Geological overview offshore

Adriatic basin is almost surrounded by the mountain chains and only one alluvial plain (Po plain) in the north. The Apennines are set on the west and southwest, the Alps on the north and northwest and Dinarides extend along northeastern boundary. These mountain chains are consequences of the compression and fracture along the edges of the Adria microplate and the results of collision with Europe and the Apennine and Adria collision plates. These geomorphological features give the main characteristics for sedimentary infill of the Adriatic basin. Their geomorphological position along the edges and geological peculiarities act as a building elements for this area.

By the end of Mesozoic, carbonate platforms of Periadriatic area move toward north and merge to the southern margin of Europe. The consequences of the collision with European plate are compressional movements, thrust folding and uplifting of Apennines, Alps and Dinarides. The result of the regional uplifts is also the end of Mesozoic Adriatic Carbonate Platform. Deposition of sediments continues in the Paleogene under the tectonic influence and Adriatic Sea takes its present shape.

Regional geological overview

Adriatic offshore area, as a part of Apulian (African) plate, has an important role in Mediterranean plate tectonics from Paleozoic orogenic phases, up to present times.

  • The cratonization of the Apulian (Adria and Dinarides) basement took place in Upper Carboniferous during the Hercynian orogenic phase. Post-Hercynian period brought peneplanation that was followed by predominantly clastic deposition during Permian and early Triassic times.
  • In Ladinian the disintegration of plates (rifting) occurred followed by the intensive Upper Triassic/Lower Jurassic (200 Ma) intrusions of magmatic rocks along deep fractures of the basement. Rifting is indicated by the igneous rocks of Jabuka, Brusnik, Vis and Palagruža islands and Vlasta-1, Rovinj-1, Susak more-1 and other wells.
  • Carnian post-rift phase is characterized by filling of the rift-depressions of various geometries. Coarse to fine-grained clastics, salts, dolomites and bituminous beds were deposited.
  • In Norian, Rhaetian and Upper Jurassic the carbonate sedimentation regime was established over the whole area, indicating the beginning of the Mesozoic carbonate platform regime. Carbonate sedimentation varied according to the intensity of regional subsidence. The maximum rate of deposition of carbonate-evaporite complex took place in the central Adriatic area in front of the frontal thrust. The main (Haupt-) dolomites and evaporites (equivalent of Burano facies) were deposited. The thickness of those sediments could reach a few thousands meters.

Adriatic offshore stratigraphy and structural units

Stratigraphic synthesis of the Adriatic offshore (Adriatic Basin and Adriatic Carbonate Platform) is represented with four main rock complexes:

  1. Siliciclastics and evaporites of Lower Triassic to Upper Triassic (Carnian) age;
    Siliciclastics and evaporites of of Lower Triassic to Upper Triassic (Carnian) age were encountered with only a few wells. They are composed of mainly clastics, evaporites, volcanics and carbonate rocks.
  2. Carbonates and evaporites of Upper Triassic to Middle Eocene age;
    In the whole area, from Upper Triassic (Norian) to Middle Eocene persisted a carbonate platform of continuous shallow water sedimentation. This sedimentation is quite uniform, with local, short living disturbances (evaporites, oligostegina limestones).
  3. Clastic and carbonate flysch sequences of Upper Eocene to Upper Miocene age;
    The final disintegration of carbonate platform started during Upper Eocene by drowning of the platform carbonates by flysch deposits.
  4. Pliocene-Quaternary clastic rocks;
    After a strong compressional event at the end of Miocene, the large transgression affected a whole exploration area, forming one unique basin of Pliocene and Quaternary to recent age.

Comparing stratigraphic development and tectonic history, the whole area is divided in several geological units, respecting stratigraphic, paleogeographic and tectonic inputs, of all penetrated rocks.

