X018

Middle to Late Ordovician Play Characterization

SUMMARY
Previously, the chance of significant reservoir in the Bir Ben Tartar/Jeffara formations is fairly low, unless
naturally fractured as demonstrated by Ordovician quartzitic sandstone on the El Franig oilfield which lies
north of the Telmzane Arch in Southern Tunisia. Recently, several wells tested gas and condensate in the
vicinity of Sud Remada Permit (Dorra-1, CEM-1 and SEA- 1 wells). Recent wells (TT2 and TT3) drilled
on Sud Remada Permit have found Bir Ben Tartar/Jeffara reservoirs with porosity values up to 25% and
permeability averaging 10mD with 44°API oil flow and development of the Bir Ben Tartar oilfield is
underway.

74th EAGE Conference & Exhibition incorporating SPE EUROPEC 2012

The Sud Remada Permit is situated in the Southeastern part of Tunisia, along Libyan-Tunisian border
and lies to the northern edge of Ghadames Basin (Fig.1) which is a large intracratonic basin extending
west to Algeria and East to Libya. The basin was affected by the Precambrian, Late Ordovician
Taconic, Late Silurian Caledonian, Permian Hercynian, Albian Austrian and Eocene to Late Miocene
Alpine Orogenies. The timing of tectonic events affecting this basin corresponds closely with
the major events in the fragmentation of Gondwanaland and Pangaea and the relative
movements of the African, Laurentian and Eurasian plates, and are summarised by (Craig J.,
et al, 2008).
• The Taconic Orogeny started at Middle Ordovician and is marked at regional scale by a
significant unconformity near the top of the Ordovician rocks.
• The Caledonian Orogeny started during the Middle Silurian and extended to the Early Devonian.
The Caledonian movements may have caused the development of the NW-SE trending faults in
the Sud Remada area.
• The Hercynian Orogeny started as early as the Middle to Late Carboniferous and it persisted
through to the Early Permian.

Hercynian movements created east-west trending arches such as the Nefusa, Jeffara and Telmzane in
NW Libya and SE Tunisia. Within Sud Remada Permit, the Triassic deposits rest unconformably on
Lower Silurian Tannezuft Formation and Upper Silurian to the north and on Upper Silurian Acacus
Formation to the south.

The Middle Ordovician Bir BenTartar and Late Ordovician Jeffara Formations constitute the main
reservoirs in the Sud Remada area. Mid-Upper Ordovician sandstone intervals are well documented as
prolific reservoirs in North Africa (Illizi Basin in Algeria; Ghadames & Murzuk basins in Libya). In
Ghadames basin, on the Tunisian side, they constitute an emerging play as indicated recently by the
successful results of several wells targeting the Ordovician.

Previously, the chance of significant reservoir in the Bir Ben Tartar/Jeffara formations is fairly low,
unless naturally fractured as demonstrated by Ordovician quartzitic sandstone on the El Franig oilfield
which lies north of the Telmzane Arch in Southern Tunisia. Recently, several wells tested gas and
condensate in the vicinity of Sud Remada Permit (Dorra-1, CEM-1 and SEA- 1 wells). Recent wells
(TT2 and TT3) drilled on Sud Remada Permit have found Bir Ben Tartar/Jeffara reservoirs with
porosity values up to 25% and permeability averaging 10mD with 44°API oil flow and development
of the Bir Ben Tartar oilfield is underway.

Source Rock Characterization

Deglacial, Early Silurian black shale represents the most important Palaeozoic source rock across the
North Africa (Le Heron & al, 2009). Because of the Hercynian unconformity, the Upper Devonian hot
shales are absent over Sud Remada Permit, and the Lower Silurian shales are partly removed in the
Northern part of Sud Remada Permit and especially over Ordovician paleohighs. So, the basal
Silurian Hot Shales and the Lower Silurian Tannezuft shales remain the main sources of hydrocarbon
in Sud Remada Permit.

