25th International Meeting on Organic Geochemistry IMOG 2011

P-037
Evolution of depositional environment and biological origin of
kerogen in Lower Miocene Cypris shale in the Eger Graben,
Czech Republic
Juraj Francu 1 , Ivana Sýkorová 2 , Bohdan Kříbek 6 , Karel Martínek 3 , Petr Rojík 5 , Achim
Bechtel 4 , Daniela Mácová 1
1 Czech Geological Survey, Brno, Czech Republic, 2 Institute of Rock Structure and Mechanics, CAS, Praha,
Czech Republic, 3 Charles University, Praha, Czech Republic, 4 Chair of Petroleum Geology,
Montanuniversität, Leoben, Austria, 5 Sokolovska uhelna a.s., Sokolov, Czech Republic, 6 Czech Geological
Survey, Praha, Czech Republic (corresponding author:juraj.francu@geology.cz)
Organic matter in a 75 m cored profile was
investigated in Lower Miocene lacustrine offshore
clays of the Cypris Formation. The purpose was to
identify organic proxies related to the environmental
changes induced by subsidence, lake level
fluctuation, climatic perturbation and bioproductivity.
The Sokolov Basin was situated in a failed rift of the
Eger Graben in the NW Czech Republic. A 30 m thick
brown coal seam produced in an open pit mine was
deposited during the Eggenburgian. It was typical by
periodical occurrence of sporinite rich layers and was
conformably overlain by up to 130 – 180 m thick
monotonous offshore lacustrine succession, the
Cypris shale of Eggenburgian to Karpatian age. At
present, the basin covers ca 20 km2 but the original
area was probably larger. Nearshore facies are rarely
preserved.
Depth (m)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
0 1 2
Pri/Phy
0 1 2
Pri/n-C 17
oxic
dysoxic
0 1 2 3 4 0 1 2
n-C19 /n-C31 n-C23/n-C31 Fig. 1. Isoprenoid and n-alkane ratios in the
Eggenburgian to Karpatian profile.
Based on the sedimentology, bulk rock data, and
biomarkers the profile provides a geochemical archive
of 19 to 17 Ma bp with several partial intervals related
to principal phases of basin evolution. The lower part
of the profile is characteristic by abrupt flooding of
older fluvial to overbank alluvial coal-bearing
sediments manifested by shale deposition. Kerogen
consists of redeposited humic matter with partly
decomposed plant tissues mixed with macrophytes
and minor algae. Pristane/phytane ratio suggests that
the water column became very soon dysoxic.
Increased n-C23/n-C31 indicates growth of aquatic
plants - macrophytes, their contribution diminished
with time. Sudden drop in lake level resulted in a thin
coal seam with high TOC and low hydrogen index
(HI). The algal bloom occurred in the middle and
upper part of the basin fill during the stagnant water
phase and resulted in HI as high as 900 mg/g TOC.
Additional two oxic events occurred in the later lake
evolution indicated by TOC and HI drop, increase in
Pri/Phy, Pri/n-C17 and lower algal molecular
signature. The final phase of lake evolution is marked
by occurrence of carbonate layer followed by
oscialting bioproductivity and organic matter
preservation.
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