Geological Evolution and Resources of the Baltic Sea Area from the Precambrian to the Quaternary - The Baltic Sea Basin - page 40

The Ordovician carbonate deposits contain generally low amounts of organic
matter; TOC is commonly less than 0.2%. Other source rocks of the Ordovician are
black shales of the Mossen formation (Middle Ordovician age) and of the Fjacka
formation (Upper Ordovician). The thickness of these black shales varies between
2.0 and 4.5 m. They have been deposited in deeper shelf zones and are characterized
by a high content of sapropelic organic matter and TOC contents of up to 14.9%
(Kaduniene 1978 ; Kaduniene et al. 1978 ) .
The Silurian is represented by a 750-1,150-m-thick succession of mainly dark
grey graptolite shales. Two parts are typical for the succession. While the lower part
with 300-m-thick shales of Llandovery-Lower Ludlow age contains up to 11.2-
16.5% TOC (Kaduniene et al. 1978 ) , the upper part contains significantly lower
amounts of organic matter. However, the distribution of Silurian source rocks is
still poorly understood in the Baltic Sea area because it is based there just on
extrapolation of data of a few offshore wells.
Fig. 2.18 Kerogen type of Cambrian, Ordovician and Silurian source rocks of the Baltic basin
(after Kanev et al. 1994 , Zdanaviciute and Sakalauskas 2001 )