Thrust Belts and Foreland Basins

Abstract

We describe the Miocene petroleum system in the context of the geology of the Northern Apennines as a system fed by multiple sources including some potential for deep oil accumulation. The presence of sources deeper than the Miocene reservoir is required by the high thermal maturity of the oils, the thermogenic nature of methane and the high ion content, in the reservoir brines, deriving from decaying organic matter. This is in contrast with the lower thermal maturity measured in the Miocene reservoir coupled with its low organic matter content. A Miocene secondary source, however, is required by the presence of a Tertiary organic marker in the oil. The deeper sources charged reservoirs of different age, geometry and sediment provenance, mostly as a function of stepwise migration of the foredeep and the overlying Ligurian units toward the foreland, which provided rapid overburden. The porosity of the reservoir was preserved in the anticlines mostly because of up-dip migration into early formed structures in the foredeep units. Therefore, the structural evolution of the area, especially the time interval between deposition and deformation of the foredeep units, is crucial for the definition of the quality of the reservoirs. Finally, the Quaternary reactivation of the thrust sheets in the foothills changed the geometry of the reservoirs, inducing new accumulations and/or dismigration from deeper and older traps.