Petroleum Systems and Prospectivity of the Gulf of Hammamet, Tunisia

Craig, A. N. Cooper Energy, Perth, AUSTRALIA

Introduction

The Gulf of Hammamet is located south-east of Cap Bon, offshore north-eastern Tunisia (Figure 1). The gulf generally consists of a shallow shelfal area, with water depths less than 200m, which extends to the east into the Pantelleria Trough where water depths exceed 400m.

The petroleum geology of the Gulf of Hammamet is poorly understood due to the lack of a regionally consistent evaluation. Previous authors have focussed on a localised area or a select subset of the stratigraphy. This paper will attempt to present a consistent structural and stratigraphic evaluation of the Gulf of Hammamet.

Figure 1: Gulf of Hammamet Location Diagram.

Hydrocarbon Occurrences

A number of oil and gas fields have been discovered within the Gulf of Hammamet (Figure 1 and Table 1) most of which are reservoired in the Middle Miocene Birsa formation sands. However, several lesser well known accumulations exist within the gulf and are reservoired in reservoirs other than the Birsa Formation. Examples of such accumulations include the Halk El Menzel oil field which is reservoired in the Ketatna Formation limestones, Lotus gas field (97% CO2) in the Cretaceous Serdj Formation, Baraka oil and gas field which is reservoired in the Middle to Upper Miocene Saouaf Formation sands and the Dougga and Tazerka accumulations in the limestones of the Abiod Formation.

Accumulation Discovery Date Reservoir Flows/Tests Birsa 1976 (Shell) Birsa 480bopd Yasmin 1976 (Buttes) Birsa

Bou Dabbous Abiod

1,900bopd Recovered oil & gas on DST Flowed minor oil on DST

Halk el Menzel 1977 (Elf) Ketatna 600bopd Oudna 1978 (Shell) Birsa *production commenced Nov 2006

at approx 20,000bopd from 1 producer / injector pair.

Tazerka 1979 (Shell) Birsa Abiod

5,700bopd 12.5 MMscfd

Neopolis 1979 (Buttes) Bou Dabbous 288bopd Cosmos & South Cosmos

1981 (BP) Birsa 2,100bopd

Dougga 1981 (Shell) Birsa Abiod

*Gas column in Birsa not tested <2 MMscfd, 80 bcpd

Lotus 1985 (Elf) Serdj 7.8MMscfd Sbeitla 1985 (Shell) Birsa 400bopd Maamoura 1987 (Eni) Oum Dhouil /

Birsa Ain Grab Abiod

18MMscfd 3,050bopd Oil recovered on DST

Zibbibo 1988 (Eni) Birsa 550bopd Zelfa 1994 (Samedan) Saouaf 4,820bopd & 7.4 MMscfd Baraka 1997 (Eni) Saouaf 4,340bopd & 4.5MMscfd

Table 1: Hydrocarbon Occurrences in the Gulf of Hammamet

Petroleum Systems

The dominant petroleum system in the gulf is the Albian-Turonian petroleum system of the Bahloul, Mouelha and Fahdene source rocks. It can be seen from Figure 3 that the critical moment of the Gulf of Hammamet petroleum systems is the late Miocene to early Pliocene where peak hydrocarbon migration has occurred. Trap retention since this time appears not to be an issue in the Gulf of Hammamet.

Source Rocks

Cenomanian-Turonian source rocks are well recognised and documented regionally in north and west Africa by Luning et al (2004) and Macgregor (1996). Within Tunisia the Cenomanian-Turonian Bahloul Formation has been documented most recently by Zagrarni et al (2008).

The Albian – Turonian has been traditionally referred to as the Fahdene Formation. Marls and shales of the Fahdene Formation being laterally equivalent to the Zebbag Formation in the south and reefal carbonates of Bordj Cedria to the west on Cap Bon. It is evident then that significant temporal and spatial variations exist in response to regional and localised tectonics.

