Hydrothermal generation of hydrocarbons in basement rocks, Southern Tuscany

Abstract

Carbonaceous material in the form of graphitic carbon, amorphous carbon and liquid hydrocarbons occurs in the metamorphic rocks of Monti Romani in Southern Tuscany. Raman spectroscopic analyses show a contrast in structural ordering between carbon in the host rocks and carbon films and nodules at the contact with high temperature mineralized veins. Microscopy and gas chromatography additionally indicate liquid hydrocarbons, with a thermal maturity at the peak of the oil window. The association of hydrocarbons with high temperature fluids together with gas chromatographic and spectroscopic data indicate a probable hydrothermal origin of the oils, from the late Miocenic sediments that fill the Tafone Graben. A model is proposed in which hydrocarbons were generated along the fault that borders the Tafone Graben and then migrated toward the basements rocks at the footwall. The presence of 25-norhopane indicates biodegradation in the depth interval between about 100 and 1500 m. The hydrothermal generation of hydrocarbons could occur in other geothermal areas in Southern Tuscany where the presence of hydrocarbons has been reported but never fully explained.