25th International Meeting on Organic Geochemistry IMOG 2011

P-045
Occurrence and geochemical characteristics of fatty acids
bound with clay minerals in muddy hydrocarbon source rocks,
Bohai Bay Basin, Eastern China
Longfei Lu 1,2 , Tenger Borzijin 1,2 , Tianzhu Lei 3 , Jingong Cai 4 , Jie Wang 1,2
1 Sinopec Research Institute of Petroleum Exploration and Production, Wuxi, China, 2 Wuxi Institute of
Petroleum Geology, Sinopec Research Institute of Petroleum Exploration and Production, Wuxi, China,
3 Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou,, Lanzhou, China, 4 Tongji
University, Shanghai, China (corresponding author:llf779712@163.com)
Sorptive protection of clay minerals is thought to play
an important role in stabilizing organic matter in
hydrocarbon source rocks and marine sediments. In
order to explore adsorption sites of organic matter
binding to clay minerals and their influences on
organic matter stability, we focus on occurrence mode
of fatty acids in clay minerals and their degradation.
Clay minerals from two lacustrine hydrocarbon source
rocks (TOC, 6.8% and 9.1%; Ro, 0.5% and 0.7%), the
third member of Shahejie Formation, Tertiary, eastern
China, were subjected to sequential treatments
including soxhlet extraction, base hydrolysis and acid
hydrolysis, and three acids moieties were obtained. Xray
diffraction (XRD) and Gas Chromatography-Mass
Spectrometer (GC-MS) were employed to
characterize clay minerals and fatty acids,
respectively.
XRD results show that the d001 diffraction peak of
smectite of clay minerals is 1.434nm and 1. 416nm.
After chloroform extracted, the d001 peak is 1.434nm
and 1.416nm, after base hydrolysis is 1.315nm nm
and 1.331nm, and after acid hydrolysis are 1.273nm
and 1.227nm, respectively. It indicates that sorption
sites of fatty acids binding to clay minerals are of
great difference, ie three occurrences of fatty acids
sorption to clay minerals: free acids extracted by
soxhlet extraction occur in micropores of clay
minerals, ―OH - ‖ labile acids obtained by base
hydrolysis adsorb to stacking edges and/or outside
surface of clay minerals, and ―H + ‖ labile acids
obtained by acid hydrolysis intercalate interlayer of
smectite.
GC-MS analysis shows that fatty acids are enriched in
clay minerals, and fatty acids of the various fractions
are different in quatity, composition and distribution.
With respect to the degradation level of the acid
moieties, the following order was obtained: ―OH - ‖
labile acids > free acids > ―H + ‖ labile acids as
indicated by (i) Percentage composition of
unsaturated fatty acids, more susceptible to alteration,
is the highest in ―H + ‖ labile acids, followed by free
acids and ―OH - ‖ labile acids, (ii) Percentage of
branched acids of ―OH - ‖ labile acids > free acids >
―H + ‖ labile acids, (iii) i+a15:0/n15:0 and i17:0/n17:0
ratio of ―OH - ‖ labile acids (>1) > free acids > ―H + ‖
labile acids and (iv) CPIA of ―OH - ‖ labile acids < ―H + ‖
labile acids < free acids.
―OH - ‖ labile acids and ―H + ‖ labile acids were
transformed from adsorption of free acids by clay
minerals, which experience a higher degradation than
free acids since deposition and in low evolution stage.
With evolution increase, free acids experience an
intense degradation while ―OH - ‖ and ―H + ‖ labile acids
experience a limited degradation for clay minerals
adsorption protection, resulting degradation level of
bound acids near or even less than free acids. Since
our samples are in a low thermal maturity stage,
degradation level of ―H + ‖ labile acids has been less
than free acids. Moreover, ―OH - ‖ labile acids occur in
stacking edges of clay minerals, part of which are not
protected well by clay minerals, leading to its
degradation level a bit higher. In addition, part of free
acids encapsulated in clay matrixes (aggregation
pores) were also obtained by soxhlet extraction,
which reduces the general degradation level of free
acids.
Our results show that the ―OH - ‖ labile acids
experienced a substantial level of degradation, the
free acids appeared a limited degradation, and the
―H + ‖ labile acids was subjected a low level of
degradation and was preserved effectively. It is
because interlayer, stacking edges and aggregation
micropores of clay minerals can influence accessibility
of substrate to bacterial and rates of penetration of
dissolved oxygen in different level. The stability of
sedimentary organic matter is influenced by the type
of adsorption sites of clay minerals, which would
result in different degradation rate and preservation
during diagenesis and hydrocarbon generation
process.
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