Italy Mesozoic oils - Implications for exploration in the Adriatic Sea

Enzo Zappaterra

Italy Mesozoic oils display great variability of physical, chemical and geochemical properties, which is generally attributed to the depositional and tectonic development of the specific basin in which the oils are found. Oils and thermogenic gases discovered to date have been generated principally from three main source bed intervals: Middle Triassic; Late Triassic- Early Jurassic (Liassic); Cretaceous.

Based on API gravity distribution, Mesozoic oils in fields/discoveries onshore-offshore can be broadly grouped into four families: Light oils to condensate: 55-35; Medium to light: 34-23; Heavy: 22-12; and Extra heavy: 11- 4 API.

All Po basin oils are light, with API gravities exceeding 36 (Malossa), with the exception of Cavone (20-23 API). Light oils occur also in Sicily (Palma) and in the So. Apennines (Monte Alpi). Light oils are contained in both shallow water platform carbonates and in deeper water pelagic carbonates.

Medium to light oils are present in the Central Apennines/Adriatic (Miglianico), in the Southern Adriatic (Aquila), and in Sicily (Mila). These oils are preferentially found in breccias/packstones within Cretaceous/Jurassic pelagic limestone sequences. The exception is Mila, which contains oil in Triassic platform carbonates.

Most of the heavy/extra heavy oils occur in the Central Adriatic (Rospo) and in Sicily (Ragusa, Gela). Significant occurrences are also in the Southern Apennines (Costa Molina, Tempa Rossa). These oils are generally reservoired in Triassic to Late Cretaceous platform carbonates, with the exception of Gianna and Elsa (Central Adriatic), which contain oil in pelagic carbonates of Cretaceous-Eocene age.

Producing levels occur within a wide range of depths, from 1300 m (Rospo) to 5600 m (Malossa). Although most light oils are found at greater depths than most heavy oils, Italy Mesozoic oils display no apparent direct correlation between oil gravity and reservoir depth, as a number of heavy oils occur at similar great depths: Costa Molina (13-21 API, 4000 m), Tempa Rossa (15-21, 5000-5300 m), Gorgoglione (18, deeper than 4000 m); and Prezioso (16, 4800 m).

Heavy oils are often found in close proximity to light oils. Notable occurrences are in Sicily, where the heavy Gela (6- 10) and Ragusa (19 API) oils are close to the light oils of onshore Irminio (36-43); in the Southern Apennines, where the Monte Alpi light oil (30-42 API) is not far from the heavy oils of Tempa Rossa (15-21) and Costa Molina (13-21); and in the Central Adriatic, where the Elsa and Miglianico oils, although in close proximity, have API gravity of 12-15 and 34-37, respectively.

Most of the Central-Southern Adriatic displays a platform-to-basin sedimentary sequence consisting of Triassic-Liassic platform carbonates overlain by Middle Jurassic-Cretaceous deep water basin micrites/marls. In contrast, an essentially uniform platform sequence (Apulian Platform) of Triassic-Cretaceous (locally Miocene) is developed along the western margin of the Central and Southern Adriatic Sea.

Both sequences have exploration potential for light and heavy oil accumulations. Thick oil columns may occur in reservoirs at the top of platform carbonate units, whereas thinner reservoirs may be developed in slope-to-basin turbidites, with correspondingly shorter hydrocarbon columns and smaller OOIP reserves. These slope-to-basin plays are prospective over large areas of the Adriatic Sea, mainly along the eastern margin of the Apulian platform, but also in areas farther to the east.

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013