Organic matter of Early Silurian succession; the potential source of unconventional gas in the Baltic Basin Lithuania

Increasing demand for alternative energy sources leads to an interest in unconventional hydrocarbons, e.g. natural shale gas. Based on initial evaluation, the Early Silurian graptolite shales are studied as prospective shale gas sources in the Baltic region. This paper addresses the occurrence, generation potential and the volume of unconventional gas in place. New organic geochemistry data and vitrinite like macerals reflectance measurements, incorporated with well logs and core data were used to reveal the potential for shale gas. The Early Silurian shales are organically rich, type II, oil–prone marine sediments. The organic matter content ranges from 0.7 up to 19.2%. Hydrocarbon generation potential ranges from 7 up to 113 kg HC/t rock, and the Hydrogen Index ranges from 136 up to 571 mg HC/g TOC. Maturity of the organic matter increases southwestwards from 0.6 up to 1.94% (Ro). The Early Silurian shales in Western Lithuania generated large amounts of hydrocarbons with an estimated volume of expelled hydrocarbons of about 13.75 × 106 kg. This mass of hydrocarbons is equivalent to 389 × 105 cubic meters of methane for a section of 1 km2 and 150 m thickness.