25th International Meeting on Organic Geochemistry IMOG 2011

P-286
Comparison of natural gases in Middle Cambrian reservoir with
hydrous pyrolysis gases from Lower Palaeozoic source rocks
from the Polish, Lithuanian and Estonian parts of Baltic Basin
Maciej Kotarba 1 , Michael Lewan 2
1 Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology,
Krakow, Poland, 2 US Geological Survey, Denver, United States of America (corresponding
author:kotarba@agh.edu.pl)
The Baltic Basin is a large NE-SW trending
depression on the NW margin of the Precambrian
East European Craton. In the onshore and offshore
Polish Baltic Basin, seven small oil and five gascondensate
accumulations were discovered in the
years1970-1999. In Lithuania, the first oil discovery
was made in 1960 and oil exploitation was started in
1990. In Estonia, no oil and gas deposits have been
discovered to date, but considerable kukersite oil
shale resources occur. A few potential source rock
horizons are present in the Lower Palaeozoic
(Cambrian, Ordovician and Silurian) section. The
Upper Cambrian-Tremadocian strata contain the best
source-rocks with low-organic sulphur, oil-prone
Type-II kerogen and are considered the most likely
source of these offshore petroleum accumulations.
Kukersite contains lacustrine Type-I kerogen.
The objective of this study is to determine
the origin of the natural gases dissolved in oil and in
gas-condensate accumulations of Middle Cambrian
sandstone reservoir of the Polish and Lithuanian parts
of Baltic Basin. The study involves molecular and
stable-isotope characterization of natural gases and
gases generated from Upper Cambrian-Llandovery
source rocks by hydrous pyrolysis. Hydrous pyrolysis
experiments were conducted for 72 h at 330 °C and
355 °C on seven thermally immature samples
representing Upper Cambrian (1 sample),
Tremadocian (3 samples), Llandovery (2 samples)
shales and Upper Ordovician kukersite (1 sample)
from Poland and Estonia. Fifteen natural gas samples
were collected from Middle Cambrian sandstone
reservoir in the Polish Baltic basin and five samples
from Lithuanian Baltic basin. Molecular compositions
of the gases and stable carbon isotope analyses of
methane, ethane, propane, butanes, pentanes and
carbon dioxide, stable hydrogen isotope analyses of
methane and stable nitrogen isotope analyses of
gaseous nitrogen were analysed.
Results of the investigation indicate that
there exist distinct δ 13 C signatures but non-distinct
molecular compositions of gases generated by
hydrous pyrolysis of the representative Upper
Cambrian-Tremadocian complex. In particular, the
trend between δ 13 C of methane, ethane, propane,
butanes and pentanes and their reciprocal carbon
number is not always linear as prescribed by some
investigators. Instead, a ―dog-leg‖ trend may exist,
which makes prediction of the δ 13 C of the source rock
kerogen problematic. Experimentally determining this
trend and establishing how it changes during
petroleum generation provides a correlation
parameter that can help differentiate between end
members of thermogenic and microbial gas input in
natural gas accumulations. This approach cannot
account for the isotopic changes that may incur during
migration and in-reservoir alterations, but it does
provide an isotopic signature of the primary gas
generated from a source rock at pre-oil-cracking
thermal maturities.
Isotopic characterization of natural gases as
compared to hydrous pyrolysis gases revealed that
these natural gases were generated by microbial and
thermogenic processes from oil-prone Type-II
kerogen contained in Upper Cambrian-Tremadocian
source rock complex. Subsequently, the gases have
undergone migration and mixing of microbial and lowtemperature
thermogenic gases. The most significant
microbial component was found in the B-3 offshore oil
and gas field. The traps within the Middle Cambrian
reservoir had already formed in middle Ordovician
through early Silurian time, when migration of
microbial methane took place. The microbial methane
was generated from immature organic matter
contained in the Upper Cambrian-Tremadocian
source-rock complex. Oil and thermogenic gas
occurred later and charged the same traps containing
the microbial gas. Carbon dioxide and nitrogen gases
are considered to be thermogenic from the source
rocks with some possible contributions from the crust.
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