25th International Meeting on Organic Geochemistry IMOG 2011

P-347
Strategies for the assessment of fluid mixing in petroleum
systems
Stefanie Pötz, Andrea Vieth-Hillebrand, Heinz Wilkes
GFZ German Research Centre for Geosciences, Potsdam, Germany (corresponding author:poetz@gfzpotsdam.de)
The issue of fluid mixing is of increasing importance
for the assessment of petroleum systems. It is now
regarded as one of the key controls on petroleum
composition, properties and quality, since essentially
all petroleums are mixtures with different components
charged from source rocks at different temperatures
[1].
Reservoired oils may derive from mixing of
petroleums expelled from similar organic facies
source rocks at different maturities, from mixing of
petroleums from two (or more) different source rocks,
from mixing of petroleums with different levels of
secondary alteration, in particular biodegradation, as
well as from any combination of the aforementioned
types. Mixing processes are mainly driven by
differences in fluid density and composition and
limited by reservoir compartmentalisation.
Additionally, migration contamination in the carrier
may be a further mechanism of petroleum mixing.
Mixing has crucial but so far widely overlooked effects
on geochemical tools conventionally used in
petroleum system evaluation such as oil-oil- and oilsource-correlation
or maturity assessment. With
regard to multiple-sourced petroleum reservoirs,
many studies focus on the identification of the
different sources based on biomarker and/or carbon
isotopic compositions of crude oils or core extracts.
Geochemical approaches to quantify the contribution
from each source rock depend on calculations using
relative peak ratios from gas chromatography or
absolute concentrations. Relative peak ratios are now
considered unreliable due to non-linear mixing
behaviour. For most of the applications described in
the literature, which are based on absolute
concentrations, end-member oils of each source rock
have to be available for geochemical treatment. This
is crucial because the availability of the correct endmember
oils is an extremely unlikely case in real
world petroleum exploration. The quantitative
determination of the contribution of mixing petroleums
to different component fractions of the resulting fluid is
even less well constrained. With this in mind improved
and reliable tools are required to assess the
relevance and effects of petroleum mixing during
petroleum system evaluation.
Inconsistencies in petroleum composition considering
different component fractions of the oil which may
have been contributed from different mixing
petroleums to a different extent are the main criterion
to identify mixed oils. An example is the determination
of different maturities for a single oil from parameters
based on light hydrocarbons, aromatic hydrocarbons
and biomarkers. In general, trace components are
less reliable for the assessment of mixing than main
components. Likewise, components varying in
concentration over several orders of magnitude are
less reliable than those varying over only one (or two)
order(s) of magnitude. The often used biomarker
approaches overestimate the proportion of the
contributing petroleum with the higher biomarker
concentrations. So far no generalisable procedures
are known that allow an unequivocal assessment of
mixing in natural petroleum fluids.
In this study mixing was investigated for crude oils of
3 different study areas (Offshore West Africa,
Offshore Norway, Western Desert in Egypt) with the
focus on quantitative compositional differences of
main and trace compounds and the carbon and
hydrogen stable isotopic composition of individual
components within the crude oils. In addition, artificial
mixing experiments have been done to evaluate linear
and nonlinear mixing behaviour and the influence of
differences in the absolute concentration of
compounds in the contributing oils on the ―endconcentration‖
in the mixed petroleum. This
contribution will discuss the implications for the
assessment of mixing processes and the use of
geochemical tools in petroleum exploration.
References
[1] Wilhelms A. and Larter, S., In: Cubitt, J.M.,
England, W.A., Larter, S.R. (Eds.), Understanding
Petroleum Reservoirs: Towards an Integrated
Reservoir Engineering and Geochemical Approach.
2004. Geological Society, London, pp. 27-35.
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