Thermal Maturity of the Silurian “Hot” Shales and Correlation with the Present Geothermal Variations in West Lithuania, Baltic Basin

The most organic-rich shales are defined in the Dobele Fm. of the Aeronian Stage of about 10 m thick in west Lithuania. This particular layer is documented in the whole Baltic Basin. Compatible shales are widely distributed in other basins referred to as similar Silurian “hot” shales. The average TOC was estimated at 6.67 wt.% (good and excellent source rock). The thermal maturity of shales was evaluated through organic geochemical techniques, including TOC determination, Rock–Eval pyrolysis, and organic petrography studies. The thermal maturity varies from T_max = 431 °C and eq.VR_o = 0.65% (early oil) to T_max = 468 °C and VR_o = 1.38% (locally up to 1.94%) (late oil and wet to dry gas generation). It is notable, most of the study area is confined to regional-scale West Lithuanian Geothermal Anomaly. Most of the geothermal features, both palaeo- and recent, recorded in lateral variation in thermal maturity of shales unravel persistence of heat flow. Locally, the Variscan tectonic activity was imprinted in thermal maturity of organic matter-rich shales (Žemaičių Naumiestis anomaly).