It is postulated that within the Gulf of Hammamet that the Bahloul and Mouelha Formations are deposited within depocentres formed by responses to the Albian-Aptian tectonic episode. With the Mouelha Formation representing the transitional transgressive facies to the peak Cenomanian-Turonian anoxia which is represented by the Bahloul Formation.

Reservoirs

Neogene

Most hydrocarbon accumulations within the Gulf of Hammamet are reservoired within the Mid-Miocene Birsa Formation sandstones, such as at Dougga, Tazerka, Oudna, Birsa, Cosmos and Yasmin fields. These sands are described as varying from being shoreface to marine and typically exhibit excellent reservoir quality (Figure 4).

The stratigraphic nomenclature of the Langhian - Messinian stratigraphy in north-eastern Tunisia is confusing. As discussed by Ben Ferjani et al (1990), historically the Ain Grab and Mahmoud Formation were assigned the Cap Bon Group name (Biely et al, 1972 and Robinson & Wiman, 1976). As proposed by Ben Ferjani et al (1990) it is better to ascribe these formations, along with the overlying Birsa, Saouaf and Beglia Formations the unit nomenclature of the Oum Douil Group (Figure 2).

Figure 2: Gulf of Hammamet Lithostratigraphy and Petroleum Systems.

Figure 3: Gulf of Hammamet Petroleum Petroleum System Event Diagram (modified after Klett, 2001).

Figure 5 illustrates a sequence stratigraphic model of the Langhian - Messinian Oum Douil Group within the Gulf of Hammamet. This model is proposed based on seismic stratigraphic interpretations. It can be observed that the individual formations of the Oum Douil Group are genetically related, beginning with the transgressive systems tract (TST) of the Ain Grab Formation, the regional flood and high stand systems tract (HST) of the Mahmoud Formation. The next genetic unit consists of the low stand systems tracts (LST) of the Birsa Formation. This in turn is overlain by the progradational HST of the Saouaf and Beglia Formations.

The transgressive Ain Grab Formation is regionally present within the Gulf of Hammamet. It is deposited on an unconformable surface, typically overlying the Oligocene. The Ain Grab Formation is best developed on structural highs where consequently it is also exhibits better reservoir qualities, such as at Tazerka and at Maamoura where it flowed 3,500bopd on test.

Birsa Formation

The Birsa Formation is the dominant reservoir within the Gulf of Hammamet, with the Birsa, Tazerka, Oudna, Yasmin and Cosmos oilfields being reservoired within these mid-Miocene aged sands.

Portolano et al (2000) defines these sands from the Birsa field as being Serravalian in age and deposited within a shoreface to lower shoreface environment. The Birsa Formation being represented by upper and middle sand units, and a lower sand unit separated by an intra Birsa carbonate unit (Portolano et al, 2000). This stacking pattern is again repeated in the Tazerka field to the north of the Birsa field, and largely similarly throughout the Gulf of Hammamet.

0

500

1000

1500

2000

2500

3000

3500

4000

0 5 10 15 20 25 30 35

Dep

th (m

BM

L)

Porosity (%)

Birsa

Abiod

Nara

Beglia

Ain Grabmin mode

max

Figure 4: Porosity – Depth plot for the Gulf of Hammamet (Note: depths mBML refers to below mudline, or seabed).

Figure 5: Sequence stratigraphic model for the Oum Douil Group.

This shoreface - lower shoreface depositional environment ascribed to the Birsa is at odds with Ben Ferjani et al (1990) which ascribes a turbiditic origin owing to abundant planktonic fauna. Recent work suggest that the Lower Birsa is indeed turbiditic, a widespread lowstand event, and that the Upper Birsa is a progradational system. This is based on observations made on modern and reprocessed seismic where the Lower Birsa is seen to thin onto structure (Figure 6) and that the Upper Birsa is less affected by structure and appears to have a westerly depositional direction (Figure 7).