The area was divided into following units:

  • Istrian Plateau Slope
  • Istrian Plateau
  • Dugi Otok Plain
  • Dugi Otok Depression
  • Compressive Blocks and Thrust
  • Central Adriatic Uplift
  • Mesozoic-Tertiary Carbonate Uplift
  • Palagruža Uplift
  • Central Adriatic Depression
  • South Adriatic Basin

Istrian plateau

The Istrian plateau is a part of the great Istrian anticline with the core in Rovinj area. Rovinj-1 well has reached Lower Triassic clastics. It belongs to the western part of the large Mesozoic Adriatic carbonate platform. The Plateau is gradually sinking toward west and its western margin is partly faulted by west-dipping normal faults. The eastern margin of the Istrian Plateau is faulted by very important NW-SE striking reverse fault that caused a regional uplift of its eastern side (Učka – Cres). In the Istrian plateau area, the sediments of Lower Triassic, Jurassic, Cretaceous, Tertiary and Pliocene-Quaternary age have been identified. The representative wells located in the plateau are Rovinj-1, Pula-1 and Susak more-1.

Dugi otok depression

The Dugi Otok Depression is situated in the Central Adriatic area between Compressive Blocks and Thrusts, Central Adriatic High and Dugi Otok Plain. The Dugi Otok depression is spreading in front of the frontal thrust, where the top of carbonates reaches the depth of 5500 m. Toward SW carbonate sediments are gradually getting shallower, and clastic sequence of the Neogene and Paleogene sediments becomes thinner. In this zone carbonate reefs appear. The depression was formed during the Paleogene and Neogene time when progressive subsidence of the Late Mesozoic carbonate-evaporite complex takes place. Due to continuous subsidence in the area of Dugi Otok Depression, the thickness flysch beds reached 3.000 m
The deep exploration wells: Jadran-13, Klara-1, Jadran-252/1, Jadran-252/2, Kristina-1, Kornati more-2 and Kruna-1 tested Lower and Upper Cretaceous carbonate sediments, clastic-carbonate deposits of the Upper Eocene, Oligocene and Miocene as well as the Pliocene-Quaternary clastic sediments.

Dugi otok plain

Dugi Otok Plain involves the area between the Istrian Plateau, Compressive Blocks and Thrusts and Dugi Otok depression. Kornati more-4 and Jadran-3 wells are located in this area. The penetrated Mesozoic-Cenozoic sediments in this area have identical stratigraphic and lithofacies characteristics as the sediments of the Compressive Blocks and Thrusts. The older part of the Lower Cretaceous is represented by late diagenetic dolomites. From Albian to Santonian/Campanian the deposition took place in lagoons and restricted shallow water environment. The anoxic conditions characterized the Albian deposition (Kornati more-4). In J-3 well area, intraplatform deposition with oligostegina limestones took place in Santonian/Campanian age.
At the end of Maastrichtian, the emersion caused a regional unconformity. The new transgression started in Lower Eocene with deposition of shallow water limestones. Open platform limestones marked progressive deepening of the carbonate platform in Middle Eocene. During Upper Eocene, Oligocene and Miocene the deepening of the area continued, replacing platform carbonates with flysch deposition. The thickness of the flysch deposits increased toward Compressive Blocks and Thrusts unit and Dugi Otok Depression. Regional emersion at the end of Miocene resulted in an irregular relief. Therefore, Pliocene overstep sequences vary in thickness between 0 and 300 m.

Compressive blocks and thrusts

This unit extends along the eastern Adriatic from northwest to southeast. Geographically, it is an extension of Outer Dinarides into the Adriatic area. It is characterized by compressive tectonics. The frontal thrust belt composed of the sequence of antiform and synform shapes cut by NE dipping reverse faults (thrusts) of generally NW-SE direction. Obviously, the compressional stress was the main mechanism that caused vertical throws and horizontal heaves. The intensity of folding is much weaker in the SW part of Kornati islands and fault planes have weaker dipping. Toward SE, structural features are more intensely expressed. They are characterized by numerous thrusts or even by groups of parallel thrusts. The direction of fault and structure trends changes into E-W to the line Split - Brač - Vis. The islands are uplifted structures caused by reverse faults of opposite vergences.
The correlation of the certain Mesozoic and Cenozoic stratigraphic members shows a distinct lithofacies differences between the northwestern and southeastern parts. The main stratigraphic characteristic of the northwestern area is a great apparent thickness of the Cretaceous carbonates. Representative wells in northwestern part are Kate-1, Ravni Kotari-3 and Dugi Otok-1. The southeastern part of this unit is characterized by complete development of Mesozoic carbonates and lack of Tertiary sediments. Representative wells are Vlasta-1 and Lastovo-1.

