Origin and significance of hydrocarbons in CO2-rich gases from Central Italy seismic areas

Abstract

(Figure presented) Tectonically active areas of Central Italy are marked by intense CO2 degassing, whose origin and role in earthquake processes are fundamental questions in geoscience. This study investigates the origin and geological controls on the geochemistry of light hydrocarbons from CO2-dominated gas emissions located in the inner sector of the Umbria-Marche Apennines (Central Italy), aiming to better understand the sources and migration pathways of geogenic fluids in the region. Our findings indicate that light hydrocarbons are predominantly thermogenic, with negligible abiotic contributions. We demonstrate that Mesozoic carbonate rocks are the primary source across the study area, though conditions of hydrocarbon formation and migration vary. Specifically, higher temperatures and open-system conditions prevail in the southern regions, likely due to thermal stress associated with Quaternary magmatism. We propose that light hydrocarbons form at crustal depths (≤5–6 km) and are transported to the surface by ascending CO2 from deeper sources. Finally, this work highlights that hydrocarbon geochemistry, combined with helium isotopes, can provide insights for reconstructing the circulation and origin of fluids in crustal reservoirs and assessing the thermal regime in tectonically active areas.