The petroleum potential of the Silurian succession in Lithuania

Petroleum potential is associated with Silurian reefal and carbonate build-ups in southern and central Lithuania where non-structural traps (reef-associated, lithologic-stratigraphic and combined) have been identified in intervals of Late Wenlock, Late Ludlow and Early Pridoli ages. Three oilfields (Kudirka, Siaures Bliudziai and Lapgiriai) have so far been discovered in this area. In addition, oil flows have been reported at wells in the Bebirva, Saukenai, Bliudziai and Pavasaris accumulations. Reefal build-ups and patch reefs dated as belonging to the Minija Regional Stage of the Early Pridoli, up to 88 m thick, are considered to be the most prospective for hydrocarbons, and all the discoveries so far made occur in this stratigraphic interval. New geochemical data indicate the presence of excellent Llandovery, Wenlock and Ludlow source rocks, composed of dark grey and black clayey marlstones and shales. Recent studies of the composition and maturation of Silurian source rocks indicate the predominance of sapropelic organic matter, present in higher amounts than in Cambrian and Ordovician source rocks. The maturity of Silurian source rocks increases from immature in eastern Lithuania to early oilphase in the centre and north, to peak oil generation in the west. Geochemical analyses of Silurian crude oils show that they are un- or only weakly biodegraded, in spite of their early emplacement (in the Palaeozoic) and the relatively low reservoir temperatures (40-50o C). The results of these geochemical studies of the Silurian source rocks and oils, incorporated with available geological data, were used to predict the hydrocarbon potential of the Silurian succession in Lithuania. The succession is estimated to have generated as much as 5.8 B tons of hydrocarbons. Previous studies indicate that 100 million tons may have accumulated in patch reefs assigned to the Minija Regional Stage. Estimated oil in-place at the known oilfields totals about 1.77 million tons.