Geological and geochemical setting of natural hydrocarbon emissions in Italy.

Hydrocarbons are contained in underground geological formations where they can slowly migrate under the action of tectonic activity. For this reason spontaneous hydrocarbon emissions may be detected on earth’s surface and they have historically attracted man’s attention, even often being the subject of health or religious cults. In nearly the whole of all Italy the various levels of interest aroused by hydrocarbons in local populations and in economic and industrial structure are recognizable in the historical and archeological documentation. During 19th and 20th centuries many hydrocarbon emissions were recorded and described but unfortunately over past few decades they have been erased by roads and towns building Scientific literature has only recently turned its attention to the databases of gas or oil natural emissions, while a possible loss of knowledge of geosites related to hydrocarbons has occurred. In recent years, some authors have recovered information about the occurrence and chemical composition of gaseous hydrocarbons bubbling in mud volcanoes. Present-day scientific literature has devoted its attention to hydrocarbon spontaneous emissions but part of the flowing methane and oil emissions have not been completely listed. A recovery attempt of the available historical and recent information on natural hydrocarbon emissions has been carried out, comparing them with the updated findings on the geological features of Italy. Surface hydrocarbon occurrences are represented by gas seeps, oil seeps and mud volcanoes. At times gas seeps are accompanied by cold or warm water springs due to gas interaction with less deep groundwater circulation paths. Mud volcanoes are mainly related to areas of tectonic compression characterized by thick sedimentary sequences. Their occurrence is limited to the continental Apenninic chain and Sicily. Some hydrocarbon seeps, sinkholes and mud volcanoes have been reported offshore within a few kilometers of the coast and their origin has been seen to be similar to continental hydrocarbon emissions. The mapping of most important gas emissions shows that the hydrocarbon domain is chiefly located in the core of the raised Apennine belt immediately behind the chain front at the boundary of and its related Plio-Quaternary foredeep, whereas CO2 emissions are located in the Apennine backdeep area. The geographic distribution of important gas accumulations in Italy does not show a highly significant correlation between surface seepages and the exploited reservoirs and it could also suggest the existence of other still unknown deep reservoirs or their small remnants difficult to be checked up. The majority of the hydrocarbon wells is characterized by biogenic gases, while thermogenic methane is predominant in surface seeping, confirming the sealed condition of most of the biogenic reservoirs and that they still have not experienced the complete evolution of organic matter towards the thermogenic terms induced by pressure and temperature