Skip to Main Content
Book Chapter

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

By
John M. MacDonald
John M. MacDonald
1
Carbonate Research Group, Department Earth Science and Engineering, South Kensington Campus, Imperial College London, London SW7 2AZ, UK
2
Present address: School of Geographical & Earth Sciences, University of Glasgow, Gregory Building, Glasgow G12 8QQ, UK
Search for other works by this author on:
Cédric M. John
Cédric M. John
1
Carbonate Research Group, Department Earth Science and Engineering, South Kensington Campus, Imperial College London, London SW7 2AZ, UK
Search for other works by this author on:
Jean-Pierre Girard
Jean-Pierre Girard
3
TOTAL, Le Centre Scientifique et Technique Jean Féger (CSTJF), Avenue Larribau, 64018 Pau, France
Search for other works by this author on:
Published:
January 01, 2018

Abstract:

Constraining basin thermal history is a key part of reservoir characterization 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 recrystallization in deeply-buried dolomitic reservoirs, through comparison with fluid-inclusion data. The studied reservoir is the Cretaceous Pinda Formation, offshore Angola, a deeply-buried dolomitized 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 c. 2000–4000 m below seafloor (mbsf) and the constituent dolomites are relatively homogeneous.

Across this depth range, fluid-inclusion homogenization temperatures for the Pinda Formation record a range of temperatures from c. 110 to 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 (c. 20–60°C) below fluid-inclusion and well temperatures for the seven samples analysed. The deepest five samples (c. 2800–3700 mbsf) record clumped isotope temperatures of around 100–120°C, interpreted to represent a deep burial recrystallization event responsible for a massive (re)dolomitization 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 recrystallization 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 that burial dolomitization occurred in the presence of evolved brine. Mineral δ18O values of c. −7 to −4.5‰ are lower than pristine Cretaceous marine dolomite and are in accordance with burial recrystallization. Clumped isotopes are therefore interpreted to record temperatures corresponding to open-system burial recrystallization events. This study shows that clumped isotopes are a valuable tool in characterizing the thermal history of deeply-buried (>2000 m) carbonate hydrocarbon reservoirs.

Supplementary material: All standard and sample data are available at https://doi.org/10.6084/m9.figshare.c.3945184

You do not currently have access to this article.

Figures & Tables

Contents

Geological Society, London, Special Publications

From Source to Seep: Geochemical Applications in Hydrocarbon Systems

M. Lawson
M. Lawson
ExxonMobil Upstream Research Company, USA
Search for other works by this author on:
M.J. Formolo
M.J. Formolo
ExxonMobil Upstream Research Company, USA
Search for other works by this author on:
J.M. Eiler
J.M. Eiler
California Institute of Technology, USA
Search for other works by this author on:
Geological Society of London
Volume
468
ISBN electronic:
9781786203687
Publication date:
January 01, 2018

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal