falling under this setting in East Africa can be subdivided into two major
categories namely the Anza basin and the East African Rift System.
3.2.1 Anza Basin
The Anza basin covers a total area of 94,220 square kilometers and is part of the
Cretaceous-Tertiary Central African Rift System which extends from the Benue
Trough in Nigeria, through Chad, the Central African Republic, the Sudan and into
the northern part of East Africa. It is a failed rift system that extends across the
Central Africa Craton. The Anza Basin form the eastern most part of this system and
has a similar trend as the oil bearing Muglad, and Melut basins of Sudan, (Figure 7).
11
Figure 7: The relationship between the Anza, Muglad, and Melut Basins
3.2.2.The East African Rift System
This system is a Tertiary to recent feature extending from the junction of the Red
Sea
and the Gulf of Aden in the north through Ethiopia and East Africa to
Mozambique in Southern Africa. Recent work, by Rice University in the United
States, indicates that this feature extends to a point located in the Indian Ocean off
the coast of eastern South Africa. The basins in the East African Rift System cover a
total area of 139,904 square kilometers.
12
Although the East African Rift System is largely a Tertiary to Recent phenomenon, in
some places the rifting is related to the Karroo volcanic activity dating back to Permo
Triassic or early Jurassic (King, 1970). In many places the structures within the rift
have been influenced by pre-existing Precambrian zones of weakness.
In East Africa, the rift system breaks up into two branches the eastern arm (also
known as the Kenya rift), and the western arm.
3.2.2.1 The Kenya Rift
The Kenya rift is part of the arm which starts from the triple junction and continues
south through the Ethiopian rift joining into the Kenya rift. Further south, it branches
into the Davis Ridge (Mongenot et.al, 1986). The northern sector of this rift segment
cuts across the abyssal plateau volcanics of Kenya. In eastern Tanzania, the rift
structures form a broad zone of faults defining a series of titled blocks with varying
orientations. South east of Mount Kilimanjaro, the para Usambara faults define a
branch of the eastern rift which trends south east to join the fault systems of the
Davie Ridge in the Indian Ocean.
The basins in the Kenya rift include the Magadi and Suguta Troughs, Turkana basin,
Lokichar (Loperot) trough, Nyanza Trough and the South Kerio Trough.
3.2.2.2 The Western branch of the East Africa Rift System.
This branch extends from northern Uganda to southern Mozambique encompassing
the major lakes of Albert, Tanganyika, Rukwa and Malawi. Its northern boundary
terminates in the Precambrian Aswa shear zone, a structure which stretches NWNSW from southern Sudan through Uganda into Kenya. In the north, the rift is
13
characterized by a N-S oriented zone that follows the earlier structural trends
through the en-echelon faults of the Albert Nile, lakes Albert, Edward, Kivu and the
Rwenzori mountains.
The E-W trending Katonga fault zone extends from north of Lake Victoria towards
the western rift. South of Lake Kivu, the faults orientations change from NNE-SSW
to NNW-SSE. Major structures in this area are the border fault systems of lakes
Tanganyika and Rukwa that join in the faults bordering Lake Malawi. The basins in
the western arm of the East Africa Rift Valley rift include the Rhino Camp Basin, the
Pakwach Basin, the northern Lake Albert Basin, the Semliki, Basin the Lakes
Edward and George Basin, the Lake Tanganyika Basin, the Rukwa basin, the
Ruhuhu Basin and the Lake Malawi Basin.
In both the Kenya Rift and the Western arm of the East African Rift Valley, the basins
are asymmetric and are bordered by curvillinear high angle border fault segments on
one side and en-echelon step faults, with minor vertical offsets and flexural
monoclines, on the opposite side (Bosworth et al, 1980 Rosendahl, 1989;
Dunkelman et al, 1989, Chapola, 1997). Although well developed grabens, exist in
the Ethiopian rift, most of the basins in East Africa are characterized by alternate half
grabens, Baker et al, 1982. Individual half grabens are separated from each other
by accommodation zones which act as transfer zones that allow switches in gross
polarity of the border fault systems.
