Join AAPG Europe and the Lithuanian Geological Society for this exciting Core Workshop. This workshop will follow on from the 'Hydrocarbon Exploration in Lithuania and the Baltic Regions' event taking place at Vilnius University on 24th – 25th October.
The Early Palaeozoic Hydrocarbon System in the Baltic Basin and adjacent territories involves Middle-Late Cambrian, the Late Ordovician (Mossen & Fjacka Formations) and the Early Silurian Graptolitic Shales source rocks and the major complexes of reservoirs, associated with Middle Cambrian sandstones, Ordovician and Silurian reefogenic and carbonate build-ups.
The major reservoirs of the Baltic Basin are:
- The Middle Cambrian (Deimena Fm). Sandstones - Producing
- The Early Ordovician (Tremadoc, Salantai Fm.) sandstones
- Late Ordovician (Early Ashgill) organogenic limestones/carbonate buildups
- Late Silurian (Late Ludlow/Pridoli) reefogenic carbonate build-ups
Cores presented from the following 3 reference wells:
1. The Middle Cambrian - the Early Ordovician quartz sandstone reservoirs
The Middle Cambrian Deimena Group sandstones comprises all the major economically important oil fields located Lithuania, Latvia, Kaliningrad district and Polish onshore and Baltic Sea offshore. The other, much less significant, potential reservoirs are the Late Ordovician carbonate build-ups of Gotland (Sweden) and Southern part of Lithuania and the Late Silurian carbonate reefogenic buildups in South Lithuania (Zdanaviciute O., Sakalauskas J. eds., 2001, Zdanaviciute, Lazauskiene 2007; Kanev et al., 1994).
The reference sections would demonstrate core from fine-grained (dominated by 0.25-0.1 mm fraction (30-75%)) quartz sandstones containing thin clay and siltstone interlayers. The sandstones are to a different degree litified by compaction and predominantly cemented by quartz and diagenetic quartz cement that has the major control on reservoir properties.
The Early Ordovician
The early Ordovician Tremadoc age (Pakerort Regional Stage, Salantai Fm.) strata distributed rather locally are a reservoir unit at the base of the Ordovician succession, comprising quartz sandstones and quartz siltstones of only 0.5 to 4 m thick. It overlays directly to the Middle Cambrian Deimena Group sandstones and together form one reservoir unit with similar reservoir properties. The formation is overlain by the Early Ordovician shales. Several small oil fields are producing from this reservoir unit in the western part of Lithuania.
2. The Late Ordovician (Early Ashgill) and Late Silurian (Late Ludlow/Pridoli) organogenic limestone and reefogenic carbonate buildups reservoirs
The Late Ordovician - The Late Silurian
The reservoir rocks within the Silurian succession are the Wenlock - Early Ludlow and Pridolian reefogenic carbonates comprising secondary dolomites and reefal limestones with thicknesses of tens of meters. Silurian sequences are locally distributed along the Eastern slope of the Baltic Basin. The Wenlockian - Early Ludlow strata are up to 28 m thick; the effective porosity ranges from 12% to 17% and average permeabilities – 12-15 mD.
The most favourable conditions for the formation of non-structural traps (reef-associated, lithologic-stratigraphic and combined) are associated with the carbonates (mainly stromoporoidal and crinoidal limestones) sucession of about 90 m thick of the late Ludlow- earliest Pridoli (Minija and Ventspils Formations). The reservoir rocks has mean porosities of 6-15 % and up to 26% and permeability ~465mD, reaching up to 2400mD. The Late Silurian reservoir rocks mainly occur in central and southern Lithuania in the central part of the basin.
3. Late Ordovician (Mossen & Fjacka Formations) and the Early Silurian Llandovery Black Shales
The Late Ordovician Shales
In the central and eastern part of the Baltic Basin the potential source rocks comprises dark grey and black shales of the Late Ordovician Late Caradoc-Early Asghill Fjacka and Mossen formations. Both units are generally thin, reaching only up to 5–10 m; the thicknesses of Fjack & Mossesn Formations are 6 m and 4 m respectively. TOC content are mostly in the 0.9 to 10 % range, with occasional higher values of up to 15 %. The source rock facies are kerogen type II and II-III.
The Early Silurian Shales
Potential source rocks in the Silurian succession are found within the Llandovery, Wenlock and, presumably, Ludlow-aged strata. The Silurian source rocks are composed of dark grey and black graptolite shales and dark grey and black clayey marlstones. Within the Baltic Basin organic matter content generally ranges from 0.7 to 9–11%, but can be as high as 16.46 % (fig. 5.5.b; Zdanaviciute, Lazauskiene, 2004). In terms of petrography, the organic matter is dominated by syngenetic, sapropelic and marine material, together with vitrinite-like particles and abundant faunal remains. Detrital sapropel is scattered as very fine-grained particles and lenses. Liptinite (up to 20%) generally occurs together with dispersed liptodetrinite in sapropelic organic matter, or more rarely as scattered particles. (Zdanavičiūtė, Swadowska 2002, Zdanaviciute, Lazauskiene, 2004, 2007, 2009).
Maturities in the area of interest attain at pre-Silurian level 1.3% Ro and around 1.0% Ro at Silurian source rock level, and reach 1.9% on the prominent West-Lithuanian local temperature high Zdanaviciute, Lazauskiene, 2004, 2007, 2009)