Central adriatic uplift

The area is tested by deep exploration wells Jadran-1, Jadran-2, Jadran-10, Ksenija-1, Koraljka-1, Kornati more-1 and Vis-1, providing data on sedimentary sequence from the Ladinian to Pliocene-Quaternary period.

The Ladinian and Carnian lithologies in the base of carbonate platform are complex and characterized by unstable depositional environment, as a result of the strong synsedimentary tectonics (rifting) followed by volcanism. The interbedded carbonates, clastics, evaporites and pyroclastics of hyper-saline anoxic lagoons are the main characteristics of the Ladinian deposition. During Carnian, dolomites and evaporites are the most common lithofacies (Kornati more-1 and Vis-1 wells). Those rocks often form diapirs during advanced compression. By the end of the Lower Jurassic, the western margin of Adriatic carbonate platform was affected by rifting related to opening of the Adriatic-Ionian Basin. Basinal sedimentation started with deposition of Upper Jurassic pelagic cherty carbonates. Lower Cretaceous pelagic limestones with chert continued to deposit in that area. During Cenomanian and Turonian the deposition of pelagic micrites with planktonic foraminifera continued. At the end of Upper Cretaceous, regression is marked with peri-reef deposits. The tectonic uplift of the unit, which took place during Paleogene and Neogene, is documented in the wells as stratigraphic gaps. During Upper Miocene the whole area was emerged. The Pliocene Quaternary overstepping sequences continuously overlie the flattened Miocene paleorelief.

Central adriatic depression and palagruža uplift

This unit is situated between Central Adriatic Uplift, Compressive Blocks and Thrusts and Palagruža Uplift. To this unit belong the wells Jana-1, Jelena-1, Palagruza-1, Patricija-1 and Perina-1.

The oldest penetrated sediments are sabkha-type early diagenetic dolomites and anhydrites of Carnian age. In Norian and Rhaetian late diagenetic supratidal dolomites prevail. Lower Jurassic deposition is characterized with sedimentation of peritidal dolomites and limestones of restricted lagoons.

The opening of the Adriatic-Ionian basin and a subsidence platform margins started at the end of Lower Jurassic. On the platform, deposition continued without interruption through the entire Mesozoic period. Due to the uplift and erosion during Laramian orogeny, final Mesozoic stratigraphic members differ from well to well. The limestones of restricted lagoons alternated late diagenetic dolomites. Regional erosional unconformity at the end of the Cretaceous is marked with deeply karstified limestone, collapse breccias and caverns filled with younger sediments.

Mesozoic-tertiary carbonate uplift

This unit is situated south of the Compressive Blocks and Thrusts. There are four wells in the area: Maja-1, Melita-1, Mirjana-1 and Dubravka More-1. Only Maja-1 well is placed in northwestern part of the unit, the others are placed toward southeast. The oldest beds in the area are coastal clastics of Lower Triassic age (well Maja-1). Overstepping sequences are Upper Triassic dolomites and Jurassic and Lower Cretaceous shallow marine dolomites and limestones. In this area there is a long period of emersion from the Lower Cretaceous until Lower Miocene. In the southeastern part of the area the sequences up to Upper Cretaceous were penetrated by Melita-1 and Dubravka More-1 wells. During Oligocene, the clastic sedimentation of open shelf and slope prevailed, represented by flysch. Miocene regressive depositional cycle is also recognized all over the unit. Clastic and carbonate sediments alternated through the rest of Miocene, terminating in Messinian with regressive sequence of interbedded carbonates, clastics and gypsum. The new transgression started in Pliocene. Similar to the other units the Pliocene Quaternary sediments are composed of shelf sand and clay deposits.

South adriatic basin

The South Adriatic Basin is situated southwest of Mesozoic-Tertiary Carbonate Uplift. Its geological setting is predicted according to Italian wells Aquila-1, Grifone-1, Rovesti-1 and Montenegro well Južni Jadran-1. The presence of Ladinian-Carnian clastic-carbonate and volcanic-evaporitic deposition is highly speculative. The basin was formed during Lower Jurassic under extensive tectonic movements. It is evidenced by wide distribution of deep sea sediments. The compressive tectonic movements after Cretaceous and in Miocene caused thrusting and uplifting of the basinal margins, and also facies differentiation. Flysch is of Paleogene and Miocene age, and during Messinian, the lagoonal sediments with evaporites were deposited.

