Geological Evolution and Resources of the Baltic Sea Area from the Precambrian to the Quaternary - The Baltic Sea Basin - page 39

of the upper part of the reservoir sections by meteoric waters during early burial
stages. The reservoir properties of the sandstones are mainly controlled by authi-
genic quartz cement which ranges from 10 to 32%. The best reservoirs are identified
in the Liepaja-Saldus ridge and the Leba ridge, where average porosities are between
14 and 18%. The good reservoir properties are mainly related to the shallow (1,100-
1,600 m) burial of sandstones. Southeast of the ridges, porosities and permeabilities
of the sandstones decrease dramatically to values between 1 and 9% (average 5%)
and <0.01 and 25 mD, respectively.
The Ordovician carbonates show, in general, poor reservoir properties; the poros-
ity is mainly 2-5% only. However, some oil shows and oil inflows were reported
from western Latvia. The corresponding reservoir layers are related to the “Porkuni
regional stage” of Upper Ordovician age. They are composed of oolitic and bio-
clastic limestones (Laskovas 1994 ) . The open porosity of the Porkuni carbonate
reservoirs of the wells E6-1 and E7-1 varies in the range of 3-24%. Best perme-
abilities are around 40 mD. Oil shows were reported from well E7-1 and an oil
inflow of 2.7 m 3 /day was reported from well E6-1. This Upper Ordovician reservoir
belt is confined to the Liepaja-Saldus ridge. Prospective resources of this area were
assessed to around 8.8 million tons (Laskovas and Jacyna 1998 ) .
In the western part of the Baltic Sea, oil is produced from Upper Ordovician
carbonate mounds at the Gotland Island (Sivhed et al. 2004 ) . Between 1974 and
1992, total oil production amounted to 100,000 m 3 . The mounds contain large num-
bers of vugs and moulds which communicate through dissolution fractures and
surfaces. Sediments represent sub-mound, intra-mound, cap and flank, and supra-
mound facies. Algae and stromatolites dominate the intra-mound facies, providing
an organic framework for the entire structure. A large field of Ordovician reefs was
identified between Gotland and Latvia, but so far no drilling has been carried out to
prove its hydrocarbon potential (Kanev and Peregudov 2000 ) .
The Silurian consists mainly of black shales and clayey marlstones, representing
a 1-km-thick source rock package. An oil show was reported from the well Nida-
44 in the Curonian Spit. It is confined to the uppermost part of the Silurian section
containing dolomite interlayers of around 7.5 m thickness with porosities from 12 to
14%. However, this is the only discovery so far. Lower and Upper Silurian reefs are
reported from Gotland and the area east of the island (Manten 1971 , Kershaw 1990 ,
Flodén et al. 2001 ) , but no evidences of hydrocarbons were reported. Still those
reef build-ups have some potential, as several oil accumulations were discovered in
Upper Silurian reefs in central Lithuania (Lapinskas 2000 ) .
2.7.3 Source Rocks
Major source rocks of the Baltic basin are Cambrian, Ordovician and Silurian shales.
The TOC content of the Lower-Middle Cambrian shales is rather low and varies
between 0.03 and 2%. The lowest values are typical for Lithuania. The well-known
alum shales (middle part of Upper Cambrian-Tremadocian) are distributed in the
western part of the basin (Buchardt and Lewan 1990 , they contain 11-12% TOC).