Geochemistry and Habitat of Oils in Italy

Luciano Novelli, Luigi Mattavelli

Most of the onshore and offshore oil occurrences found in Italy have been systematically analyzed by different techniques, i.e., capillary gas chromatography, biological markers, and stable isotopes composition. On the basis of the above analyses, ten different groups of oil have been identified and geographically located. Subsequently, the influence of the various geological settings on generation and migration of these different groups of oils was investigated and is discussed here.

In a foredeep regime, the remarkably fast heating rates, due to the rapid burial during late Tertiary, caused a rapid generation of oil mainly in the Triassic carbonate source rocks. Such generation, combined with the high over-burden pressure and a contemporaneous development of an intense tectonic compression, resulted in the expulsion of immature, heavy oils. Examples of this are evident in the central Adriatic Sea, southern Italy, and southeastern Sicily.

Due to its complex geological and tectonic history, the Alpine-Apennine chain behaved differently with regard to oil generation and migration in different areas. In fact, the high temperatures reached by the Mesozoic source rocks underneath a stack of allochthonous thrust sheets and the insulating "thermal blanket effect" exerted by the same sheets on other younger source rocks above gave rise to generally light oils. Furthermore, in this unique geological setting, the most external thrust sheets locally acted as reservoirs of the foredeep regime. Foreland sequences acted as both reservoirs of the foredeep oils and as "generative kitchens" of liquid hydrocarbons if suitable source rocks were present and adequate burial was reached.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.