Geochemistry and habitat of natural gases in Italy

Abstract

Systematic chemical and isotopic analyses were carried out on natural gases from 92 fields which represent more than 90% of the original gas reserves of Italy. A quantitative assessment of the various genetic gas types indicates that most of the gaseous hydrocarbons (80%) were generated through bacterial activity and/or diagenetic processes while only a minor amount (20%) was produced by thermal degradation of organic matter. The formation of the gas accumulations was geologically controlled by the Neogene tectonic phases and by the sedimentary events associated with the growth of the Apennines and Southern Alps. In fact, about 80% of Italian natural gas reserves have been found in the Apennine foredeep while the remaining part is equally distributed in the folded belt and in the foreland. Furthermore, the influence exerted by tectono-sedimentary characteristics is emphasized by the fact that thermal degradation of organic matter was the only process responsible for most of the gas generated in the Apennine folded belt (97% thermogenic gases). On the other hand, the high rates of subsidence, turbiditic sedimentation and very early compressional tectonics in the foredeep favored processes leading to the accumulation of very large quantities of biogenic gas (82%). Moreover geochemical and geological data as well as mathematical modelling demonstrate that migration and accumulation of Italian natural gases mainly occurred later than 5 MY ago and, in some cases, these phenomena are still active. In fact, in the multipay zone gas fields of the Northern Adriatic, a kind of steady equilibrium is reached between loss of gas through the thin cap rocks (often less than 1 m) and continous replenishment from the bacterial fermentation and/or low temperature thermochemical reactions.