These accommodation zones are structural highs of varying complexity, and they are
made up of oblique-slip transfer ramps and monoclines, Chapola, 1997.
14
3.3 Intracratonic Basins
The intracratonic basins of East Africa include the Hoima and Kyoga Basins in
Uganda. Both of these basins were formed as a result of sagging but their formation
is closely linked to the rift structures adjacent to each of them. Neither of them has
attracted significant petroleum exploration effort. The Nyanza trough and the Usanga
basin in Kenya and Tanzania respectively also have significant sagging in their
formation.
4.0 The Petroleum Potential of the Sedimentary Basins in East
Africa
Economic accumulations of gas have been discovered in two of the 15 major basins
of East Africa. They are the Songo Songo Gas field in the Rufiji trough and the
Mnazi Bay Gas discovery in the Ruvuma Basin. The areas around these two basins
can therefore be classified as petroleum provinces, in contrast to the other basins,
where economic potential remains unproven. In addition to the two gas discoveries,
several factors point to the potential for more discoveries in the region.
4.1.0 Surface Seepages and Petroleum shows in wells.
a) Oil seepages
Oil seeps have been found in six of the fifteen basins in East Africa. These are the
Kibuku, Kibiro and Paraa oil seepages in the Albertine Graben, the sublacustrine
flow of asphalt in the Lake Tanganyika basin, the Msimbati oil seep in the Ruvuma
basin, the Wingayongo oil seep in the Rufiji trough,the Tundaua oil seep on the
15
Pemba Island of the Coastal Basin and the Tarbaj Hill oil seep in the Mandera basin.
Figure 6. The geochemical characteristic of these oil seeps are described in Table 1.
Figure 8: The Paraa Oil seepage on Victoria Nile (top) and the Tundaua oil
seepage on Pemba island (bottom) in the Albertine Graben and Coastal Basins
respectively
16
Table 2: Geochemical data of the oil seepages of the
sedimentary basins in East Africa
Name of Seep
Kibiro
Kibuku
Msimbati
Paraa seep
Organic Matter Type
Algal type 1
Marine/Terrestrial
Depositional Environment
Lacustrine
Estuarine/Bay
Degradation
Extensive
Moderate
Moderate
Source Rock Maturity
Early to
Middle
Mature
Early Mature
Moderate to
Mature
Source Rock Age
No older than
Jurassic
Post Early
Cretaceous
% Sat:Arom:Polars
22:17:60
50:25:20
--
Carbon Isotope (PPT-PDB)
-23.8
-27.9
--37:23:40
Tarbaj Hill
Tundaua
Wingayongo
Marine
Carbonate
Lacustrine
Campani
an
Bajocian
Description
Non-Marine
Carbonate
Reducing
Late
Cretaceous
Lacustrine
--
GEOCHEMICAL RATIOS
Pr/Ph
--
Steranes %C27:C28:C29
35:42:23
6.2-6.7
28:24:48
Ts/Tm
1.46
0.3
1.63
Diasterane Index
1.43
--
1.47
Ratios defined:
1. Ts = 18α(H)-Tisnorneohopane
2. Tm = 17α(H)-Trisnorhopane
3. Pr = Pristane
4. Ph = Phytane
5. Diasterane index = Ratio of C27 βαα(20R + 20S) to C27 ααα(20S + 20R)
17
(b) Petroleum shows in wells
Out of the 60 wells drilled in East Africa, oil and gas shows have been encountered
in over 30 of them. Table 2 shows the petroleum shows, which have been
encountered in the wells, drilled in the different basins of East Africa.
The presence of these oil seeps and occurrences in the wells does not only confirm
the generation of hydrocarbon in the respective basins but it also confirms
hydrocarbon expulsion and migration.
18
Table 3: Oil and gas shows encountered in deep Wells drilled in
the basins of East Africa.
