25th International Meeting on Organic Geochemistry IMOG 2011

P-095
Natural petroleum fractionation – what do we really mean?
Chris Cornford
IGI Ltd, Bideford, United Kingdom (corresponding author:chris@igiltd.com)
Fractionation is defined as the separation of a mixture
into some or all of its component parts. Petroleum can
be considered a mixture in terms of molecular types
(homologues), carbon number (‗size‘ of molecule) &
stable isotopic composition (typically carbon +
deuterium). Petroleum fractionates at a number of
points during the life of a petroleum system: during
generation from kerogen in the source rock, during
expulsion from the source, migration and phase
separation on the way to a primary trap, during
remigration from trap to trap (stacked reservoirs or
Gussau displacement), and finally to survival within
the reservoir to the present day (various alteration
processes). At low temperatures biochemical
(bacterial) processes can also lead to isotopic
fractionation. The re-mixing of fractionates (e.g.
thermogenic with biogenic methane or fresh with
biodegraded oil) is also an additional complication.
Processes leading to fractionation at the bulk,
molecular and isotopic levels are listed in Table 1.
Table 1: A summary of processes potentially
leading to compositional or isotopic fractionation.
Term used Process
Pressure (buoyancy)
Effect (proven
or predicted)
Migrational
fractionation
gradient leading to
fluid flux of a single
phase through low
permeability/high
tortuosity porosity or
fractures
Strong light
molecular and
isotope
fractionation 2
Adsorption
geochromatography
Migrating petroleum
molecules partition
between fluid phase
and solid kerogen &
minerals
Strongest for
migration
through coals;
gas isotopic
fractionation?
Phase
separation 1
P-T control - starts at
bubble or dew point
boundary and
continues
progressively with
up-dip migration
Strong bulk,
medium
molecular but
little isotopic
fractionation 2
Diffusion 1
From high to low
concentrations so
requires gradient
Transient
compositional
effects, esp. for
Water
washing
Requires active flux
of external fluid
gases 4
Strong
compositional
Gas
washing/
stripping 1
Biodegradation
Biogenic
gas
Gravity
segregation
(water +/- dissolved
solids i.e. ‗salinity‘)
Requires active flux
of external fluid (gas,
typically methane but
could be CO2, N2)
Preferential bacterial
attack on � 13 C3
(residue) and isotopic
fractionation of CH4
and CO2 products
(fractionate)
Biochemical
fractionation of CH4
(a bacterial metabolic
waste product) with
respect to substrate
(proto-kerogen)
‗Heavy‘ molecules
separate to base of
accumulation or
reservoir
but minor
isotopic
fractionation 3
Strong
compositional
but little
isotopic
fractionation 5
Bulk &
molecular
changes +
isotopic
fractionation of
gas & CO2
Isotopic
fractionation
only since
product is pure
methane 6
Mostly bulk
and molecular
fractionation;
can be seen for
gases
1 a confusion of these processes is commonly termed
‗evaporative fractionation‘; fractionation it is but
evaporation it is not! 2 common to most oils; 3 examples
from foredeep basins; 4 examples from top and base
seal of deltaic sands; 5 stacked reservoir sands; 6 can
be mistaken for products of diffusional fractionation
The presentation reviews the geochemical literature,
quantifies these processes using simple physical
chemistry, and shows that the results of the
calculations agree with composition and isotope
gradients observed in exploration and production
wells.
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