4. Geology of the Albertine Graben

GEOLOGY  OF THE THE ALBER  ALBERTINE TINE GRABEN

The three (3) Domains of  of the the Alb  Albert ertine ine Graben depicts varying trends: The Central  Central Domain Domain is characterized  by  Orthogonal  extension while the Northern and  Southern Domains underwent Oblique underwent  Oblique Extensions. The sub-basins ar aree bounded  bounded by  by steep steep border  faults  faults that  that are are characterized  characterized by  by a a series of  of en en echelon linked  faults  faults with alternating alternating pol  polari arity  ty  formin  forming g accommodation accommodation zon  zones. es.

STRUCTURAL SETTING

T

he Albertine Graben forms the Northern-most extension of the Western arm of the East African Rift System (EARS). The Graben stretches from the border between Uganda and Sudan in the north to Lake Edward in the south, a total distance of over 500km with a variable width of 45 km (Figure 6).

therefore have evolved in a strike slip dominated regime similar to the other basins developed in the Central African Rift System (Figure 6).

Structurally, the Albertine Graben can be Structurally, divided into three domains: the northern domain that trends in a NNESSW direction and encompasses the Lake Turkana

RED SEA

AFRICA  S Z  CA

The Albertine Graben

A  s   w  a   S    h  e   a  r   Z    o  n   e  

Lake Albert

The Graben is a Cenozoic rift basin formed and developed on the Precambrian orogenic belts of the African Craton. Rifting was initiated during the late Oligocene/Early Miocene. The Graben has undergone several tectonic episodes of both extensional and compressional regimes. Evidence of  stress regimes oblique and perpendicular to the boundary faults is seen through the geometry and orientation of the fault systems defining the basins in the Graben.

The Gregory Rift

UGANDA Lake Victiria                      H                     C                       N           A                      R                      B                      N                      R                      E                      T           S           A                      E

RWANDA DEMOCRATIC REPUBLIC OF CONGO

BURUNDI

W       E       S       Lake Tanganyika T       E       R       N       B       R       A       N       C       H      

N

KENYA

TANZANIA

INDIAN OCEAN

Lake Rukwa

250 km

0

LEGEND

ZAMBIA

Volcanics RiftSediments NormalFaults

The Albertine Graben terminates at a known sinistral strike slip shear zone, the Aswa shear zone. Other shear zones can be interpreted in the south of the Graben, around Kivu and Rukwa areas in the Tanzania part of the East African Rift. The Albertine Graben could

Cenozoic Strike slip Faults Location ofthe  Albertine Graben aben EastAfrican rift

MALAWI

L                     a                k                    e              M                      a                 l a                          w                i                   

MOZAMBIQUE

Survived Mesozoic Basin Mesozoic Strike Slip Fault

CASZ

Central African Shear Zone

ZIMBABWE

Figure 6: The Albertine Graben and the East African African Rift System 6

PETROLEUM EXPLORATION AND PRODUCTION DEPARTMENT

GEOLOGY OF THE ALBERTINE GRABEN

M

BUTIABA WAKI

FT

PLAYS, PRODUCTION AND SHOWS

WELL TD 4058' BASEMENT

 __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __ 

OIL AND POSSIBLE GAS

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100

       Y        R        A        N        R        E        T        A        U        Q

                     E                      N                      E                      C 200                      O                      T   S                     I

                     E                      L                      P 300                      Y                      L                      R                      A                      E

0

500

0

0

0

O

O

0

0

O

0

O

O

0

0

O

O

O

O

O

0

O

0

0

O

0

O

0

0

0

O

0

O

 __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __ 

 __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __ 

0

O

1500

0

0

0

0

0

O

0

   N LACUSTRINE SANDS    O    I    T AND    A    M CONGLOMERATES    R    O    F

THE SURFACE. THE OIL SEEPS MAY BE OF TWO KINDS. -THE SUDANESE HIGH WAX, HIGH POUR POINT. SOURCED FROM

   O    S    I

400 0

SEEPS ARE KNOWN ON

0

O

0

0 O

0

O

0

0 O

O

0

0

O

0 0

0

O

0 O

1000

O

0

O

0

O

0

0

0

O

O

0

O

0

0

0 0

O

0

 __ __ __ __ __ __ __ __ __ __ 

LACUSTRINE SHALES. - A MORE PARAFFINIC

   A    K

SEEP? SOURCED FROM MORE MARINE SHALES.

