The
attraction of the thrust zone in the N1 to N7 blocks was with
analogies to giant sized oilfields in the thrust belt of the
southern Apennines of Italy with ~2 BBO reservoired in
Cretaceous carbonates, and gas fields e.g. Gagliano in northern
Sicily with 700 BCF reservoired in Numidian sandstones.
Oil and gas
fields in fractured carbonates to the south in Tunisia, e.g.
Belli, Sidi El Kilani, and Miskar provide further support for
the area to be prospective. Various source rocks of Lower
Cretaceous to Eocene age source the oil and gas for the fields
in Italy and Tunisia.
Northern
offshore and onshore Tunisia corresponds to a fold thrust belt
linking the North Africa Atlas and the Sicily Apennine chains,
all belonging to the peri-mediterranean orogenic arc that place
during the Cenozoic following the collision between the African
and Europeen plate. This NE/SW trending thrust is made of two
major domains:
-the Numedian-Tellian
Domain which corresponds to a vertical and lateral successive
allochtonous unit of Oligocene-Miocene Numidian flysch and
Cretaceous-Eocene sandstone and carbonates deposits. These
thrusted sheets were mainly displaced during the Langhian-Turonian
compression and folded again during the Late Miocene-Pliocene
shortening. Detachment levels are represented by
either”allochtonous” Triasic salt resulting in different
diapirism phases or Cretaceous-Paleogene shale (Fig.1).
The Northern
Tunisian has subjected during Mesozoic-Tertiary time to two
major tectonic events:
■ the tethyan
rifting and margin growth started during the Middle Triassic
with an NE/SW to N/S opening direction which led to the geneisis
of rifts, tilted blocks, horsts and grabens. These basins were
subsequentely fiolled by later Triassic sequences.
Following the
rifting stage which covering a long period from the late
Jurassic to Paleogene, the area of northern Tunisia was a
passive margin and a thick succession of mainly carbonates and
shales, with pulses of turbiditic sandstone deposition as
encountered in the Lower Cretaceous of the RAJA-1 well
(Fig.2).
■ following the “passive margin” depositional episode,
the area of northern Tunisia started to experience the beginning
of a compressive regime which culminated in the southward
directed thrusts of the Oligo-Miocene Numidian deep water
turbidite deposits with previously deposited Tellian (Mesozoic-Paleogene)
section in the early-middle Miocene, the “Atlassic” event. An
earlier stage of folding was experienced in the late Eocene, the
“Pyrenean” event, which is evidenced by folds that can be
observed in tectonic windows within the Numidian outcrops of
northern Tunisia.
Within the
section of up to 3500 metres of northerly provenanced Numidian,
a possible petroleum system exists with very thick shale
intervals with minor source rock potential, with interbedded
turbidite sandstones and sealing shales.
Bitumen
associated with the Numidian sandstones has been typed to the
Ypresian Bou Dabbous Formation. Copper, lead and zinc
mineralisation is associated with many of the faults, where it
is associated with Triassic evaporates and dolomitic deposits.
The latter probably acted as a major décollement surface during
the early-middle Miocene thrusting episode.
■
Post-thrusting, the deposition of middle to late Miocene shales
and
sandstones is associated with igneous activity of Middle
Miocene age.
Major uplift of
northern Tunisia is evidenced from the varied maturity levels of
onshore “Tellian” source rock outcrops and in the offshore
RAJA-1 well. The age of significant uplift is proposed to be
from Middle Miocene to present and related to the collision of a
microplate (“Kabylie” like) with the stable African craton.
During the uplift associated with the main period of the
thrusting episode, continued collision resulted in the clockwise
rotation of Sicily away from northern Tunisia, leading to
transtensional tectonics and accompanying volcanism.
Faulting with
NW-SE trends associated with this latest stage of tectonic
development is clearly seen from the onshore geology and in the
seismic and potential field’s datasets. It is possible that
offshore seepage is coincident with this recent fault set.
The Cretaceous
section provides the main petroleum system for the area, with
the Abiod / Aleg carbonate reservoirs (Campanian), juxtaposed to
the source intervals of the Moualha (Late Albian) and Bahloul (Turonian),
with good regional sealing shales of the El Haria Formation
(Fig.3).
● The Eocene
provides another good petroleum system, where the Bou Dabbous
fractured carbonate reservoir, is juxtaposed to more mudstone
source rocks rich intervals in the Bou Dabbous, with extensive
regional sealing shales of the Souar Formation.
● The Numidian
represents a promising target and is proven play in Sicily. The
source rocks correspond to the shaly matrix of the sand bars,
which represent the reservoir levels. Sealing is controlled by
the cretaceous and Eocene allochtonous units, the
intraformational shales and the Late Miocene-Pliocenes sequences.
The
expulsion-migration time period of these source rocks is Miocene-Pliocene,
and considering the large associated structure and closures.
The main oil
seepages that have been reported are at locations south of the
main thrust front, and will be briefly discussed.
Live oil seeps
at Kef Bou Dabbous, located 15 km west of Testour, and at Djebel
Ech Cheid, located 20km SSW of Testour have been typed to the
Mouelha Formation. Seepages closer to the Numidian “thrust”
front are present at Djebel Bou Goutrane and Kasseb Dam, located
15 km west of Beja and have been typed to the Bou Dabbous
Formation.
In the
offshore area evidence of seepage is indicated from Infoterra’s
satellite images, and have been interpreted as located along a
late extensional fracture trend related to transtensional
movements. Repeatability of the “seeps” has not been verified.
GEOGRAPHICAL
SETTING