NO
1
WELL
East Lika-1
2
Kimbuji
East-1
Kimbiji
main-1
Kisangire-1
Kiwangwa1
Lukuliro-1
Mafia-1
Makarawe1
Mandawa-7
Mbuo-1
Mita
Gamma-1
Mnazi Bay1
Pemba-5
Ras
Machususi1
Songo
Songo-1
Songo
songo-2 to
9
Tan Can-1
Zanzibar-1
Loperot-1
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
39
Sirius-1
Dodori-1
Pate-1
Mararani-1
Kipini-1
Hagarso-1
Pandagua1
Wal Merer
Simba
Maridadi
1-8
Kofia
offshore
Ndovu
Duma
Hothori-1
Endela
Waki- 1 well
TYPE OF SHOW
Oil stain and cut fluorescence in Middle and Lower
Jurassic
Good gas show in Eocene andPpaleocene
BASIN
Mandawa Basin
Poor gas show
Dar-es-salaam Platform
Middle Jurassic bitumen stain
Trace oil, show high mud gas in MiddleJurassic
Dar-es-Salaam Plat form
Ruvu Basin
Gas shows in lower jurassic Madaba series
Oil and gas shows in Lower Tertiary
Trace oil, high mud gas in Middle Jurassic
Rufiji trough
Coastal Basin
Ruvu Basin
Free oil in Nondwa Shale and Mbuo claystone
Oil fluorescence
Oil stain, streaming flurescence
Mandawa Basin
Mandawa Basin
Mandawa Basin
Gas discovery in Miocene/Oligocene sands, oil shows in
Upper Cretaceous/Upper Tertiary
Minor oil and gas shows in Eocene/Oligocene
Mud gas at base of Tertiary
Ruvuma Basin
Coastal Basin
Coastal Basin
Gas discovery in Lower Cretaceous
Rufiji Trough
Development in Lower Cretaceous
Rufiji Trough
Minor gas in Paleocene
Minor gas in Eocene limestone
Oil recovered in a Drill stem test
Dar-es-salaam Platform
Coastal Basin
Tertiary rift basin
Paraffinic oil discovered in a DST
Wet gas tested in sands of Paleocene age
Wet gas
Oil and gas shows in Tertiary strata
Oil and gas shows in Cretaceous strata
Oil and gas shows in Cretaceous strata
Gas shows in the Tertiary
Anza Basin
Lamu Basin
Lamu Basin
Lamu Basin
Lamu Basin
Lamu Basin
Lamu Basin
Gas shows in the Cretaceous
Gas shows in Tertiary and Cretaceous
Gas shows in the Tertiary
Lamu Basin
Lamu Basin
Lamu Basin
Gas shows in the Tertiary
Lamu Basin
Gas shows in the Cretaceous
Gas shows in the Cretaceous
Gas shows in the Cretaceous
Gas shows Paleogene
Oil shows
Anza Basin
Anza Basin
Anza Basin
Anza Basin
Albertine Graben
Dar-es-Salaam Platform
19
4.1.2 Source Rocks
The sparse data acquired in the basins of East Africa has led to the generally poor
knowledge about the source rock potential in the region. Some major source rocks
have however been reported in the different basins and these include.
(a) The Bajocian Makarawe Shales
These are oil and gas prone Lower Jurassic shale formations which
reportedly entered the generation window in the Upper Cretaceous in the
Ruvu Basin. They have also been reported in the Ruvuma basin where they
have TOC values of up to 12%.
(b)
The Nondwa formation evaporites.
These are Lower Jurrasic evaporites which have been reported in the Rufiji
Trough and Mandawa Basins.
(c) Karroo Beds
These are Permo Triassic shales which are quite well distributed across East
Africa. They have been reported in some of the basins of the East African Rift
System, the Lamu Basin, the Mafia Songo Songo Area and the Mandera Basin.
(d) Other source rocks reported include the Neocomian shales of the Anza Graben,
the Tambach, Poi and Kapkiamo shales in the Kenya Rift and the Mid to Late
Tertiary formations of the Albertine Graben.
20
4.1.3 Reservoir Rocks
Good reservoir quality sandstones have been encountered in outcrop and in
wells in the different basins in the region. Some of the reservoir rocks
reported include:
1. Karroo Sandstones
These are continental, through deltaic, to braided fluvial sandstones which
are distributed in many basins of East Africa. The basins where good
reservoir quality Karoo sandstones have been reported include the Ruvu
Basin, the Dar-es-Salaam Platform, the Selous Basin, the Rufiji Trough, the
Mandera Basin, the Ruvuma Basin, the Anza Basin together with the basins
of the Kenya Rift and Interior Rift Basins of Tanzania.
