The role of post-orogenic normal faulting in hydrocarbon migration in fold-and-thrust belts. Insights from the central Apennines, Italy

The reconstruction of the hydrocarbon migration and entrapment history is fundamental for reducing risks and identifying traps and migration pathways during oil exploration and exploitation in fold-and-thrust belts, which host 14% of the world's discovered oil reserves. Within fold-and thrust belts, faults and folds have fundamental roles in controlling hydrocarbon migration, entrapment, and/or leakage. Past studies have mainly focused on the role of syn-orogenic thrusts, tear faults, and folds in controlling hydrocarbon migration and entrapment. On the contrary, little attention has been dedicated to the role of normal faults in hydrocarbon remobilization during the post-orogenic extension. We combine field and subsurface data with geological evidence from the Ripi oil field, central Apennines (Italy), and propose a conceptual model for hydrocarbon remobilization along post-orogenic normal faults and potential entrapment within shallow reservoirs. Results show that post-orogenic normal faults promoted up-dip hydrocarbon remobilization from oil traps generated during fold-and-thrust belt development. Where pre-orogenic deposits are still covered by syn-orogenic deposits, hydrocarbons moved up-dip along post-orogenic normal faults and impregnated high-porosity sandstone lenses, generating discontinuous and heavy oil reservoirs. On the contrary, where syn-orogenic deposits were eroded in response to orogenic uplift, hydrocarbons leaked at the Earth's surface. Results from this study can be applied to reduce risks during hydrocarbon exploration and appraisal in similar tectonic settings, such as Hellenides and Carpathian fold-and-thrust belts, as well as the southern Apennines of Italy, where the Tempa Rossa and Monte Alpi oil fields, two of the largest onshore oil fields in Western Europe, were discovered in the last decades.