No abstract

A series of oils and potential petroleum source‐rock samples has been analysed from exploration wells on‐and offshore Lithuania. Despite the limited amount of data, the results indicate the possible existence of partly‐exhausted source rocks within the Cambrian succession. Furthermore, possible source rocks are present within the Ordovician succession, and excellent source rocks occur within the Silurian in several wells. The source rocks are all present within a few hundred metres of stratigraphic succession, and the thermal maturity roughly follows the actual depth of burial, despite the fact that most of the subsidence and maturation took place in the latest Palaeozoic. The most important reservoir rocks are Middle Cambrian sandstones, but petroleum accumulations also occur in Ordovician limestones and Silurian reefal carbonates. Petroleum accumulations in Lithuania probably result from the pooling of oil derived from several sources, with the Lower Silurian (Llandoverian) shales being the most important single contributor.

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 oil‐phase 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 (40‐50°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.

Some 36 oilfields, all producing from Middle Cambrian (Deimena Group) sandstones, are located in the central Baltic Basin in an area covering onshore Lithuania and Kaliningrad (Russia) and the adjacent offshore. This paper presents new data on the composition of crude oils from fields in this area and reviews the reservoir properties of the Deimena Group sandstones.Twenty-one crude oil samples from fields in Lithuania and Kaliningrad were analysed by standard techniques including GC and GC-MS. The oils had densities of 790.5 to 870.0 kg/m 3 , and had low asphaltene (<2.2%) and sulphur (<0.44%) contents. The gasoline fraction (b.p. >200 o C) ranged from 12-34%. The saturated hydrocarbon content was 35.3 to 77.8%, and the ratio of saturate to aromatic hydrocarbons was 2.1-5.2, indicating long-distance migration or high thermal maturities. GC analyses of saturate fractions indicated a composition dominated by n-alkanes with a maximum at C 13 -C 15 and reduced abundance in the C 20 -C 35 range. The analysed crude oil samples are characterized by relatively low concentrations of steranes and triterpanes.Biomarker data indicated an algal origin for the precursor organic matter and a clasticdominated source rock. Sterane isomerization ratios imply that the oils are in general relatively mature. Exceptions are samples from the Juzno Olempijskoye and Deiminskoye fields, Kaliningrad, which were early mature. Oil from well Gondinga-1 (Lithuania) was lightly fractionally evaporated and has a relatively higher density, higher viscosity, higher asphaltene content and lower content of saturated fractions.Stable carbon isotope ratios of crude oils and saturated and aromatic fractions were analysed. Whole oils showed little carbon isotope variation, but there were significant differences in δ 13 C ratios for saturated and aromatic fractions. The geochemical data show differences in oil sourcing and indicate the possible existence of different kitchen areas in the Kaliningrad region.Vertical and lateral variations in Deimena Group reservoir properties are controlled by variations in quartz cementation. In fields in western Lithuania, sandstone porosity ranges from

Four minor oil discoveries have been made in West Lithuania in recent years. Studies of the oil composition show that its physical and chemical properties (density, viscosity, petrol content, etc.) and the group composition of hydrocarbons (content of saturated and aromatic hydrocarbons, tars and asphaltenes) mainly depend on the formation conditions and distances of migration between the kitchen and accumulation areas. According to the distribution patterns of n-alkanes and isoprenoids, the examined oils are comparable and generated from sapropel organic matter. There are certain differences in biomarker and carbon isotope data, indicating oil generation from different source rocks containing organic matter of different catagenesis.