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The sedimentary sequences of the Italian region were deposited mostly on the African continental margin and in the contiguous ocean that was generated by Jurassic spreading. The lithology and thickness of the carbonate deposits were controlled from the Triassic to Early Cretaceous by tensional tectonics that preceded and accompanied the oceanic opening. In later times, the prevalently clastic sedimentation was conditioned by the position of the areas subject to erosion and by the morphology of the basins or, in other words, it was the consequence of the compressive tectonics and the Alpine and Apenninic orogeny. Six major structural units can be identified: the Island of Sardinia, a fragment of the European continental margin that escaped the Alpine compressive tectonics; the Tyrrhenian Sea, a Miocene-Pliocene tensional area with an oceanic-type
crust; the Southern Alps fold and thrust belt; the Apennine fold and thrust belt, where nappes of regional extension are present; the Foredeep; and the Foreland. The majority of the gas reserves discovered in Italy are located in Pliocene sandy reservoirs of the external Apennines and the foredeep. They originated during diagenesis through the action of bacteria on the immature organic matter contained in coeval shales. Most of the oil and part of the gas have a thermogenic origin from Mesozoic source rocks, mainly black shales deposited within Middle and Upper Triassic restricted basins. These shales matured during the Neogene, and the oil therefrom migrated into the adjacent Mesozoic carbonate reservoirs or, where tectonization was more intense, into Tertiary clastic reservoirs. Als , it is probable that a minor part of the oil found in Tertiary sandy reservoirs might have been originated by the organic matter contained in the shales of the Miocene Flysches. Presently (1982), gas and oil are the most important energy resources of Italy, covering about 9.8% of the national energy demand. Petroleum exploration, which started about a century ago and which has been intense since 1950, has not yet exhausted the possibility of additional interesting discoveries that appear to be conditioned mainly by the progress of exploration techniques.
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