American Oil and Gas Reporter - June 2014 - 54

SpecialReport: Global Opportunities
Not only can this new dataset reveal the broad structure of the
entire Adriatic, the high quality of the data allows the imaging
of deep, Upper Triassic source-rock-bearing basins that had been
unseen previously. This is crucial for exploration hydrocarbon system modeling, enabling oil and gas exploration companies to assess the basin's hydrocarbon potential for the first time. Multiple carbonate platforms can be imaged and mapped accurately,
leading to the identification of high-porosity/permeability play
systems, particularly around platform carbonate margins.
Despite lying between the dramatic thrust belts of onshore Croatia (the Dinaric thrust belt), and Italy (the Apulian thrust belt), the
surprising simplicity of basin structure is revealed to be dominated more by halokinetic (salt movement) tectonics, forming large,
simple traps that have the potential for sizable oil discoveries.
An initial study of the new datasets across the Adriatic margin has led to the startling realization that although the Mesozoic
(carbonate) and Cenozoic (clastic) play systems offshore Croatia are similar to those on the Italian margin, the seismic data indicate that the Croatian offshore has a superior hydrocarbon system, given the asymmetry of the Adriatic caused by the timing
of opposing thrust systems, and most importantly, increased halite
presence below the Mesozoic carbonates in the east.
Regional Geology
The oldest sequences penetrated by wells offshore Croatia are
Triassic evaporates. Within the Adriatic margin, these are predominantly anhydritic to the west and halites to the east. The recognition and understanding of mobile halite distribution in the east
reveals the controls on the structural development of the overlying carbonate platforms both onshore and offshore Croatia.
Well information, outcrop studies onshore, gravity and magnetic studies, and the new 2-D data have confirmed the presence
FIGURE 3
Four Structural Development Stages
Of the Croatian Adriatic Margin

54 THE AMERICAN OIL & GAS REPORTER

of a thick Triassic syn-rift sequence composed of halite, dolomites,
sabkha carbonates and black organic-rich shales. These latter sediments-deposited as thick, algal mats in the sabkha environment
both in Late Triassic syn-rift grabens and within halokinetically controlled rafts-are the dominant source rock for the region.
The Upper Triassic (Burrano formation) is the key source for
the multibillion-barrel oil fields of the Monte-Alpi, Cerro Falcone,
etc., Apulian thrust belt trend onshore Italy, and is responsible for
much of the oil onshore to the east in Croatia and the Pannonian Basin. Both onshore Italy and Croatia, the many seeps have
been typed to a Late Triassic source rock, which itself is exposed
in many places.
Offshore Croatia, it will be the ability to map the Triassic minibasins and model the generation and migration pathways that will
unlock the oil potential of this margin. Such oil was recovered
from Late Triassic carbonates in the deepest well drilled to date
offshore Croatia (Vlasta-1). However, source basins and migration pathways have been elusive with legacy seismic data, since
the key reflectors were largely not imaged.
On the Italian margin, the Triassic evaporates are largely anhydritic. However, on the eastern margin, Triassic halite loading
by Mesozoic carbonate platform deposition created large diapiric structures that dominate Central Adriatic prospectivity. Some
of these salt diapirs are imaged penetrating the seabed. Such features extending above the density equilibration level must be connected to a deeper halofer, and it is reasonable to envisage thick
salt facilitating the decollement of Dinaric thrusts.
The heterogeneities of the Triassic salt sequence not only determined the structural fabric of the region, but provided the
palimpsest controlling the subsequent Mesozoic deposition. This
provides the dominant prospectivity of the Croatian margin, and
Jurassic and Cretaceous platform carbonate margin deposits.
Thick Carbonate Platforms
During the Jurassic and Cretaceous, the Adriatic Basin comprised a wide, shallow-water carbonate shelf, distal from any clastic input from the Late Triassic until the Eocene. Steady subsidence of the margin resulting from the accumulation of thick carbonate platforms persisted, largely unbroken for close to 130 million years. That persistence was perfect for developing platformmargin facies at the edges of localized basins on the shelf associated with deep-seated rifts.
These are interesting for explorers, since they often are sites
of higher-energy deposition, with reef buildups on the shelf margin, and accumulation of porosity-retaining talus debris sloping
down the often steep slopes leading off the margins into the deeper basins. In places, reworked carbonate-storm deposits were deposited along the base of the platform margin slope in the form
of carbonate-turbidity deposits.
At the end of the Eocene, the collision of the northern margin of the carbonate platform with the European mainland created the Po Valley, which began to drain the Southern Alps and
deposit clastic sediments into the Adriatic. These clastics largely killed off the carbonate margins, and subsequent deposition
is dominated by clastics derived from the Po.
In the Oligocene, the continued collision created the west-verging Dinaric thrust belt (similar to that in the Ionian Basin to the

American Oil and Gas Reporter - June 2014

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