A genetic model of hydrocarbon-derived carbonate chimneys in shelfal fine-grained sediments: The Enza River field, Northern Apennines (Italy)

Methane Derived Authigenic Carbonate (MDAC) concretions have been recently exposed in the Pleistocene Argille Azzurre Fm. outcropping along the Enza riverbanks, Northern Apennines foothills, Italy. The relations between the sediment deposition, the coeval tectonic deformation and the MDAC concretions have been investigated to unravel the mechanism of methane migration through the sediments and the processes leading to MDAC occurrence. The biogenic methane responsible for the formation of MDAC chimneys and slabs has been generated in the organic-rich Pliocene sediments located in the Po Plain subsurface. The gas migrated up-dip towards the Northern Apennines foothills. The MDAC cements are mainly composed of dolomite, whose precipitation requires thousand years and the absence of SO4 in the pore fluids. According with the sedimentation rates of the MDAC-hosting Pleistocene succession, the Sulphate Methane Transition Zone migrated upwards faster than time required for dolomite precipitation. Therefore, the connate water migrating together with the methane could have acted as second DIC source, allowing the MDAC formation also in the methane-rich zone.