25th International Meeting on Organic Geochemistry IMOG 2011

P-372
Transformation of organic matter of sedimentary rocks in model
experiment with the flow of supercritical CO2-fluid
Sara Lifshits, Olga Chalaya
Institute of Oil and Gas problems SD RAS, Yakutsk, Russian Federation (corresponding
author:s.h.lifshits@ipng.ysn.ru)
According to our model of oil generation the
sedimentary rocks with a dispersed organic matter
under the conditions of oil window can be regarded as
a natural ―mechanochemical reactor,‖ which is
triggered for oil production by the flow of plutonic
fluids in the supercritical (SC) state [1]. The SC fluid
evidently performs the function of not only a solvent,
but also a medium in which chemical reactions occur.
The fragments of kerogen molecules and bituminous
substances entrained by the SC fluid will experience
very strong deformation stresses because they are
comparable in size to the micropores and microcracks
of petroleum mother rocks (5–10 nm). This
mechanical energy will probably suffice for C–C bond
cleavage in high molecular substances; i.e., for
initiating mechanochemical reactions. We can thus
assume that solution and migration in a flow of SC
fluid are accompanied by mechanochemical
transformations of DOM and form the light-volatile
components of oil.
To check this oil formation model, we performed
experiments to study the possibility of hydrocarbons
transferred and transformed in a SC fluid medium.
The samples used for supercritical fluid extraction
(SCFE) included bituminous limestone and clay sand
soil. As a supercriticall medium, we chose CO2
because it is one of the main components of plutonic
fluids. Hot chloroform extraction of the samples was
performed simultaneously with SCFE.
The presence of relict hydrocarbons in the
supercritical extract and the distribution of the
individual hydrocarbons close to that in the starting
rock indicate that the supercritical fluid can dissolve,
transfer, and accumulate petroleum like organic
substances. The organic substance of the
sedimentary rock sample under study has already
been transformed in dia- and catagenesis. As a
consequence, the transformations that can occur in
the micropores and microcracks of sedimentary rocks
in the SCFE are less pronounced. To study all
possible transformations, we performed an
experiment on SCFE of a soil sample because soils
contain a geochemically immature organic substance.
The extract was dissolved in hexane and studied by
GC/MS. Figure 1 presents the mass fragmentograms
(m/z=57) of the hydrocarbons fractions of the
hydrocarbon fractions of the chloroform and
supercritical extracts of the clay sand samples. The
compositions of the hydrocarbon fractions of the
supercritical and chloroform extracts differ
substantially. The distribution of the individual
hydrocarbons in the SC extract became close to that
for oil contaminated soils. Thus, the ratio of the odd to
even normal alkanes in the supercritical extract was
1.5 versus 5.2 in the chloroform extract. The ratio of
relatively low molecular n-alkanes to high molecular
ones was twice higher (0.32%) than the ratio in the
chloroform extract (0.15%). The amount of isoalkanes
in the supercritical extract (43.0%) was almost eight
times larger than that in the chloroform extract (5.5%).
The maximum of n-alkanes shifted from the high
molecular region nC31 to the medium molecular one
nC25.
Fig. 1. Mass-fragmentograms (m/z = 57) of the alkane
hydrocarbons of the (a) chloroform and (b) supercritical
extracts of soil.
The data obtained suggest that the supercritical
extraction is accompanied by a possibly
mechanochemical transformation of the natural
organic substance. As a result, the composition,
structure, and distribution of individual hydrocarbons
of the supercritical extract change toward
geochemical maturity characteristic of petroleum
mother rocks [2]. The formation of oil hydrocarbons in
nature can thus result not only from the evolution
changes in the organic matter of sedimentary rocks in
dia- and catagenesis, which lasts for millions of years,
but also probably from fast mechanochemical
transformation in a flow of the supercritical fluid,
where the micropores and microcracks of petroleum
mother rocks play the role of nanomechanical
reactors. The proposed mechanism allows the time
scale of oil field formation to be revised from millions
to possibly hundreds of years.
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