Beglia and Saouaf Formations

These formations form secondary reservoirs in a number of Birsa accumulations within the gulf. They also form the primary reservoirs in the Maamoura, Baraka and Zelfa discoveries. The Beglia Formation is described onshore as being largely fluviatile and shallow marine in origin and Serravalian to Tortonian in age (Ben Ferjani et al, 1990). The Saouaf Formation is paralic, lagoonal to deltaic and forms the main to upper part of the Oum Douil Group (Ben Ferjani et al, 1990). It is typically a shale prone interval with many intercalations of sand and siltstones.

Paleogene

The Oligocene reservoirs in the Gulf of Hammamet constitute the siliciclastic Fortuna Group and limestones of the Ketatna Formation.

The siliciclastics of the Fortuna Group are restricted to the western portion of the gulf, grading laterally to finer grained facies of the Salammbo Formation. The Ketatna Formation limestones appear to be restricted to structural highs and are characterised in the upper part by biohermal facies (Sebei et al, 2007).

1.0sec

2.0sec

VKS07-26

1kmNW SE

Lower Birsa Formation

Hammamet West-2

Figure 6: 2D TWT seismic section over the Hammamet West structure illustrating the onlap nature of the lower Birsa Formation.

1.0sec

1.5sec

80-135R

0.5kmW S

Tazerka-1

Upper Birsa Formation

Figure 7: 2D TWT Seismic section over the Tazerka structure illustrating the westward thinning of the upper Birsa Formation over the structure.

Eocene reservoirs include the Upper Halk el Menzel Formation and the Lower Eocene Bou Dabbous Formation. Fractured Bou Dabbous Formation is the significant reservoir onshore Cap Bon such as at Belli, Al Manzah and Beni Khalled fields.

Cretaceous & Jurassic

Cretaceous to Jurassic reservoirs predominantly comprise the Maastrichtian Abiod Formation, the Albian Serdj Formation, the Lower Cretaceous to Upper Jurassic Sidi Khalif Formation and the Jurassic Nara Formation.

The Abiod Formation is characteristically a chalky limestone. It is typically represented by upper and lower limestone units, separated by a middle shaly member. The Abiod Formation evidently possess some reasonable matrix porosity (Figure 4), however permeability is typically quite low. Fracture porosity and connected pore volume is required for the Abiod Formation to be a viable offshore reservoir.

Several Abiod accumulations are present within the Gulf of Hammamet (Table 1) where hydrocarbon flows have been established on test. However, a number of potential hydrocarbon accumulations also occur where petrophysical evidence for a hydrocarbon column exists, but no flow has been achieved. Examples include Jaafar-1 and Hammamet West-2.

Interpretation & Mapping

A regional database of some 30,000km of 2D seismic and 29 wells were used in this study. Wells were critically reviewed and detailed post-mortems compiled. Particular attention was paid to failure analysis as this formulates the assessment of risk for future exploration activities.

Based on this well data a regional stratigraphic framework was constructed. This framework was based on a sequence stratigraphic approach which is independent of the variations in lithology and hence the vagaries stratigraphic nomenclature. In this approach well and seismic picks are assigned an age and correlated regionally as time surfaces.

A number of key regional framework events were interpreted over the Gulf of Hammamet. The TM16 event (Ain Grab Formation) is an excellent seismic marker throughout the Gulf of Hammamet. Figure

8 is a TWT structure map of the TM16 marker for the Gulf of Hammamet. This figure clearly shows the structural grain of the gulf.

Figure 8: TM16 (Ain Grab Formation) TWT Structure Map.

Tectonics & Structure

The structural setting and structural styles of the Gulf of Hammamet have been variously described by Ben Ferjani et al (1990), Burollet (1991), Bedir et al (1998), Patriat et al (2003), Bedir (2005) and Corti et al (2006) and are shown in Figure 9.

Figure 9: Generalised Structure Map of the Gulf of Hammamet.

The Gulf of Hammamet structure is strongly modified by the Alpine Orogen. Figure 10 illustrates the proximity of the gulf to the contractional processes in the Western Mediterranean that give rise to the Tellian Atlas.