2. Middle- Upper Jurrassic Limestones
These are shelf-derived carbonates which have been reported to be
distributed in the Ruvu Basin, Rufiji Trough, Anza Basin, Lamu Basin and
Mandera Basin.
3. Middle-Upper Jurrassic Sandstones
These are shallow marine tidal sandstones which have been reported in wells
drilled in the Dar-es-Salaam Platform, Ruvuma Basin and the Turkana Basin
of the Kenya Rift. Figure 9.
21
4. Lower Cretaceous Sandstones
These are Neocomian, Aptian and Albian sands which were largely deposited
in a shelf environment. These are the sandstones which provide the reservoir
for the Songo Songo gas field in the Rufiji Trough Basin. They have been
reported in the Dar-es-Salaam Platform, Coastal Basins and the Lamu Basin.
5. Tertiary Sandstones
These are mainly Mid to Late Tertiary fluvial-lacustrine sandstones, which are
distributed in the basins of the East African Rift system.
The basins where these sandstones have been reported include the
Albertine Graben, the Nyanza and South Kerio troughs. Figure 9.
6. Good reservoir quality progradational sandstones deposited during the Tertiary
have also been reported in the Lamu Basin.
22
Figure 9: Outcrop of the Kisegi sandstones in the Albertine Graben and the
Jurassic limestones in the Lamu Basin (bottom).
23
4.1.4 Structure
Different basins in the region display a variety of structuring. However since all the
major basins in the region were derived mainly from extensional tectonics, the basins
have a preponderance of structures related to titled fault blocks, hosts, rollovers and
down to basin listric faulting. These extension tectonics related structures are
predominant in the lower section of the basins of the passive margin and in the entire
section of the continental rift basins (Figures 10 & 11).
.
Figure 10: Structural Cross Section through the North Lokichar and North
Kerio Basins, Kenya Rift
24
Figure 11: East-West structural section across the Albertine Graben
In basins like the Mandawa and Lamu Basins where salt exists, post salt structures
are derived mainly from salt movements. These include structures like listric faulting
and traps against salt walls and diapirs which have been identified in the Mandewa
Basin (Figure 12).
Figure 12: Structural cross section acrosss the Seleous and Mandawa Basins
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5. THE DEVELOPMENT OF PETROLEUM EXPLORATION IN EAST AFRICA.
5.1 Exploration History
5.1.1 The earliest efforts to establish the presence of economic reserves of oil in
East Africa were first undertaken in the Albertine Graben during the 1930’s.
This was a follow up of the discovery of oil seepages in the Graben during the
1920’s. Several shallow stratigraphic wells were drilled adjacent to these
seepages. A deep well was drilled near Butiaba in 1938 by the African
European Investment Company to a depth of over 1,200m. The well revealed
several oil shows and recovered free oil on test but no economic reserves of
oil were established. Subsequent geological surveys carried out in this area
during the 1940’s and 50’s established the presence of sedimentary
sequences of shale and sandstones in outcrop.
5.1.2 In the early 1950’s, British Petroleum (BP) and Shell started exploration
activities in the Lamu Basin and in the Coastal Basins of Zanzibar, Pemba
and Mafia. This work culminated into the drilling of 8 exploration wells in the
Lamu Basin and 4 wells in the Coastal Basins.
None of those wells
encountered hydrocarbons in sufficient quantities to justify further drilling.
5.1.3 Steadily rising oil prices mainly due to the Arab oil embargo of 1973 led to
renewed petroleum exploration activity in East Africa in the early 1970’s.
Texas Pacific took acreage in the Lamu Embayment and drilled the Hagarso
well to a depth of over 3000m. Chevron and Esso also took acreage in the
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Anza Graben during this period where they drilled the Anza and Bahati wells
in the South Anza Basin. Interest intensified in the mid-1970s and up to nine
companies including BP-Shell, Whitestone, Oceanic, Wainoko, Shell
International, Total, Agip, Cities Services and Unacol took acreage in the
Mandera Basin, Anza Graben and Lamu Basin.
