MEDITERRANEAN SEA POTENTIAL SEEN IN AREA SOUTH OF MALTA

William F. Bishop
Consulting geologist
Houston

Godwin Debono
Office of the Prime Minister
Valletta, Malta

Seismic data and stratigraphic projections indicate that an entirely different facies exists in Area 4 in the Mediterranean Sea south of Malta than the continuous carbonate sequence of the Malta platform.

Japan National Oil Corp., in September 1989 under authority of the government of Malta, conducted a 3,615 line km geophysical survey (seismic, gravity, magnetics) in Area 4, which comprises about 13,000 sq km and is 40 km south of Malta (Fig. 1).

Bathymetric elements in Area 4 (Fig. 2) generally reflect underlying geologic structure.

The Linosa-Medina trough is the southern extremity of a graben system that trends southeastward from Linosa Island and probably is continuous with the Medina Channel graben. The Malta Channel graben separates Area 4 from the Malta platform on the north.

Between these grabens is the prominent horst of the Malta seamount. The positive Medina bank feature is bisected by a local graben, along the north side of which is the poorly defined ridge of the West Medina bank. The broad, featureless Melita rise represents the eastern edge of the extensive Tunisian platform, and nearby is the local horst of the Luzzu plateau.

The Melita-Medina Channel graben separates the rise from the Medina bank. Water depths vary from 150 m on the latter to more than 700 m in the Linosa-Medina trough.

MALTA AREA 4

Area 4 is part of the Pelagian block of the Central Mediterranean (Fig. 3). This extensive, largely submarine platform is bounded on the east by the Ionian abyssal basin along the Sicily-Malta escarpment, a system of normal faults of post-Miocene age that probably follows an older crustal fracture zone.

On the west is the north-south axis of Tunisia, a major geomorphic and structural feature, separating the Sahel or onshore portion of the Pelagian block from the Atlasic folds.

The Gafsa-Jeffara system of normal faulting/flexuring divides the Saharan platform on the south from the Ashtart and Tripolitania basins. The northern limit of the Pelagian block is the Calabrian fore-arc thrust zone, and on the northwest is the compressive Magrebid trend of Cap Bon.

The Pelagian block was a stable promontory of the African margin throughout the plate tectonic history of the Central Mediterranean. Crystalline basement is continuous between Sicily and North Africa, and continental crust underlies the entire Pelagian realm.

Stretching and thinning in the Sicily channel rift complex in late Miocene to Recent times results from transtensional dextral shear, and the dominant mode of deformation appears to be development of pull-apart basins. The Jarrafa graben is a similar feature of post-Miocene age.

STRATIGRAPHY INFERRED

The only well drilled within Area 4, MB-1 (Fig. 2), reached TD of 1,211 m in Lower Tertiary dolomite, and deeper stratigraphy must be inferred from that of offshore Tunisia and Sicily and from wells in Maltese waters north of the study area (Fig. 4).

Porous Triassic and Jurassic shallow-water carbonates, which are reservoirs in southeastern Sicily, were penetrated in one well off Malta and are expected in Area 4.

Proven source rocks of the Triassic/Jurassic Noto and Streppenosa formations were deposited in the Ragusa basin, and a similar pull-apart basin, indicated seismically in the southern part of the study area, should contain equivalents to these units. Deepwater limestones and marls of the Modica and Buccheri formations also should be present.

Several wells off Malta penetrated the Upper Jurassic/Cretaceous Naxxar formation, which is mostly restricted-shelf dolomite with scattered limestone intervals indicating periodic freshening. About 400 m of chalky, pelagic limestone beneath a tongue of Naxxar dolomite at AQ-1 show that this well was near the southern edge of the Malta platform during late Cretaceous time.

Seismic interval velocities indicate the rest of Area 4 to contain a Cretaceous platform-basin transitional facies similar to that off Tunisia, where deeper water Sidi Kralif, Fahdene, Bahloul, and Aleg formations contain source-rock intervals. Porous, intertidal dolomite, shelf-edge calcarenite, and rudist-reef reservoirs sealed by interbedded shale and marl should be present.

A Paleogene wedgeout, indicated seismically in the southwestern part of Area 4 (Fig. 5), should contain nummulitid banks, similar to those of Metlaoui reservoirs in Tunisia and Libya. An equivalent of the Bou Dabbous source rock is expected.

