Tectonic Setting of the Petroleum Systems of Sicily

Petroleum systems in Sicily are divided between the Neogene fold and thrust belt of the Sicilian accretionary prism and its foreland. By far, more production has been established in the foreland on the Ragusa platform, where generally heavy oil from Triassic and Jurassic source rocks has been trapped in similarly aged fractured carbonate mound reservoirs. Traps are structures recurrently rejuvenated since the Cretaceous. Although models of source maturation and migration history emphasize Pliocene–Pleistocene charge related to subsidence in the foreland, occurrences of light oil and exploration data suggest intermittent periods of charge since the Mesozoic. Hydrocarbon deposits in the immediate vicinity of the thrust front, in the narrow foredeep depression, are also heavy and have characteristics in common with those more remote in the foreland from the front; they do not appear to have been significantly influenced by or mixed with oils from within the fold belt, although variations in their chemistry have yet to be investigated in terms of source correlation. Thrust sheets in the fold and thrust belt carry similar Mesozoic stratigraphy to the foreland, namely, a series of carbonate platforms and basins with potentially similar source rocks, and can reasonably be expected to have undergone a similar pre-Neogene history. Commercial hydrocarbons in the fold belt, however, are limited to thermo- and biogenic gas-charged Tertiary reservoirs and questionably Tertiary-sourced oil offshore to the west. These all currently are viable exploration plays. Numerous seeps and tar deposits indicate that at least one petroleum system was active in the fold belt, but only one tar occurrence links that system to the Mesozoic carbonate section. Recent structural studies have indicated that a two-phase history of thrust emplacement and its redeformation by duplexing on lower detachments characterizes much of the Sicilian fold and thrust belt and affords exploration targets that have yet to be satisfactorily tested. Furthermore, structural petrological and fluid-inclusion studies in Neogene-thrusted rocks of the lower portions of the frontal fold belt indicate that a light hydrocarbon system used Neogene detachment surfaces as a preferential migration route in at least part of the accretionary prism. Insofar as the foreland may give an insight into the early history of the fold belt platforms and basins, a general sense of the hydrocarbon migration history in the fold belt might include the following: (1) early maturation and migration of a variety of oils into carbonate mound reservoirs during a long period of time in the latter half of the Mesozoic and Paleogene (like in the foreland plays), bolstered by (2) a charge of immature oil generated during rapid depression in the foredeep immediately adjacent to the moving thrust front (yet to be demonstrated), and (3) generation of higher maturity oil and gas in the thrust wedge during stacking of thrust sheets and remigration and mixing with the earlier generated fluids (an untested play).