The geologically young Northern Apennines contain various natural seeps documenting a variability of reservoirs and fluids in terms of origin, age and evolution. Hydrocarbon and saline water seepages are controlled by the structures of the chain and the foredeep. In the geological record, the witness of these natural seepages are represented by authigenic carbonates. Their isotopic content allows to understand the carbon source and the processes of carbonate precipitation. The better known regional examples of Methane Derived Authigenic Carbonates (MDAC) and chemoherm build-ups, clustered in the Upper Miocene and Lower-Upper Pliocene successions, are formed in deep water (> 300 m) sediments. Occurrences in shallow water (shelf) settings have never been reported. Our case study describes the palisade chimneys field, recently exposed along the Enza riverbanks in the Northern Apennines in the Argille Azzurre Formation (blue mudstones) of Pleistocene age. The paleontological content (mainly bivalves) documents that this unit was deposited in an open marine muddy shelfal setting, in a bathymetric range of about 50-70 m, with local anoxic condition, documented by benthic foraminifer assemblages. The succession is biostratigraphically dated at the Early Pleistocene (Calabrian, nannoplancton zone MNN19e). A number of subvertical, metric-high columnar chimneys, ranging in diameter from 10 to 35 cm, in association with discontinuous planar carbonate concretions, intervening along the bedding planes, outcrop within the mudstones. Such concretions are clearly formed by the moderate cementation of the hosting pelitic sediments. The tubular concretions show central conduits running along the whole length, locally filled by sediments. The grain size within the concretions is homogeneous, both in the chimneys and planar concretions and irrespective of the bedding planes, suggesting a process of transport and homogenization prior to the cementation. Within the Enza river succession, some gas leakage still occurs, showing δ13C isotopic values of about -70‰, indicating a biogenic origin of the methane, generated within the Plio-Pleistocene mudstones. On the contrary, numerous hydrocarbon seepages along the same Northern Apennines foothills are characterized by thermogenic methane, generated at greater depth and presently trapped within Tertiary reservoirs. This documents the role of the different geologic context as the main control on gas generation and migration. The on-going research is focused on the petrographic, geochemical and biogeochemical characteristics of these concretions to unravel the different fluids involved in their formation, their concretioning modality and the fluid flow history.
|Publication status||Published - Dec 2012|
|Event||2012 AGU Fall Meeting - San Francisco, United States|
Duration: 3 Dec 2012 → 7 Dec 2012
|Conference||2012 AGU Fall Meeting|
|Period||3/12/12 → 7/12/12|