Three exploration wells
were drilled offshore in the Lamu Basin and although several indications of
gas were recorded, none were judged to be economic.
In the Coastal Basins, Amoco joined Agip to drill 3 wells onshore and 3 wells
offshore between 1973 and 1982. One of the offshore wells, Songo Songo 1,
resulted in the discovery of the Songo Songo gas field. Another offshore well
also resulted in the discovery of the Mnazi Bay gas field of similar size.
Tanzania Petroleum Development Corporation (TPDC) drilled 8 wells around
the Songo Songo discovery to delineate the field, which was found to hold
one-two trillion cubic feet of gas.
5.1.4 The high oil prices of the early 1980’s led to additional exploration activity in
the region. This included funding of an aeromagnetic survey across the
Albertine Graben by the World Bank. Shell, Esso, IEDC and BHP also took
licenses covering the Ruvu and Selous Basins where they acquired over
6,400 km of seismic data and drilled 5 exploration wells: Ruaruke North-1,
Liwale-1, Lukuliro-1, Makarawe-1 and Kiwangwa-1, none of which
encountered economic reserves of oil.
5.1.5 During 1983, the international oil industry was drawn to the East African Rift
System by the results of the aeromagnetic survey funded by the World Bank
and the results of a seismic survey conducted over the Rukwa and Lake
Tanganyika basins conducted by Duke University, as part of Project PROBE
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scientific study. The study confirmed the presence of thick sequences of
sedimentary rocks in both basins and indicated good structures. These
results together with the existence of an oil seep on Lake Tanganyika led
Amoco to acquire acreage in the Rukwa Basin and drill two wells there,
namely Galula-1 and Ivuna-1. These two wells were also not successful.
5.1.6 Oil prices collapsed in the mid 1980’s and there was little additional
acquisition of acreage by oil companies. Shell and Texaco which were still
holding acreage in the Mandawa, Mafia and the Ruvuma Basins respectively
continued to carry out work albeit at a slower rate. This work led to the drilling
of an additional exploration well in each of these basins. Petro-Canada drilled
an aid funded well on the Dar-es-Salaam Platform and another one on the
Garissa High of the Lamu embayment. Total, Amoco and Walter International
drilled eight wells in the Anza graben during the late 1980’s despite the low
prices of oil. This was mainly to follow up the relationship which had been
established between the Anza Graben and the Rift Basins in Sudan where
substantial quantities of oil had been discovered in the Unity and Heglig
fields. It is anticipated that the Anza Graben and the Abu Gabra Rift Basin
were continuos Rift Basins from the Late Jurassic through much of the
Cretaceous, suggesting that similar sedimentary packages occur in both
basins. None of the wells in the Anza Graben encountered commercial
reserves of oil.
5.1.7 Exploration interest was low during the early 1990’s mainly as a result of the
low international prices of crude oil. These prices began to pick up in the late
1990’s and this has led to renewed interest resulting in several companies
being licensed in the different basins of East Africa.
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5.2 Current status
5.2.1 Licensing
There are over 10 oil companies licensed in East Africa at the moment and carrying
out exploration work. These include Antrim Resources in the Costal Basin of Pemba
and Zanzibar, Ndovu Resources in the North East of the Songo Songo Gas Field,
dana and Afrex Petroleum offshore Lamu, Heritage Oil and Gas Limited in the
Semliki Basin and Hardman Resources together with Energy Africa in the Northern
Lake Albert Basin. Negotiations are also at advanced stages with Petrobras of Brazil
and Shell International for licenses in the Tanzanian deep offshore.
5.2.2 Exploration and development work
Large scale seismic data was recently acquired in the deep offshore basins of the
southern part of the East African coast and the data acquired is being used to attract
companies in the area. Seismic data is being planned in the northern parts of the
deep offshore.
