Source rocks

In the Baltic Region three prominent source rock intervals are found within the Lower Palaeozoic succession within Upper Cambrian to Tremadocian, Ordovician (Caradocian) and Lower Silurian (Llandoverian) succession. The table (see fig.) illustrates identified and potential source rock intervals for Latvia and the entire Baltic Region.

Upper Cambrian to Tremadocian deposits constitute excellent source rocks, being time equivalent to the Alum Shale of Bornholm and Scania. These organic rich shales have been eroded in the eastern parts of the Baltic Syneclise, including Latvian area. At the same time, the southwestern part of Latvia’s offshore area is geographically close to the Leba High offshore Poland, where Alum Shales are identified in all wells. Therefore potential source rocks could occur in this region. In the southern part of Latvia, organic rich shales are found within the Late Tremadocian succession.

Caradocian black shales, referred to as the Fjäcka and Mossen Formations, constitute excellent source rocks, TOC values up to 16% and pyrolysis yields of 65-75 kg/ton. A total thickness is about 5-10 m.

A prolific source rock interval occurs within the Llandoverian deposits (Dobele Formation). These shales display excellent source rock properties in Latvia; TOC values may reach 15-17% with pyrolysis yields of up to 70 kg HC/ton rock. A thickness is about 5-25 m.

The type of organic matter in these source rocks is similar, irrespective of geologic age. The kerogen type, which dominates, is typically marine Type II; the variations observed being ascribed to original richness and thermal maturity variations only.

Thermal maturity

In the Polish, Kalinigrad district and Lithuania areas these shales are thermally mature, being immature to very early mature in Latvia. Therefore, hydrocarbon occurrence in Latvia is dependent on migration pathways and distances from the main kitchen areas.