Carbon isotope stratigraphy using carbonate cements in the Triassic Sherwood Sandstone Group: Corrib Field,west of Ireland |
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| Institution: | 1. Ministry of Education Key Laboratory for Coast and Island Development, Collaborative Innovation Center of South China Sea Studies, School of Geographic and Oceanographic Sciences, Nanjing University, Nanjing 210093, PR China;2. State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Resources, Collaborative Innovation Center for Exploration of Strategic Mineral Resources, China University of Geosciences, Wuhan 430074, PR China;3. State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, PR China;1. A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia;2. Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia;1. Cambridge Carbonates Ltd, 1 rue de varoux, 21120 Marey sur Tille, France;2. GEOPS, Univ. Paris-Sud, Université Paris Saclay, Rue du Belvédère, Bât. 504, 91405 Orsay cedex, France;3. Sorbonne Universités – UPMC Univ. Paris 06, Institut des Sciences de la Terre de Paris, ISTeP UMR CNRS 7193, 4, place Jussieu, F-75005, Paris, France;4. UMR 6282 Biogéosciences, Univ. Bourgogne Franche-Comté, CNRS, 6 Bd Gabriel, 21000, Dijon, France;1. The Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, China;2. The Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia;3. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;4. Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan;5. Beijing SHRIMP Center, Chinese Academy of Geological Sciences and Department of Geosciences, University of Mainz, Germany |
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| Abstract: | Carbon stable isotopes from carbonate minerals (mainly dolomite) from six wells from the Lower Triassic Sherwood Sandstones of the Corrib Gas Field, Slyne Basin, west of Ireland, allow stratigraphic correlation. The results also provide information on palaeoenvironmental change during the deposition of these continental redbed sedimentary rocks. The Triassic reservoir rocks have been buried to > 4000 m and heated to > 165 °C and now contain methane-rich gas. Although the oxygen isotopic signal has been at least partially reset during burial and heating, a primary carbon isotopic signal appears to have survived diagenesis. The carbon isotope ratio varies from ? 3.2‰ to + 2.1‰. All six wells show similar stratigraphic changes when all the carbon isotope data are plotted relative to a major playa horizon. δ13C increases from about ? 3‰ at the base of the Sherwood to about + 2‰ 170 m above the base. δ13C then decreases to about ? 2‰ for the next 70 m and remains steady for the following 50 m. The top 20 m of the Sherwood contains carbonate with a δ13C values decreasing to about ? 3‰. The occurrence of a stratigraphically-correlatable carbon isotope pattern implies that the primary evolution signal has been preserved. The change in δ13C correlates with indicators of aridity and biological stress such that the highest δ13C values are in sedimentary rocks deposited in a playa lake (arid times); these rocks contain the greatest quantity of dolomite cement. Conversely, the lowest δ13C values correspond to sedimentary rocks deposited from well-developed rivers (relatively humid times) from the lowest quantity of dolomite cement. The same carbon isotope evolution has been found in another well in the Slyne basin and in Belgium, suggesting that the palaeoenvironmental isotope signal in the Triassic sedimentary rocks of the Corrib Field may have a regional significance. |
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