Figures 11-13 illustrate the effect of the Alpine – Atlassic orogeny on structure within the Gulf of Hammamet. In the north of the gulf in the area of Kelibia and offshore, significant thrusting is evident (Figure 11). This thrusting has resulted in several high angle SW-NE orientated thrust structures that as yet are undrilled. Both thrust and sub-thrust plays have significant potential.

Further south into the Hammamet Trough and on the northern edge of the stable Halk el Menzel platform, significant low angle thrusts and thrust sheets are apparent (Figure 12). These structures appear to form a series of lobate thrust fronts with a E-W to SW-NE trend. This trend hosts the Cosmos-Cosmos South and Yasmin fields. This thrusting appears to be soled in the Cenomanian-Turonian section.

The Halk el Menzel Platform appears to have been a stable structural zone since the Cretaceous. Data quality over the platform is good and it is weakly faulted. To the south and west of the Halk el Menzel Platform significant Mio-Pliocene structuring is evident. In the west the dominant structural feature is the NW-SE trending Pliocene Grombalia-Jriba Graben. Little compressional structuration is evident, however, some wrench features are to be expected. Seismic data over the southern termination of the graben is poor and partially masks structure making structural hypotheses difficult.

Figure 10: Thrust fronts in the western Mediterranean (modified after Jolivert et al, 1999)

The E-W trending Kuriate Graben defines the southern margin of the Halk el Menzel Platform. Atlassic compression and inversion of the graben is evident (Figure 13), resulting in the formation of the Kuriate Islands and surrounding shoals. The bounding Teboulba Platform and Mahdia Arch have undergone significant footwall uplift and tilting resulting in Pliocene erosion.

Conclusions & Recommendations

As part of a geological and geophysical regional review of the Gulf of Hammamet Bargou and Hammamet blocks, a structurally and stratigraphically consistent interpretation and assessment has shown that significant exploration potential still exists within the Gulf of Hammamet.

A number of sizeable structures remain, as yet undrilled. The dominant plays within the gulf remain within the Miocene. However, other plays such as the Mio-Oligocene and the Maastrichtian possess exploration potential. This study has gone some way toward unravelling and understanding the complex structure and geology of the gulf.

Figure 11:TWT seismic section over high angle thrust feature, northern Gulf of Hammamet.

Figure 12: TWT seismic section over the Cosmos – Begonia thrust trend, central Gulf of Hammamet.

Figure 13: TWT seismic section over the Kuriate Graben, southern Gulf of Hammamet.

References

Bedir, M., Bobier, C. & El Manaa, S. (1998). “Subsurface Neogene Clay Diapirs in Northern Tunisia: Basin Structuration and Petroleum Implications.”, in ETAP Memoir 12, pp29-45.

Bedir, M. (2005). “New Seismic Neogne Clay Diapirs and Hydrocarbon Implications in the North-Eastern African Margin of Tunisia.”, in Martinelli. G. & Panahi. B. “Mud Volcanoes, Geodynamics and Seismicity.”, Springer, pp1-15.

Ben Ferjani, A., Burollet, P. F. & Mejri, F.(1990), “Petroleum Geology of Tunisia.”, ETAP.

Biely, A., Rakus, M., Robinson, P. & Salaj, J. (1972), "Essai de Correlation des Formations Miocenes au sud de la Dorsale Tunisienne.", Note du Service Geologique Tunisie (38), pp73-93.

Bizmuth, H., & Hooyberghs, H., J., F, (1994), “Planctonic Foraminifera and Biostratigraphy of the Oligocene and Neogene in the Korba 1 Oil Well.”, Bull Centres Rech Explor-Prod Elf Aquitane (18)2, pp489-528.

Burollet, P., F. (1991). “Structures and Tectonics of Tunisia.”, Tectonophysics (195), pp359-369.