The basal Silurian “Hot Shales” are 20 to 40m thick, and were deposited in a very low-energy, anoxic
environment and are organic-rich and highly radioactive.
At the Bir Ben Tartar Field, geochemical analyses of Tannezuft Formation indicate very good source
rock characteristics with TOC values ranging from 3.5 to 21%, S2 reaching 46.54mg of HC/g of rock
and HI values up to 270mg of HC/g of TOC. These values indicate an oil/gas prone type II kerogen.
Tmax values range from 431 to 444°C suggesting an early stage of oil maturity. Consistent with the
high TOC values and relatively low thermal maturity of the samples, a high petroleum potential (S2)
74th EAGE Conference & Exhibition incorporating SPE EUROPEC 2012
Copenhagen, Denmark, 4-7 June 2012
is observed in most of the samples of the Silurian “Hot shales” which rates them as good to very good
petroleum source rock (Vecoli et al 2009).

The maturation of the Silurian source rock is primarily a function of the amount of the erosion related
to Hercynian Orogeny. Maturity level of Silurian shale decreases from south to north. The Silurian
shales maturity ranges from an early mature stage at the Bir Ben Tartar Field to a dry gas stage to the
South. In fact, it seems that the Tannezuft formation maturity level is lower over resistant Ordovician
paleohighs such as Bir Ben Tartar structure than in the surrounding areas where the Tannezuft
formation might be thicker, more subsiding and as a result more mature.
At the end of the Carboniferous, the oil migrated out of the Silurian hot shale both upward toward the
Late Silurian sandstone reservoir and downward toward the Ordovician sandstone reservoirs. The oil
migrated updip from the southwest to the northeast along carrier beds and filled the Bir Ben Tartar
structure created during the pre Carboniferous movements.

Middle to Late Ordovician Reservoir characterization

The Middle to Late Ordovician succession is made of several lithofacies and parasequences including
a variety of prograding lower-middle shoreface and glacio-marine deposits. The major drop of sea
level is recorded at the end of the Lower Caradocian as a response to the Taconic tectonic event.

- Stratigraphic framework

Palynological investigations indicate that the Late Ordovician Jeffara and Middle Ordovician Bir Ben
Tartar Formations (at Bir Ben Tartar Field) are separated by a major stratigraphic gap covering
approximately 20 MA (Fig.2). This correlates well with the Taconic unconformity to the south in
Libya and Algeria.
The lower unit of Bir Ben Tartar reservoir consists of thick beds of micaceous-rich sandstones with
subordinate siltstone and shale, while the upper unit is composed of heterolithic facies comprising
mudstone, bioturbated siltstone interbedded with sandstones and oolitic facies. The Jeffara Formation
rests unconformably on the Taconic Unconformity and is represented by two distinct units. The first
unit is composed of glaciogenic related deposits while the second is mainly composed of micaceous
sandstones with thin interbedded shales and siltstones (Fig.2).

- Facies and depositional environments

The sandstones of Bir Ben Tartar lower unit and the Jeffara sandstones both display hummocky cross
stratifications, planar parallel laminations and climbing ripples and were deposited in inner shelf
setting above the fair weather wave base. They are interpreted as laterally extending prograding shelf
bars. The increase of the clay content and the abundance of bioturbation within the siltstone and
mudstone deposits of upper unit of Bir Ben Tartar may suggest a short-lived sea-level rises and the
evolution of the inner shelf setting to more distal setting (lower shore face to offshore transition). The
sandstone layers encased within this dominantly silty to muddy unit are thought to be deposited
during storm events.
The Jeffara Formation comprised of conglomeratic horizons and diamictites are considered to be
glacial related deposits. The diamictites are thought to have been deposited during a glacial retreat
period.

