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Geological Modeling of the Offshore
Orange Basin, West Coast of South Africa
by: Curnell Campher
1 2
2 3
Supervisors: D. Van der Spuy, G. Kulmann, R.di Primio, and R. Domoney
1
1
2
3
Petroleum Agency, PO Box 1174, Parow 7499, South Africa.
GeoForschungs Zentrum, Telegrafenberg, 14473 Potsdam, Germany.
University of the Western Cape Private Bag X17, Bellville 7535, South Africa.
Introduction
The Orange Basin is the largest and youngest of the offshore the potential multi-trillion cubic feet of natural gas reserves. 10 for basin modeling. The aim was to reconstruct the
South African basins (Figure 1A) and covers an area of roughly This poster presents results and conclusions of the basin geological history including the basin formation, heat-flow
130 000 square kilometres relevant to the 200m isobaths modeling conducted in the study area located within the South regimes over time, sequence stratigraphy, depositional
(Gerrard and Smith, 1982). The basin is classified as under- African exploration blocks 3A/4A and 3B/4B (Figure 1B) regimes and the subsequent maturity evolution of two
explored with one well drilled for every 4000 square kilometres. covering an area of roughly 97 kilometres by 150 kilometres. identified post-rift source rocks in the Orange Basin namely,
Despite this, the basin has confirmed hydrocarbon reserves in The methods followed included the use of Petrel for seismic the older Barremian to Early Aptian, and the younger
the form of large gas fields (Ibhubesi and Kudu gas fields) with interpretation and well correlation and PetroMod IES version Cenomanian to Turonian source rock.
K-H1
K-F1
A-l1
A-L1
A-A1
N
Block 3B/4B
Block 3A/4A
X'
X
N
Figure 1 (A): Location of the Orange Basin with exploration license blocks 3A/4A and 3B/4B
Figure 1 (B): Location of the Study Area with well locations in exploration
license blocks 3A/4A and 3B/4B
Rifting
Km
0
5
10
SW
X
C
(Jungslager 1999)
4
Tert iary
Ca m -
Maas
Seismic gas chimneys
and seeps
3
K/T
Santonian - Coniacian
F l
u
F l
u
80 Ma
Flu
Turonian
Cen
m - u Alb
67 Ma
u A
pt - l Alb
13At1
+
+
93
Ma
103 Ma
3
+
22At1
+
+
+
Medial
Hinge
17At1
+
1
+
4
4
2
+
15At1
14At1
+
+
?
+
+
6At 1
1At1
1
+
2
+
+
+
+
+
+
+
1
+
2
+
+
+
+
Gas
seep
+
NE
+
Margin
Hinge
Hauterivian
RIFTED
CONTINENTAL CRUST
T
A-J
Graben
X'
Time
Drifting
• Onset of full drift signified by
13At1 unconformity
(Brown et al 1995)
• Sequence described as thick
Sedimentary wedge
• Contain large growth, slump
Structures and associated toe
Thrusts along shelf edge
Early Drift
• Between Mid Aptian- Early
Aptian (Brown et al 1995)
• Corresponding to
unconformities 6At1 and 13At1
• Classified as
intermediate succession.
• Barremian-Early Aptian
source.
• Rifting was initiated along
the west coast during the Late
Jurassic (Muntingh 1993)
• Divergent passive margin
with underlying grabens and
half grabens.
• Rifting continued until the
Late Hauterivian marked by
the 6At1 unconformity
(Muntingh 1993)
Figure 1C: Generalized cross section through the Orange Basin indicating basin evolution from rifting,
early drifting to complete drifting.
• Source rock association with
Global Cenomanian - Turonian
Anoxic event.
Basin evolution
The Orange Basin sediments can be categorised into three phase and are later influenced by eustatic sea level change, sedimentation barrier (Figure 2A). The onset of full drift
units, namely the syn-rift, early drift and complete drift tectonics and sediment influx. The thickness map of the 6A resulted in a northward shift of the depocentre (as indicated
sediments, corresponding to the active rifting, onset of drifting sequence indicates depocentres towards the middle of the on the thickness map of 13A (Figure 2A)) as the affect of
and complete drifting phases of the Orange Basin's evolution study area trending sub-parallel with the coast (Figure 2A). basement architecture diminished. The 22A thickness map
(Figure 1C, 2B). Sediment thickness maps indicate the These depocentres can be attributed to basement grabens and shows a thin interval on the shelf due to Late Tertiary erosion.
development of depocentres influenced by areas of adequate half grabens which also trend sub-parallel to the west coast of This contributed to the development of a depocentre in the
accommodation space. These depocentres are commonly South Africa. The 6A sequence thins westward due to the distal part of the basin as eroded sediment was redeposited
developed within basement grabens during the initial rifting Agulhas Columbine Arch (basement high) providing a basinward (Figure 2A).
erosion
Two erosional events have been incorporated in the basin illustrated as two erosional maps 17At1 and 18At1 indicating over a much larger area as indicated on the 22At1 erosional
model. The first and more intense of these occur during the the aerial extent and intensity of the erosion (Figure 2C). The map (Figure 2C).
Early Paleogene eroding an estimated thickness of 960 meters second, less intense erosional event of the Early Miocene,
down to the 17A sequence (Figure 2C). This erosional event is eroded an estimated thickness of 420 meters of sediments
Petroleum Agency SA, PO Box 1174, Parow 7499, South Africa
0345