Reservoir
rocks and hydrocarbon occurrence
All
prospective reservoir units in Latvia, as well as in the whole Baltic
Region, are found within the Lower Palaeozoic sequence.
Cambrian
In
Polish, Kalinigrad and Lithuanian areas, a significant number of HC
accumulations occur in this sandstone reservoir. In Latvia, this
reservoir unit is represented by Deimena
Formation, which contains the oil pool discovered at
Kuldiga. Middle Cambrian sandstone has good reservoir
properties. Its thickness varies from 50 to 90 m, with open porosity 15
to 25 % and permeability several hundred mD.
Lower
Cambrian reservoir in Latvia is represented by the Ventava and Ovishi
Formations, the reservoir properties of which are similar as
for the Deimena Formation, but oils have not yet been discovered within
these deposits to date.
In
addition, there are units with both proven and potential reservoir
properties within the Ordovician and Silurian successions.
Ordovician
The
Ordovician reservoirs are represented by limestones, silt- and
sandstones.
Most
promising of them are Late Ashgillian Saldus
Formation bioclactic/oolitic limestones, which have open
porosity up to 10-20% with permeabilities up to 40 mD, the thickness of
these limestones reaches 10-15 m. Numerous oil shows and minor oil flows
were obtained from many wells.
Potential
reservoirs are associated with Ashgillian Jonstorp
Formation reefal/biohermal limestones, which are oil-bearing
on the Swedish island of Gotland.
Other
potential reservoir units occur within the Upper Llanvirnian
and Llandeilian detrital and fine-grained limestones, oil shows
are evident in cores from many wells.
The
Kallavere Formation
is a reservoir unit at the base of the Ordovician succession, comprising
sandstones. Together with the Deimena Formation form one reservoir unit
with similar reservoir properties.
Within
the Lower Ordovician the Kriukai Formation, a minor reservoir
units occurs, comprising silt- and sandstones, where oil stained rocks
have been identified in several onshore wells. Porosities are up to 16%
and permeability several tens of mD.
Silurian
The
potential reservoirs within the Silurian succession are Ludlovian
and Pridolian reefal carbonates. They occur along the eastern and
northern flanks of the Baltic Syneclise and
in Lithuania they are proven hydrocarbon reservoirs. Similar
Upper Silurian reefs are identified by seismic in the northern part of
offshore Latvia.
