North Africa Paleozoic Clastic Plays: General Setting and Geometrical Features
G. Serafini & E.Trincianti
(1)
(1)
Eni Exploration & Production Division
ENI E&P has been active in North Africa exploration activity for decades. The geoscience
knowledge acquired in such a period allows to produce a summary of the most important
clastic plays throughout North Africa.
The North Africa Paleozoic clastic reservoirs range from the Cambrian up to the Middle
Devonian.
The Cambro-Early Ordovician consists of 2 depositional seqences with the same vertical
stacking pattern. It is arranged into an aggradational cycle passing upward into a Fu and Cu
cycles. The overall geometry is quite tabular with thickness variations due to the subsidence
rate variability
L
a
t
e
ORD.
E
a
r
l
y
M
i
d
d
l
e
ORD.
CAM.
Fig. 1 - Cambro-Ordovician stacking pattern and facies
The Late Ordovician glacial sequence is well expressed in the Murzuk Basin (Libya). The
glacial play concept consists of troughs cut in preglacial strata by ice sheets, infilled by
sediment during the ice retreat.
Glacial
Pre-glacial
Fig. 2 - Ordovician glacial valley
The glacial sequence internal architecture complexity and variability implies that to provide
predictive models in the subsurface, extremely good seismic data are needed.
An example of such a sedimentary setting is the El Feel Field in the Murzuq Basin (Libya).
The petroleum system consists of the Ordovician sandstone reservoir of the Mamuniyat Fm.
charged and sealed by the Silurian Lower Tanezzuft Hot Shale. The physical stratigraphic
framework of the Ordovician section was built all over the field by integrating, well cores, well
logs, paly data and seismic evidences and, according to the regional geological context, the
overall sedimentary setting can be ascribed to flood-dominated glacial fluvial systems.
N
S
A
B
C
D E
F
Flattened at Top Hot Shale
Onlap
TMC
Top Basement
Erosional surface
Truncation
Fig. 3 - 3D line through El Feel Field
By integrating palynological analyses with sedimentological and seismic data, the field
geometry and architecture was reconstructed suggesting that Mamuniyat Fm. might be the
infilling of a large scale physiographic element whose edges are not recognisable within the
3D survey boundaries.
The Silurian-Devonian plays are primary exploration targets in the Ghadames basin (TunisiaLibya).
Main Main
Sequence Sedimentary
Sediment
Stratigraphy
Accomodation
Sequence
ary
Surfaces
Stratigrap
Accommo-
GR
0
1 50
+
II°TS
Idealized Vertical
Idealized
Sedimentary
Vertical
Succession
Sedimentary
Main
Main
Lithostratigraphic
Lithostratigraphi
Units
c
TAHARA
TAHARA
II SB
frasnian mfs
II°MFS
Frasnian
MFS
givetian
mfs AOUINET
AOUINETE
OUENINE
ba
s e F3 AOUENNINE I-IV
Givetian
i nte rv al
FS
III°TS
III°SB
ba s e F4
i nte rv al
III°SB
EMGAYET SHALES
SHALE
EMGAYET
II°lTS
III°SB
III°SB
III°SB
II°eTS
OUAN KASA
III°SB
III°SB
III°SB
II° SB
III°SB
III°SB
TADRART
TADRART
OUAN KASA
D
E
V
O
N.
ACACUS
ACACUS
III°SB
S
I
L
U
R.
UPPER TANEZZUFT SDST.
TANNEZZUFT
TANEZZUFT SH.
hot shales
II SB
ORDOVICIAN
250 ft.
ORDOVICIAN Formations
Fig. 4 - Silurian-Devonian stratigraphic scheme
The Silurian Acacus Fm. represents an overall progradational deltaic setting shaling out from
Libya to Tunisia. Its progradational character is well documented both on seismic and in the
wells (CU stacking pattern).
Fig. 5 - Acacus Fm. geometry and stacking pattern
The Devonian Tadrart Fm. is the sedimentary response to the Caledonian orogenesis. It is
characterised by sandy fluvial systems developing tabular and continuous sand sheets all
over the basin.
Fig. 6 - Tadrart Fm. outcrop
Top Tadrart records an overall transgression making the marine facies of the Ouan Kasa Fm.
overlie the fluvial Tadrart. The deltaic mouth bars developing at this time show a more
irregular geometry and a more patchy distribution with respect to what was deposited at
Tadrart time.
Fig. 7 - Ouan Kasa Fm. mouth bar
The Middle Devonian sst. are gas bearing reservoirs in Southern Libya. They are the
response to a minor tectonic activity occurring between the Caledonian and the Hercynian
orogenesis and affecting the structural highest areas. For this reason the sst. distribution is
usually fan shaped with an overall shaling out both to a downcurrent and lateral direction.
Eventually, the Paleozoic sequence is cut off by the Hercynian unconformity whose
morphology accommodates the Triassic fluvial systems.
Aknowledgments
A special thank to ENI E&P management to allow such a publication and to all the North
Africa subsidiaries for all the effort they produce to provide data and information in order to
keep the regional knowledge constantly updated.