In addition, constitution of the pipeline from Songo Songo to Dar-es-Salaam is being
undertaken as part of the gas field. Plans are underway to drill additional exploration
wells around the Mnazi Bay gas field.
Drilling of an exploration well has just been completed in the offshore part of the
Albertine Graben and plans are underway for a seismic survey to be undertaken
over Lake Albert.
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6. Analogue
The sedimentary basins of East Africa are found in settings, which are anologous to
some of the world's most prolific petroleum provinces.
This means that the
environments and conditions, which lead to the generation and accumulation of oil
and gas in those petroleum provinces, are very similar to those in East Africa.
6.1 Passive Margin Basins.
The basins of the passive margin of East Africa developed in much the same way as
the basins along the passive margin of West Africa. The sequences deposited during
the rifting associated with continental breakup, the salt deposition in some of the
basins and the passive margin development of both sides of the continent have quite
a lot in common. The discovery of some of the world's biggest oil and gas fields in
West Africa over the years is very well known. The basins along the Nigerian,
Gabonese and Congo coastlines have been producing oil for well over 30 years.
Angola, Ivory Coast and others along the same coastline discovered and
commenced oil and gas production about 10 years ago. Most recently Equatorial
Guinea and Mauritania have also joined the list of producing states after discovering
large amounts of oil.
As discussed earlier, the tectonic development of the deep offshore basins of East
Africa together with the size and nature of structures observed in the seismic data
acquired in these basins is a positive indicator towards the petroleum potential of
these basins. It is pertinent to note that discoveries along this coastline to date, i.e.
the gas discoveries in Mozambique, Songo Songo and Mnazi Bay attest to this
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potential. The oil and gas discoveries in Madagascar and the Bombay High could
also serve as a mirror image of the relationship between Brazil on the South
American Continent and West Africa.
6.2 Continental Rift Basins
Petroleum production is well established in rift basins around the world which are
similar to those in East Africa. There are numerous productive analogues of the
types of reservoirs in the East African rifts and these include transgressive deltaic
sandstones such as the Brent (in the North Sea), Karrem (in the Gulf of Suez) and
lacustrine turbidites such as those found in the Abu Gabra (Sudan), Cabinda (West
Africa) and Reconcavo (Brazil) basins.
The Anza Graben is part of the Central African System in which large amounts of oil
and gas are not only being produced, but are still being discovered, in the different
parts of Africa like Sudan, Chad, Niger and Nigeria. The Anza basin is not only
similar in age to the oil producing Muglad Basin of Sudan, the two basins also have
similar trends and are actually in close proximity.
The Tertiary basins of the East Africa Rift Valley have in the past been considered to
be too young for effective petroleum generation and accumulation, but the prolific
production of oil from sedimentary basins of similar age and type is now very well
established world-wide. The Yishu Graben of China, the Cambay basins of western
onshore India, and the Central Summatra basin of Indonesia are all Tertiary Rift
Basins where oil reserves have been established and production is going on. This no
doubt gives a good indication of the petroleum production potential of Tertiary Rift
basins in East Africa.
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8.0 Conclusion
There are over 15 major sedimentary basins in East Africa and these basins fall in
four tectonostratigraphic settings of Passive Margins, Central African Rift System,
East African Rift System and Intracratonic basins.
Sedimentary basins cover almost 30% of the area of East Africa.
They are
distributed across the entire region extending from east to west and from north to
south.
Geological and geophysical surveys together with exploration drilling has been
intermittently undertaken to varying degrees in these basins over the last seventy
years in an effort to search for oil and gas. Although this effort has been minimal
given the size of the areas covered by these basins, it has so far yielded the
discovery of two gas fields of Songo Songo and Mnazi bay in the Ruvuma and Rufiji
basins respectively.
In addition to the discoveries, all the basins of East Africa display good source rocks
conducive for the generation of oil and gas, together with good to excellent reservoir
rocks and structures conducive for the accumulation of oil and gas.
East Africa therefore offers definite opportunities for discovery of oil and gas. Its
sedimentary basins are analogues to those in other parts of the world where large
volumes of oil and gas are already being produced. Renewed exploration effort is
required to realise the petroleum potential of East Africa.
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