Catalano, S., De Guidi, G., Romagnoli, G., Torrisi, S., Tortorici, G. & Tortorici, L. (2008). “The migration of plate boundaries in SE Sicily: Influence on the large-scale kinematic model of the African promontory in southern Italy.”, Tectonophysics (449), pp41-62.

Corti, G., Cuffaro, M., Doglioni, C., Innocenti, F. & Manetti, P. (2006). “Coexisting geodynamic processes in the Sicily Channel.”, in Dilek, Y. & Pavlides, S (eds), “Postcollisional tectonics and magmatism in the Mediterranean region and Asia.”, Geological Society of America Special Paper 409, pp83-96.

Gomez-Gras, D. & Zoghlami, K. (2003) “Provenance of the Upper Un it Sandstones of the Fortuna Formation of North-Eastern Tunisia.”, AAPG International Conference Expanded Abstracts.

Hooyberghs, H., J., F. (1995) “Synthèse sur la Stratigraphie de l’Oligocène, Miocène et Pliocène de Tunisie.”, Notes du Service Gèologique de Tunisie (61), pp63-72.

Hsu, K. J., Cita, M. B., Ryan, W. B. F. (1973). “The Origin of the Mediterranean Evaporites.”, Initial Reports of the Deep Sea Drilling Project, U.S. Government Printing Office, Washington DC, pp1203-1231.

Jolivert, L., Frizon de Lamotte, D., Mascle, A. & Sèranne, M. (1999). “The Mediterranean Basins: Tertiary Extension within the Alpine Orogen – an Introduction.”, In Durand, B., Jolivert, L., Horvath, F. & Sèrane, M. (eds), The Mediterranean Basins: Tertiary Extension within the Alpine Orogen., Geological Society, London, Special Publications, 156, pp1-14.

Klett, T.R. (2001). “Total Petroleum Systems of the Pelagian Province, Tunisia, Libya, Italy and Malta – The Bou Dabbous-Tertiary and Jurassic-Cretaceous Composite”. USGS Geolgical Survey Bulletin 2201-D

Luning, S., Kolonic, S., Belhadj, E. M., Belhadj, Z., Cota, L., Baric, G. & Wagner, T. (2004) “Integrated depositional model for the Cenomanian – Turonian organic-rich strata in North Africa.”, Earth Science Reviews (64), pp51-117.

Macgregor, D. S. (1996). “ The hydrocarbon systems of North Africa.”, Marine and Petroleum Geology, Vol 13(3), pp329-340.

Messaoudi, F. & Hamouda, F. (1994). “Evènements Structuraux et Types de Piègs dans l’offshore Nord-Est de la Tunisie.”, ETAP 4th EPC Proceedings, pp55-64.

Negra, M. H. (1998). “ Origins of Changes in Abiod Formation Potential Properties.”, ETAP Memoir 12, pp251-265.

Patriat, M., Ellouz, N., Dey, Z., Gaulier, J. & Kilani, H., B. (2003). “The Hammamet, Gabes and Chotts Basins (Tunisia): A Review of the Subsidence History.”, Sedimentary Geology (156), pp241-262.

Portolano. P., Schein, L. & Simonnot, A. (2000). “3D Geological Modelling of the Birsa Oil Field.”. 7th ETAP EPC Proceedings, pp365-379.

Robinson, P. & Wiman, S. K. (1976), "A Revision of the Stratigraphic subdivision of the Miocene Rock of Sub-Dorsale Tunisia.", Note du Service Geologique de Tunisie (42), pp72-86.

Sebei, K., Inoubli, M. H., Boussiga, H., Tlig, S., Alouani, R. & Boujamaoui, M. (2007). “Seismic Stratigraphy, Tectonics and Depositional History in the Halk el Menzel Region, NE Tunisia.”. Journal of African Earth Sciences (47), pp9-29.

Zagrarni, M. F., Negra, M. H. & Hanini, A. (2008). “Cenomanian-Turonian facies and sequence stratigraphy, Bahloul Formation, Tunisia.”, Sedimentary Geoogy (204), pp18-35.