25th International Meeting on Organic Geochemistry IMOG 2011

P-016
Iron isotopic compositions of crude oils – development of a new
extraction method and first results
Christian Ostertag-Henning 1 , Ulia Hammer 1 , Friedhelm von Blanckenburg 2
1 Bundesanstalt fur Geowissenschaften und Rohstoffe, Hannover, Germany, 2 Deutsches
Geoforschungszentrum GFZ, Potsdam, Germany (corresponding author:Christian.Ostertag-
Henning@bgr.de)
Metals in oils may contain valuable information about
the redox conditions during the deposition and early
diagenesis of the source rocks (e.g. [1]). But generally
only data of a limited number of metals - mainly V
and Ni - are used for the interpretation and for
correlation purposes. As [2] stated for a series of oils
from the German Molasse basin, ―in contrast to the
contents of S, V, Co, Ni and Se, those of Cr, Fe, Zn,
Br and I show strong oscillations within the different
oil fields.‖ – even if these oil fields belong to the same
oil family. This invokes the importance of in-reservoir
alteration of oils on the content of some metals. For
Kuparuk oil field on the North Slope of Alaska, [3]
noted a close covariance of strong biodegradation of
the oils with the occurrence of glauconite and hence
Fe III -rich intervals in the reservoir. They argue for an
anaerobic biodegradation mainly by ferric-iron
reducing bacteria.
To unravel potential information of the iron isotopic
composition about the extent of biotic and abiotic
processes involving the reduction of iron and
concomitant oxidation of hydrocarbons a method for
the determination of the iron isotopic composition of
crude oils and asphaltenes was established. The acid
digestion of samples cleaned of traces of formation
water and inorganic particles was performed in
microwave reaction vessels, the purification of the
resulting dissolved iron was ensured by ion-exchange
chromatography and coprecipitation (cf. [4]). The
complete extraction of iron from the hydrocarbon
phase or the asphaltenes was verified using standard
NIST 1084a. The concentrations of iron and other
metals have been measured by ICP-OES, the isotopic
composition of iron by MC-ICP-MS.
The iron isotopic composition of different oils
analyzed encompasses a very large range of values
up to +0.8 permil � 56 / 54 – a remarkable high value (cf.
[5]). One likely explanation for the strong enrichment
in 56 Fe is the dissimilatory iron reduction by bacteria in
the reservoir. Additional information on processes
during secondary migration might be revealed if
analysis of related oilfield waters are available – as [6]
have discussed.
Fig. 1: Range of iron isotopic compositions of sediments and
diverse biological materials – and the investigated crude oil
samples.
References
[1] Lewan, M. (84) Geochim. Cosmochim. Acta 48: 2231-8.
[2] Ellrich, J.; Hirner, A. & Stärk, H. (85) Chem.Geol. 48:313-
23.
[3] Holba, A.; Wright, L.; Levinson, R.; Huizinga, B. &
Scheihing, M. (04) In: Cubitt et al. (ed.): Understanding
Petroleum Reservoirs: Towards an integrated reservoir
engineering and geochemical approach. Geol. Soc. Spec.
Publ. London, 237: 53-88.
[4] Guelke, M. & von Blanckenburg, F. (07) Envir. Sci. and
Technol. 41: 1896-1901.
[5] Dauphas, N. & Rouxel, O. (06) Mass Spectrom. Rev. 25:
515-50.
[6] Busigny, V. & Dauphas, N. (07) EPSL 254:272-87.
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