25th International Meeting on Organic Geochemistry IMOG 2011

P-065
Molecular maturation of Bitumen-1 versus Bitumen-2: a case
study from the Oligocene Enspel Formation
Christian J. Illing 1 , Christian Hallmann 2 , Roger E. Summons 2 , Harald Strauss 1
1 Westfälische Wilhelms-Universität Münster, Münster, Germany, 2 Massachussetts Institute of Technology,
Cambridge, United States of America (corresponding author:c.illing@uni-muenster.de)
Sedimentary biomarkers can be utilized as a major
source of information on past organismic diversity and
environmental conditions. The bitumen that contains
these molecules is released from macromolecular
kerogen during thermal maturation and is typically
extracted from powdered sediments. Already in 1970
it was recognized that typically-extracted bitumen
does not constitute 100 % of the extractable organic
matter, and that a second bitumen, extracted after
digestion of the mineral matrix with mineralic acids, is
frequently characterized by a differing chemical
signature (Smith et al., 1970). This concept was
revived recently by Sherman et al. (2007) and
Hallmann (unpublished), who suggested that the
mineral-occluded portion of the bitumen (bitumen-2) is
significantly less endangered by contamination issues
than the free bitumen. Little is however known on the
origin and nature of this bitumen-2. Does it represent
a subset of bitumen, adsorbed to clay minerals; is it
unexpelled bitumen that is still tightly associated with
the kerogen? And why do chemical signatures
between bitumen-1 and bitumen-2 differ? Is this due
to contamination of bitumen-1 or do molecular
parameters evolve differently in these two organic
matter pools?
To answer some of these questions, we focused on a
natural maturity sequence. This project examined a
series of lacustrine sediments from the Oligocene
Enspel Formation (Germany) to increase our
understanding of the information contained in mineraloccluded
bitumen (bitumen 2). Enspel is a maar-like
lake whose sediments were covered by a lava flow.
This induced a thermal maturation sequence
decreasing from the top of the succession
(overmature) to very immature sediments a few
meters below the lava flow. The environmental
conditions and the geochemistry of those sediments
are well known (e.g. Lueninger & Schwark, 2002).
We analyzed 6 shale samples that span a
stratigraphic interval of 1.2m from the base of the lava
flow. Besides their interest for studies short-lived
thermal stress in respect to conservation of typical
biomarker ratios (Peters et al. 2005) these samples
permit comparisons of how the molecular signature of
bitumen-2 changes in comparison to that of bitumen-1
over the same stratigraphic interval.
The analytical approach generally followed a protocol
set up for biomarker analysis in Precambrian
sedimentary rocks (Hallmann et al. 2011), despite the
fact that the samples used here originated from
outcrop. The samples were first rinsed with organic
solvents to remove surface contaminants and
subsequently powdered, extracted with
dichloromethane/methanol (93:7). The first extraction
step used an ultrasonication method (100 mL solvent,
2x 15 min. at 50°C) that was repeated 6 times. To
ensure a maximal extract yield and a minimum
amount of remaining free bitumen, the samples were
subsequently extracted 13 times by ‗accelerated
solvent extraction (Dionex ASE 200). Subsequent to
this exhaustive extraction, samples were treated with
hydrochloric and hydrofluoric acid in Teflon bottles to
remove the majority of the inorganic mineral matrix.
Remaining heavy minerals, sulfides and neofluorides
were not removed to minimize alteration of the
organic matter. After the digestion, the kerogen
concentrate was extracted three times by
ultrasonication with hexane to obtain the mineraoccluded
bitumen (bitumen-2). Bitumen-1 was
fractionated in to saturated hydrocarbon, aromatic
hydrocarbon and polar fractions. Individual
compounds were analyzed by SIM and MRM
methods.
This data set shows interesting trends with regard to
the question on the differences between the two
bitumen fractions. Those results help to improve our
understanding of biomarker information contained in
bitumen II.
References:
Hallmann, et al. (2011) In: Topics in Geobiology 36
Lüniger & Schwark (2002) Sed. Geol
Peters et al. (2005) The Biomarker Guide
Smith et al. (1970) GCA
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