- Diagenesis and Reservoir Quality

Detailed petrographic analyses revealed that whilst the primary control on the distribution of reservoir
quality is the primary sedimentary fabric and architecture there is a complex diagenetic overprint that
strongly influenced porosity and permeability preservation, both loss or enhancement.
The main diagenetic processes that modified the sandstone primary textures and mineralogy, took
place during near-surface eodiagenesis, mesodiagenesis and late diagenesis.
• Quartz overgrowths are common within the clean sandstones lacking early chlorite coatings.
74th EAGE Conference & Exhibition incorporating SPE EUROPEC 2012
Copenhagen, Denmark, 4-7 June 2012
• Dissolution of siderite and the enhancement of the reservoirs properties
• Fracturing of well cemented quartz arenites of the upper Bir Ben Tartar unit, particularly those
situated close to the Taconic unconformity.
• The coatings occur as continuous to discontinuous layers around the quartz grains which inhibited
late quartz overgrowth and compaction through burial.
The lower unit of Bir Ben Tartar (BBT-A) and the Jeffara sandstones, which are interpreted as shore
face prograding facies contain moderate to high clay content (up to 25 weight %) with micas. Porosity
values range between 8 and 11% and the permeability is less than 1mD. Consequently these
sandstones display fair reservoir quality due the low permeability values. The increase in the porosity
values, up to 26%, observed within upper Bir Ben Tartar unit (BBT-B) is associated with chlorite as
well as meta-stable carbonate cements (siderite) that when dissolved created oversized secondary
porosity.
Combination of core-plug petrophysical data with petrography allowed us to define six key rock types
within Middle to Late Ordovician reservoirs. Among the 6 rock types defined within the Ordovician
of TT2 and TT3 wells, those have significant petrophysical properties are:
• The Rock Type-1 which consists of quartz arenite with chlorite coatings presents high quality
reservoir rocks (15.7 to 26 %- average = 20.3%). It is mainly present within the Upper Bir Ben
Tartar unit.
• The Rock Type-3 consists of siderite cemented quartz arenite having fair to good reservoir quality
where siderite cement was dissolved.
• The Rock type-6 is represented by the chamositic and sideritic oolithic facies which when leached
developed secondary vuggy pore systems.

Conclusions

Recent drilling at Bir Ben Tartar Field has demonstrated recoverable oil 44oAPI from Mid-Late
Ordovician Bir Ben Tartar and Jeffara sandstone reservoirs.

Porosity and permeability has been developed within these reservoirs because of the presence of clay
coatings on the quartz grains inhibiting quartz cementation and the dissolution of siderite cements due
to the meteoric telodiagenesis occurring during the last Ordovician uplift (Taconic unconformity).

Figure 1 Sud Remada Permit Geological location map (After F.J. Ghautier & al, 2003)
74th EAGE Conference & Exhibition incorporating SPE EUROPEC 2012
Copenhagen, Denmark, 4-7 June 2012
Figure 2 Stratigraphy of the Middle to Late Ordovician succession in TT-2 well

References

Craig J. & al. (2008): Structural Styles and prosectivity in the Precambrian and Palaeozoic
Hydrocarbon Systems of North Africa. In: Salem, M.J. (Ed.). The Geology of East Libya 4,
51–122. (Gutenberg Press, Malta).
F.J. Gauthier & al (Anadarko-Algeria), 2003, The structural & tectonic evolution of the Berkine-
Ghadames basin (1st North Africa/Mediterranean Petroleum Geosience and Exhibition (Tunis
– 2003)
Le Heron D.P, & al, (2009): Ancient glaciations and hydrocarbon accumulations in North Africa and
the Middle East. Earth Science Reviews 93, 47-76
Vecoli M., Riboulleau A. & Versteegh G.J.M., (2009): Palynology, organic geochemistry and carbon
isotope analysis of a latest Ordovician through Silurian clastic section from borehole TT-1,
Ghadames Basin, southern Tunisia, North Africa palaeoenvironmental interpretation.
Palaeogeog. Palaeoclim. Palaeoecol. v.273, p.378-394.

74th EAGE Conference & Exhibition incorporating SPE EUROPEC 2012