Shallow-water hydrocarbon seepages in the Pleistocene Argille Azzurre Formation: the chimneys field of Enza river, Northern Apennines, Italy

The geologically young Northern Apennines contain various traces of hydrocarbon seepages, which are linked to fluid migration triggered by the tectonic evolution of the chain and the foredeep. The most known examples, related to comparatively deep water conditions (> 300 m) are clustered in the late Miocene and early-middle Pliocene successions of the region, while their occurrence in shallow water (shelf) settings is very rarely reported. A case in point is represented by the palisade chimneys field recently exhumed along the Enza river banks in the Northern Apennines (Reggio province). Here, a number of modally vertical, metric-high columnar chimneys ranging in diameter from 10 to 35 cm, affect the Argille Azzurre Fm, biostratigraphically dated at the Early Pleistocene (nannoplancton zone MNN19). Such columnar concretions are clearly formed by the moderate cementation of the pelitic sediments. Macropaleontological content (mainly bivalves) documents that this unit was deposited in an open marine muddy shelfal setting in a bathymetry range of about 50-70 m. Within the Enza river Argille Azzurre succession some biogenic gas leakages still occur. Preliminary studies suggest that the methane migration can be related to the geologic setting of the foothills in this area, which shows a gentle compressive deformation. On the contrary, numerous hydrocarbon seepages along the northern Apennines foothills are characterized by thermogenic methane generated at depth and presently trapped within Tertiary reservoirs. This suggests to take into account the different geologic context as the main control on gas generation and migration. The on-going research is focused upon the petrographic, geochemical and biomaker characteristics of these chimneys to unravel the typology of fluids involved in their formation, their concretioning modality and the fluid expulsion history.