Petroleum system

Considering all geological and geochemical data aggregated in whole area of Croatian Adriatic offshore, three petroleum systems could be developed.

  • First petroleum system (Pliocene – Miocene Biogenic Gas System) has been recognized in the northern part of the Adriatic offshore with the most significant biogenic gas fields are Ivana, Ika, Marica and Izabela. The oldest sediments are Jurassic and Cretaceous karstified platform carbonates, unconformably overlain by Pliocene and Quaternary clastic deposits. The producing horizons are situated at depths of 600 to 1400 m and related to structural and stratigraphic traps. The most frequent reservoirs are moderate consolidated sands and siltites, interbedded with clays and shales in place. Shales are the main source rocks with kerogen of type III which generate biogenic gas in the environment with a very low geothermal gradient. Only the Ika field gas is accumulated in the Cretaceous fractured and karstified carbonate rocks.
  • Second petroleum system (Cretaceous Petroleum System) has been presumed in the central and southern part of the Adriatic offshore. The source rocks with good generation potential are evidenced by well samples from Upper Jurassic - Lower Cretaceous evaporite complex related to the black laminated limestones deposited in anoxic hypersaline lagoons and Upper Cretaceous pelagic carbonates. They are interbedded with stromatolitic dolomites and anhydrite of the sabkha type. Reservoirs are build up carbonates, slope and ramp carbonate and basin carbonates.
  • Third petroleum system (Middle Triassic Oil System) has been presumed in the Central and Southern Adriatic offshore with source rocks deposited in intrabasin areas from Ladinian-Carnian: shale, limestone and stromatolites (hypersaline anoxic lagoon- sabkha), kerogen type II-III (terrestrial origin). Reservoirs are build up carbonates, slope and ramp carbonate and basin carbonates.

Source rocks

  • Ladinian-Carnian: shale, limestone and stromatolites (hypersaline anoxic lagoon- sabkha), kerogen type II-III (terrestrial origin), poor to fair generating potential, early to middle stage of thermal maturation;
  • Albian-Lower Cenomanian - laminated miliolid-ostracodal limestone and stromatolites, (restricted anoxic lagoon), kerogen type II prevailing (oil prone kerogen with high sulphur content), fair to good potential for generating oil, early stage of catagenesis;
  • Turonian-Senonian laminated oligostegina limestone (intraplatform basin) and laminated limestone with stromatolites (restricted tidal flat), kerogen type I-II (marine origin, oil prone), good to very good generating potential, immature to marginally mature;
  • Pliocene and Pleistocene shale, probably source rock for biogenetic gas, type III kerogen (terrestrial origin), good generating potential, mature.

Reservoirs

  • Middle Triassic clastics, breccia, sandstone, siltstone and pyroclastics (Vls-1 well);
  • Upper Triassic and Lower Cretaceous late-diagenetic dolomites with intercrystalline porosity;
  • Cretaceous and Eocene limestone with fenestral, inter and intraskeletal porosity;
  • Cretaceous and Eocene limestone with vuggy porosity;
  • Oligocene-Miocene calcarenites (Irma-1 well);
  • Plio-Pleistocene sands and sandstones.

Seals

  • Triassic shales, marls and evaporites (anhydrite,halite and gypsum);
  • Upper Jurassic and Lower Cretaceous evaporites (anhydrite and gypsum);
  • Cretaceous and Eocene argillaceous limestone;
  • Paleogene-Neogene flysch deposits;
  • Pliocene and Pleistocene shales.

Carbonate platform plays

  • Karstified top of carbonates (structural-stratigraphic traps);
  • Carbonate platform edge (reefs);
  • Cretaceous carbonate breccias (tectonic activity zones);
  • Traps on the carbonate platform margin (slope).

Cenozoic plays

  • Gas accumulation in flysch sandstone layers - structural and stratigraphic traps;
  • Clastic deposits of the Miocene basinal margin - structural and stratigraphic traps;
  • Plio-Quarternary sediments of distal delta, structural and stratigraphic traps.

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