NPA is currently carrying out a regional
structural interpretation of the basins along the Central
Africa Rift System. Using the NPA archive of Geocover digital
mosaics (30m Landsat TM tiles covering approximately 60 by
40) we are producing modern structural interpretations and
models and integrating gravity and seismic data where available.
The NW-SE trending Muglad basin is the largest of the Central
African rift basins. It extends across an area of 120,000 km2
and is up to 200 km wide and over 800 km long. Locally the
basin contains up to 13 km of Cretaceous to Tertiary sediments.
The two large hydrocarbon accumulations, the Unity and Heglig
fields, have a combined recoverable reserves of 250-300 million
barrels of oil. Several smaller fields have also been discovered.
The
interpretation has taken place digitally on-screen in a Geographic
Information System (GIS) package A GIS viewing package
is supplied to the client enabling them to view the imagery
and the interpretation interactively.
Regional Tectono-Stratigraphy
Three major episodes of extensional tectonism are recognized
in the Sudanese Basins:
1. Early Cretaceous.
2. Late Cretaceous.
3. Paleogene.
Each boundary is locally expressed by an angular unconformity.
Commercial hydrocarbons are sourced from the Lower Cretaceous
and are reservoired in Upper Cretaceous sandstones. Traps are
dominantly structural and are associated with extensional roll-over
anticlines (see above).
Regional Bouguer gravity has been integrated into the study.
Gravity gradients over the Muglad basin outline the main basinal
lows and basement highs and show the regional basin architecture.
Note the Heglig - Toma South - Unity Field ridge alignment,
the latter two on the margins of the gravity high. Similarly,
El Toor and Munga to the east are also seen to be aligned along
the margins of a NW-SE trending gravity high.The NPA gravity
interpretation shown above maps the switch in the polarity
of the main listric faults in the basin and the location of
the intervening accommodation zones.
Building
upon the regional image interpretation NPA have carried out
number licence studies in the Muglad basin. The interpretations
are at 1:100,000 scale and map the main controlling listric
faults, transfer faults and structural highs that may be associated
with roll-over anticlines.
The Central Horst is a major topographic high through the
block. The horst divides the block structurally, with the faults
on the west trending NW-SE and those to the east trending NNW-SSE
(see below). Gravity and geo-seismic data show that there is
a correlation of data-sets this horst ties-in with the image
interpretation.
Geo-seismic section through the southern
Muglad basin
Crestal collapse graben (white squares) in the hanging-wall
of the major (primary) listric extensional faults. This is
the type of structure upon which Unity field has been located
(see above) and may provide an analogue for other targets in
the Muglad basin.
Unity Field
Unity Field is the largest in Sudan
with estimated recoverable reserves of 150 M barrels of oil.
Imagery & Interpretation
To the right is the Landsat ETM image (15m resolution) of
Unity Field and the 1:100,000 scale interpretation overlay.
The NW-SE
trending extensional faults are clear to see on the imagery.
Additional fault trends mapped in the area are ENE-WSW and
NE-SW, these trends may reflect transfer faulting. Bottom right
is the lithostratigraphic correlation between Kaikang-1 and
Unity-2. Productive reservoirs in Unity are the
upper Bentiu Sandstone and sands in the lower Darfur Group.
The geo-seismic cross-section across Unity shows the listric
nature of the primary detachment faults. The ramp/flat nature
of the basement has led to the development of a series of
roll-over anticlines with superimposed crestal collapse graben.
The
Unity wells seem to have been drilled upon such a structure.
Building
upon the regional image interpretation NPA have carried out
number licence studies in the Muglad basin. The interpretations
are at 1:100,000 scale and map the main controlling listric
faults, transfer faults and structural highs that may be associated
with roll-over anticlines.
