Fluid sources and stable isotope signatures in authigenic carbonates from the Northern Apennines,Italy |
| |
| Institution: | 1. Department of Applied Geology, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia;2. Department of Earth Sciences, University of Western Australia, 35 Stirling Highway, Crawley, Perth, WA 6009, Australia;3. Australian Research Council Centre of Excellence for Coral Reef Studies, School of Biological Sciences, University of Queensland, Brisbane, Queensland, Australia;4. School of Earth Sciences, University of Queensland, Brisbane, Queensland 4072, Australia;1. Florida Institute of Technology, Physics and Space Sciences Department, Melbourne, Florida, 32901, USA;2. Florida Institute of Technology, Systems Engineering Department, Melbourne, Florida, 32901, USA |
| |
| Abstract: | Authigenic carbonates are frequently associated with methane cold-seep systems, which extensively occur in various geologic settings worldwide. Of interest is the relation between the fluids involved in their formation and the isotopic signals recorded in the carbonate cements. Along the Northern Apennines foothills (Italy), hydrocarbons and connate waters still seeping nowadays are believed to be the primary sources for the formation of fossil authigenic carbonate found in Plio-Pleistocene marine sediments. Four selected outcrops of dolomitic authigenic carbonates were analysed to compare signature of seeping fluids with fractionation of stable carbon and oxygen isotopes recorded in the carbonate.Along the foothills, deep methane-rich fluids spontaneously rise to the surface through mud volcanoes or are exploited in wells drilled nearby to the fossil Plio-Pleistocene authigenic carbonates. The plumbing system providing fluids to present-day cold seeps was structurally achieved in Late Miocene and Plio-Pleistocene. δ13C values of methane, which vary from −51.9 to −43.0‰ VPDB, indicate that gas composition from the deep hydrocarbon reservoirs is relatively uniform along the foothills. On the contrary, δ13C in fossil authigenic carbonates strongly varies among different areas and also within the same outcrop.The different carbon sources that fed the investigated carbonates were identified and include: thermogenic methane from the deep Miocene reservoirs, 13C-enriched CO2 derived from secondary methanogenesis and microbial methane from Pliocene successions buried in the Po Plain. The δ13C variability documented among samples from a single outcrop testifies that the authigenic carbonates might represent a record of varying biogeochemical processes in the hydrocarbon reservoirs. The sources of stable oxygen isotopes in authigenic carbonates are often ascribed to marine water. Oxygen isotopic fractionation in the dolomite cements indicates that marine pore water couldn't be the sole source of oxygen. δ18O values provide a preliminary evidence that connate waters had a role in the carbonates precipitation. The concomitant occurrence of active cold seepages and fossil record of former plumbing systems suggests that generation and migration of hydrocarbons are long-lasting and very effective processes along the Northern Apennines foothills. |
| |
| Keywords: | Authigenic carbonates Cold seep Oxygen and carbon stable isotopes Petroleum system Methane migration Connate water |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
