Geological Evolution and Resources of the Baltic Sea Area from the Precambrian to the Quaternary - The Baltic Sea Basin - page 41

The kerogen of all mentioned shales can be mainly classified as type II.
Irrespective of source rock age, kerogens show a rather uniform trend on the
T max -hydrogen index plot (Fig. 2.18 ) . Organic maturity increases from the east to
the west, exceeding reflectance values of 2.0% Ro in the western part of the Polish
offshore and 4.5-5.0% Ro in the Rügen area (Hoth 1997 , Hoffmann et al. 2001 ) .
2.7.4 Oil and Gas Generation
One-dimensional modelling of HC generation was carried out for selected offshore
wells. Thereby burial history was calibrated with sonic and density well log data
(Šliaupa et al. 2002a ) and organic maturity data (Brangulis et al. 1993 , Buchardt
et al. 1997 ) .
The burial reconstruction indicates that maximum burial depth exceeded 4.5 km
in the southwestern Baltic basin by the end of the Devonian (Fig. 2.19 , well A8-1).
Hydrocarbon generation started during Late Silurian time, the period with the maxi-
mum subsidence rate. Cambrian and Ordovician source rocks lost their hydrocarbon
generation potential by the end of the Silurian. Modelling results show that the
hydrocarbon generation from Silurian shales lasted up to the beginning of the
Carboniferous. It is therefore inferred that the structures of the Leba ridge were filled
by migrating hydrocarbons from the west during the latest Silurian and Devonian
times.
Further west, the Lower Cambrian sandstones from Bornholm, and in particu-
lar the Hardeberga sandstone, contain a substance that has been interpreted to be
pyrobitumen. It causes the dark colour seen at many outcrops (Møller and Friis
1999 ) . The presence of pyrobitumen indicates the former presence of migrating
hydrocarbons. Petrographic observations show, even though the sandstones are now
extensively compacted, that only low amount of diagenetic cement was formed
during hydrocarbon generation and migration (probably during the Silurian).
The modelling results of the well B2-1, located on the Leba ridge, show that the
oil generated there during Devonian-earliest Carboniferous time. In this area, only
between 7 and 17% of the HC potential of the Silurian, Ordovician and Cambrian
shales was realized.
Intensity of oil generation was also rather low in the eastern part of the Baltic
basin due to both low burial and heat flow (40-50 mW/m 2 ). The oil generation
started in latest Devonian-earliest Carboniferous time. In the area of well B8-1,
only Cambrian and Ordovician shales entered the oil window (Fig. 2.19 , well B8-1),
but only 9 and 6% of the oil generation potential was realized. In the D6 area, oil
generation started not before Mesozoic time (Fig. 2.19 , well D6-1).
West Lithuania was and is characterized by an anomalous heat flow reaching
70-90 mW/m 2 today. This has caused a more intense hydrocarbon generation com-
pared to the eastern part of the Baltic Sea. Cambrian, Ordovician and Silurian source
rocks entered the oil window in western Lithuania during the Middle Devonian time.
Maximum hydrocarbon generation took place in the Early Carboniferous.