 __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __ 

500

1720'

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 __ __ __ __ __ __ __ __ __ __ 

600

2000

FLUVIO LACUSTRINE

 __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __ 

   E    N    E                   Y    C                   R    O    I 700                            AI    L    P 2500          T    O                   R    T                   E    E 800                   T    N    E    C    O    I 900    M 3000    R    E    W 1000    O 0    L

   N SANDS AND SHALES    O    I    TWITH SOME BITUMINOUS    A    M SHALES    R    O    F    I BITUMINOUS SHALE    G    E    S    I    K

 __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __ 

 __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __ 

 __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __ 

 __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __ 

O

O

0 O 0

O

0

0

0

O

0

O 0

O

0

OO

0

0

0

O

       C        I        S        S        A        R        U        J

1100

3500

 __ __ __ __ __ __ __ __ __ __ 

 __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __ 

1200

 __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __   __ __ __ __ __ __ __ __ __ __ 

4000

0

0

0

OO

0

O

0

O

O

3320'

       N BITUMINOUS SHALE        O        I        TFLUVIO LACUSTRINE CLASTICS       I       APOSSIBLY EQUIVALENT TO UPPER        M

 __ __ __ __ __ __ __ __ __ __ 

0

O

IN THE WELL AT 3850' A SAND

WITH ASPHALTIC OIL WAS FOUND, NO TEST.

      K        RJURASSIC STANLEYVILLE FORMATION       A        OOF THE CUVETTE CONGOLAISE.        F       W

0

4008'

BASEMENT

Figure 7: A Section through the Waki-1B well demonstrating the stratigraphy of the Albertine Graben

Rhino Camp and Pakwach basins, the central domain in a NE-SW direction where the Lake Albert (ButiabaWanseko and Kaiso-Tonya areas) and Semliki basins are located and the southern domain in a NNE-SSW direction where the Lakes EdwardGeorge basin is found (Annex-3). The Graben trends in a NE-SW direction through most of its length, probably following the pre-existing basement fabric.

STRATIGRAPHY

T

he stratigraphy of the Albertine Graben dominantly comprises of  fluvial-deltaic and lacustrine deposits. The stratigraphy of the area has been greatly influenced by tectonism coupled with climate and Lake level fluctuations.

A number of workers including Wayland (1925), Bishop (1965) and Pickford (1993) have given varying descriptions of the stratigraphy. Studies by PEPD and the oil companies operating in the country are currently underway in an attempt to harmonise these descriptions and produce a unified stratigraphy for the Graben. Generally, the oldest units, the Proterozoic is well exposed on the rift flanks and shoulders of the Albertine Graben. It is composed predominantly of high-grade metamorphosed and igneous rocks of Pre-Cambrian age. Seismic data points to a Pre-Cenozoic rift sedimentary section in some parts of the Graben, mainly restricted to the Lake Albert depocentres. The syn-rift section of the Albertine Graben was first recorded in the WakiPETROLEUM EXPLORATION AND PRODUCTION DEPARTMENT

1B well, which was drilled in 1938. A section of this well is shown in Figure 7. This well penetrated both the Mid and Upper Cenozoic syn-rift sections which are characterized by conglomerates and sandstones. The well went through the Mesozoic prerift section before reaching basement. The Kisegi and Kaiso formations of the Cenozoic are also quite well exposed in the Graben, consisting mainly of  intercalation of conglomerates and sandstones deposited in fluvial and shallow lacustrine environments. The Kisegi Formation in some cases overlies the meta-quartzite Basement. Although the Lower Cenozoic may overlie the crystalline basement in many parts of the Graben, there is a possibility that it overlies a Mesozoic section (Karoo) in some parts of the Graben. 7