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The Hydrocarbon Potential of the Baltic Basin: Geochemistry of Source Rocks and Oils of the Lower Paleozoic Succession

O. Zdanaviciute1, J. Lazauskiene2, A. I. Khoubldikov3, M. V. Dakhnova4, and T. P. Zheglova4
1Nature Research Centre, Institute of Geology and Geography, Vilnius, Lithuania
2Vilnius University, Vilnius, Lithuania
3Lukoil-Kaliningradmorneft Plc., Kaliningrad, Russia
4All-Russia Research Geological Oil Institute, Moscow, Russia

The Baltic Basin, located on the western margin of the Easter European Craton is bounded by the Baltic Shield to the north-west and north and by the Teisseyre-Tornquist Fault Zone to the SW. Several tens of relatively small oil and gas fields occur there over a wide stratigraphic interval, ranging from the crystalline basement through the entire Lower Paleozoic succession up to the Devonian sandstones. Commercial hydrocarbon accumulations have been discovered in NW Poland, Kaliningrad region of Russia, Lithuania and the adjacent offshore. Furthermore, oils are known from several wells of West Latvia and Sweden (on the island of Gotland) and seepage of oil occurs in several areas in Central Sweden (close to Lake Siljan, Kinnekulle). All of aforementioned oils are of the Lower Palaeozoic origin, since excellent oil-prone source rocks occur in the Cambrian, Ordovician and Silurian succession of the Baltic Basin.

The Middle Cambrian sandstones are the most important reservoir units comprising all the producing oil and gas fields so far discovered throughout the Baltic Basin. Some petroleum potential within the basin also is associated with Silurian reefs and carbonate build-ups located in southern and central parts of Lithuania and the Upper Ordovician carbonate mounds in island of Gotland.

The main task of this study was to investigate of organic matter and crude oils from Kaliningrad and Lithuanian territory, to determine it’s differences and explain hydrocarbons genesis and migration pathways of oil in the Baltic Basin. These two administrative regions have been selected to illustrate the diversity of the hydrocarbon potential of the Baltic Basin, as more than 700 deep wells have been drilled and more than 45 small hydrocarbon accumulations discovered within this area.

The hydrocarbon potential is related to the several stratigrapfic intervals of the organicrich source rocks. In the south-eastern and central part of the Baltic Basin the Cambrian shale and clayey siltstone, Ordovician black argillite of the Mossen and Fjacka Formations as well as Silurian dark grey clayey interval of the Llandovery–Ludlow age are considered as the major source rocks. The organic matter of the source rocks is of similar composition – it represents typical marine Type II (based on Rock-Eval analysis data). The Rock-Eval screening pyrolysis, biomarker data, reflectance of vitrinite-like macerals and conodont color alteration index show considerable variations of the source rocks maturity. Thermal maturity in this area ranges from immature in the eastern part of Lithuania and Kaliningrad region to “oil window” in the south-western part of the study area. In some places (wells Ramučiai-1, Pajūris-1 and others) the anomalously high maturity of organic matter, indicating the lower part of the condensate/wet gas zone have been recorded, most probably being related to the locally increased paleo-temperatures.

Twenty-one crude oil samples were analyzed by standard techniques including GC and GC-MS show that oils of the Baltic Basin have densities from 790.5 to 870.0 kg/m3 (46.7– 30.4 oAPI), and low asphaltene (<2.2%) and sulphur (<0.44%) contents. The saturated hydrocarbon content varies from 35.3 to 77.8% and the ratio of saturate to aromatic hydrocarbons ranges in 2.1–5.2, indicating long-distance hydrocarbons migration or high thermal maturities. GC analyses of saturate fractions indicate a composition of oils dominated by n-alkanes with a maximum at C13–C15 and reduced abundance in the C20–C35 range. Crude oils also are characterized by relatively low concentrations of sterane and triterpane biomarkers. A significant feature of the oils is increased content of tricyclic triterpanes, comparatively low hopane/sterane ratios, and low proportions of extended hopanes. These oils were generated by marine source rocks (shale containing algal/bacterial kerogen) and have a relatively high level of maturation (except two samples which are of lower maturity). It is interesting to note that the oils of the Baltic Basin are not biodegraded, despite their early emplacement (the Lower Paleozoic age) and the relatively low present reservoir temperatures. The studied oils are very similar in terms of stable carbon isotope composition – d13C values for crude oils differ only by 1‰. Results of biomarker and stable carbon isotope analyses allow three genetic oil groups to be distinguished in the Kaliningrad region. The oils in these groups appear to be confined to tectonically distinct areas suggesting that the hydrocarbons were derived from different kitchens (Zdanaviciute et al., 2012).

The Early Silurian graptolite shale also is studied as prospective unconventional shale gas sources in the Baltic Basin. 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 in the West Lithuanian. The estimated volume of expelled hydrocarbons might have been as high as 390 × 105 cubic meters of methane for a section of 1 km2 and 150 m thickness (Zdanaviciute and Lazauskiene, 2009).

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013