Exploration Country Focus
The Italian landscape is largely dominated by the Alps and the Apennines chains, which arose during Tertiary as a result of the interaction between the European and the African tectonic plates. Alps bound Italy on the north, while Apennines traverse the entire peninsula and form the backbone of the country.
The two chains and their southern and eastern sectors constitute a thrustbelt- foredeep-foreland system (fig. 2) whose evolution was very complex and led to the creation of a wide range of geological scenarios. In this framework several petroleum systems have developed, some of which are of paramount economic importance and make Italy the most important hydrocarbon province of the southern Europe.
These petroleum systems can be classified as associated with three main tectono-stratigraphic systems (fig. 3):
- Biogenic gas in the undeformed terrigenous Plio-Quaternary foredeep wedges;
- Thermogenic gas in the thrusted terrigenous Tertiary foredeep wedges;
- Oil and thermogenic gas in the carbonate Mesozoic substratum.
Figure 3. Tectono-stratigraphic cycles and hydrocarbon occurrences
Biogenic Gas in the Terrigenous Plio-Quaternary Foredeep Wedges
Several biogenic gas fields have been discovered in the Plio-Quaternary successions of the late foredeep basins of the Apennines, in both central and northern Adriatic Sea and in the Po Plain (fig. 4). These successions mainly consist of thick turbidite sequences with alternating shale and sand layers, which often constitute interbedded combinations of sources and reservoirs. Traps are most commonly structural and range from thrusted anticlines of the inner foredeep margins (e.g. Cervia Mare and Porto Corsini Mare: fig. 6b), to gentle folds draping the underlying morphology of the carbonate substratum of the outer foredeeps. Stratigraphic traps are not rare and are mainly related to the turbidite sands terminations along the basins borders.
The exploration of the biogenic play started after the World War II in the central Po Plain and progressively moved eastward and offshore. Starting from the early eighties the seismic DHI technologies became the key support for any successful exploration in this play. The most important discoveries occurred in the Adriatic Sea, where hundred of seismic bright spots were successfully tested and put into production. Among them the Barbara gas field stands out for its giant size.
Today the exploration of the biogenic play is very mature and limited to the search of small accumulations near the existing fields. Some potential is left in thin layered reservoirs below the seismic resolution.
Thermogenic Gas in the Thrusted Terrigenous Tertiary Foredeep Wedges
This system includes older turbidite foredeep basins of the Southern Alps and of the Apennines, whose successions have been tectonically involved in the accretionary prisms of the two chains (fig. 4). The deep burial of these units allowed an early thermogenic generation of gas from the deepest organic rich pelitic cycles of the prisms. Gas occurrences related to this petroleum system have been found in the Southern Alps and the Northern Apennines but the most important discoveries were made in the Southern Apennines, namely in Calabria (Luna field) and in Sicily (Gagliano, Bronte and Fiumetto fields: fig. 6d). The exploration of this play is made difficult by the generally poor seismic imaging, due to the structural complexity of the related geologic framework. Also, discoveries occasionally provide only minor rewards in terms of amount of gas in place (small traps) and reservoir quality.
Oil and Thermogenic Gas in the Carbonate Mesozoic Substratum
This system involves the Mesozoic carbonate substratum of the foredeep/foreland area and of the external thrust belts (fig. 5). It has the largest oil and gas accumulations of the Italian region (e.g. the giant oil field of Monte Alpi – Cerro Falcone in Val d’Agri in Southern Apennines).
Several different plays are present which are essentially related to the three main phases of the Tethyan crustal stretching, i.e. the Middle Triassic, Late Triassic/Early Jurassic and Early Cretaceous stages. Traps for hydrocarbons are also highly varied, as they were shaped by the interference between the Mesozoic extensional phases and the subsequent Tertiary compressional events.
Beginning with the most recent play, the Cretaceous is associated with the late extensional reactivations which occurred in the Tethyan post rift stage and were driven by the first mobilization of the Triassic salt. It is restricted to long lasting Cretaceous platforms, the main ones being the Apulian, Bagnolo and Dalmatian. Extension created new accomodation space within these platforms in which basinal sequences accumulated. A major anoxic peak occurred during the Albian-Cenomanian time, leading to the deposition of the main source rock.
Due to its shallowness and to the low permeability of the associated platform reservoir, this play is more successfully explored in the highly fractured frontal structures of the thrust belt (e.g. Monte Alpi and Tempa Rossa oil fields: fig. 6c).
The Late Triassic/Early Jurassic play system is linked to the main phase of the Tethyan rifting and is the most explored of the three systems, both in the foreland and in the thrust belt, and from Lombardy to Sicily. The source rocks involved are terrigenous or mixed carbonate/terrigeneous, and were deposited during the anoxic stage that preceeded the spread out of the Jurassic basins. Due to the discontinuity of the regional and local seals,reservoirs are located in a wide chronostratigraphic range, from the coeval platform units up to the topmost carbonate units, and even in the overlying terrigenous sequences (Nilde field, Miocene, Sicily). The related hydrocarbon accumulations mainly occur in the reactivated structures of the foreland margin: Malossa and other Po Plain gas and condensate fields; all Mid and South Adriatic oil fields (e.g. Aquila field: fig. 6c); Gela and all other Sicilian oil fields (fig. 6d).
The Middle Triassic play is linked to the earliest stage of the Tethyan fragmentation. Due to its depth it can be pursued only in the foredeep and foreland regions, whereas in the thrust belt it is generally too deep and overcooked.
Reservoirs involve the dolomitized platform units of Triassic age, which were charged by mainly carbonate source rocks deposited in the confined basins created by the rifting. Traps are also mostly provided by Mesozoic structures redeformed by the Tertiary compression. The Villafortuna – Trecate oil field in Northern Italy is the largest oil accumulation pertaining to this play (fig. 6a).