Later Paleogene deposition probably was similar to that of offshore Tunisia, where shale and argillaceous limestone of the Souar and Salammbo formations are effective seals.

The Globigerina limestone, Blue Clay, and Upper Coralline limestone formations of Miocene age are present in MB-1 and probably in the rest of Area 4. Thickness of Pliocene-Quaternary fora-miniferal mudstone/claystone was controlled by post-Miocene rifting, and thick isochron anomalies correspond to the bathymetric depressions of Fig. 2.

SEISMIC HORIZONS

Data quality of the new seismic is good in the Tertiary and fair in the deeper Mesozoic. The strongest and most consistent reflectors are the top of Miocene and top of Cretaceous unconformities.

A basal Miocene reflector, which follows structurally the top of Miocene, also is ubiquitous in the survey area, but a Paleogene reflector of fair continuity and amplitude (Fig. 5) is seen only on the Melita rise.

An intra-Cretaceous horizon of poor continuity but strong character, structurally discordant with the top of Cretaceous, is of mappable quality.

The top of Jurassic reflector (identified by correlation with wells that are farther north) is fairly continuous and strong except where it is very deep.

Probable Triassic phantom reflectors also are seen.

POTENTIAL TRAPS

Seismic interpretation has delineated a variety of potential traps, including simple and faulted anticlines, horsts, fault blocks, and the very impressive Lower Tertiary wedgeout. Stratigraphic traps are expected in carbonate reefs and bar/bank deposits of the Cretaceous-Paleogene interval.

Restored seismic sections indicate that some structures began to form by the end of the early Cretaceous. Most important structures were formed by the end of Cretaceous time, and post-Miocene faults do not necessarily breach them. Within the Early Jurassic rift basin in the Melita-Medina channel is a series of tilted fault blocks terminating in the overlying Tertiary section.

Potential reservoirs include Triassic carbonates (Fig. 6) of the Gela formation productive at Gela and Ragusa fields, Fig. 1) and Jurassic carbonates of the Siracusa (Vega, Perla) and Nara formations (El Biban), expected on the flanks of the rift basin and on intra-basinal highs.

In the Cretaceous platform-basin transitional zone, stratigraphic plays in reefal facies of the Zebbag formation (Isis, Miskar) and structural plays in Cretaceous carbonates (El Biban, Ezzaouia) should be present.

Substantial oil flows were tested in wells of northern Tunisia from uppermost Cretaceous Abiod chalk, but its equivalent may be too shallow or unseated. A low-velocity interval at the base of Tertiary on the Melita rise should contain Metlaoui nummulitid bank facies (Ashtart, Bouri).

OTHER POSSIBILITIES

Equivalents to Triassic/Jurassic Noto and Streppenosa source rocks are expected in the rift basin of the Melita-Medina channel.

Based on the geothermal gradient in AQ-1 just north of Area 4, the top of the oil window should be at about 3,000 m, and since the entire Jurassic section is below 3,500 m, it should be capable of peak oil generation.

Other potential sources, equivalent to the Cretaceous Bahloul and Fahdene formations, should be present in the platform-basin transitional area off the Medina bank and Malta platform.

These organic-rich facies should have reached maturity during the Miocene in the Melita-Medina channel and southern Medina bank. Maturation in the Linosa-Medina trough probably took place in post-Miocene time when subsidence reached a depth of 3,000 m.

The Tertiary Bou Dabbous equivalent, inferred to be present only in the southwestern part of Area 4, is at less than 3,000 m, but post-Miocene time has been a period of active tectonics in the Pelagian block, as shown by the Sicily channel rift complex with its volcanic islands and submarine volcanic centers. Therefore, it is possible that higher geothermal gradients with be present off the stable Malta platform.

CONCLUSION

Seismic data and stratigraphic projections indicate that an entirely different facies exists in Area 4 from the continuous carbonate sequence of the Malta platform.

Early Mesozoic source rocks may be present in the Melita-Medina graben, and Cretaceous source rocks, reservoirs and seals should occur in the platform-basin transitional zone.

An early Paleogene wedgeout is expected to contain nummulitid reservoirs. Parts of the area have a burial history favoring the onset of hydrocarbon generation from the end of Cretaceous time to the present.

Large structures representing numerous play types are present, and the potential for discovery of major hydrocarbon reserves in Area 4 is excellent.