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