Testing clumped isotopes as a reservoir characterization tool: a comparison with fluid inclusions in a dolomitized sedimentary carbonate reservoir buried to 2-4 km

MacDonald, J. M. , John, C. M. and Girard, J.-P. (2018) Testing clumped isotopes as a reservoir characterization tool: a comparison with fluid inclusions in a dolomitized sedimentary carbonate reservoir buried to 2-4 km. In: Lawson, M., Formolo, M.J. and Eiler, J.M. (eds.) From Source to Seep: Geochemical Applications in Hydrocarbon Systems. Series: Geological Society, London, Special Publications (468). Geological Society of London, pp. 189-202. (doi: 10.1144/SP468.7)

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Constraining basin thermal history is a key part of reservoir characterisation in carbonate rocks. Conventional palaeothermometric approaches cannot always be used: fluid inclusions may be reset or not present while δ18O palaeothermometry requires an assumption on the parent fluid composition. The clumped isotope palaeothermometer, however, is a promising technique for constraining the thermal history of basins. In this study we test if clumped isotopes record temperatures of recrystallisation in deeply-buried dolomitic reservoirs, through comparison with fluid inclusion data. The studied reservoir is the Cretaceous Pinda Formation, offshore Angola, a deeply-buried dolomitised sedimentary carbonate hydrocarbon reservoir. It provides an ideal test case as samples from industry wells are available over a relatively wide burial depth range of ~2000-4000 metres below sea floor (mbsf) and the constituent dolomites are relatively homogeneous. Across this depth range, fluid inclusion homogenisation temperatures for the Pinda Formation record a range of temperatures from ~110-170 °C, increasing with depth. These closely match present-day ambient well temperatures, indicating recent resetting of the fluid inclusions. Clumped isotopes, however, record temperatures significantly (~20-60 °C) below fluid inclusion and well temperatures for the seven samples analysed. The deepest five samples (~2800-3700 mbsf) record clumped isotope temperatures around 100-120 °C, interpreted to represent a deep burial recrystallisation event responsible for a massive (re)dolomitisation of the reservoir. The lower clumped isotope temperatures (65 and 82 °C) of the shallower (2055 and 2740 mbsf) samples are interpreted to represent physical mixing of two dolomite generations due to incomplete burial recrystallisation of an early shallow dolomite. Determination of temperature through clumped isotopes allows calculation of the parent fluid δ18O values. In the five deepest samples, the fluid δ18O values of 3.7-6.5 ‰ cluster around the modern-day porewater composition (5 ‰) suggesting burial dolomitisation occurred in the presence of evolved brine. Mineral δ18O values of ~-4.5 to -7 ‰ are lower than pristine Cretaceous marine dolomite and are in accordance with burial recrystallisation. Clumped isotopes are therefore interpreted to record temperatures corresponding to open-system burial recrystallisation events. This study shows that clumped isotopes are a valuable tool in characterising the thermal history of deeply-buried (>2000 m) carbonate hydrocarbon reservoirs.

Item Type:Book Sections
Glasgow Author(s) Enlighten ID:MacDonald, Dr John
Authors: MacDonald, J. M., John, C. M., and Girard, J.-P.
Subjects:Q Science > QD Chemistry
Q Science > QE Geology
College/School:College of Science and Engineering > School of Geographical and Earth Sciences > Earth Sciences
Research Group:Solid Earth Research Group
Publisher:Geological Society of London
Published Online:14 December 2017
Copyright Holders:Copyright © 2017 The Authors
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher

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