GEOLOGY AND HYDROCARBON PROSPECTS OF LATVIA

Astrids Freimanis
Latvian Department of Geology
Riga, Latvia

Lazar Margulis, Arnis Brangulis, Sergei Kanev, Raisa Pomerantseva
Institute of Marine Geology & Geophysics
Riga, Latvia

Oil prospects in Latvia are associated with the Baltic syneclise.

Latvia occupies about one fourth of that large tectonic depression; zones of oil accumulation continue there from adjacent areas: the Telshai rampart (Lithuania) and the Leba nose (Polish offshore).

The oil prospects in separate areas are determined by their position regarding the sources of oil generation-the Gdansk-Kura and Liepaya depressions.

The most prospective areas are the Liepaya-Saldus zone of highs and the Pape-Barta trough. The Liepaya-Saldus zone was situated so that the hydrocarbon migration path crossed it. It probably is an important oil accumulation zone.

The Latvian onshore area is 64,300 sq km, and the offshore covers about 27,000 sq km. Oil exploration in Latvia started after World War II.

The first oil deposit (Cambrian) was discovered there in 1964, leading to the start of exploration in other parts of the Baltics.

Some 11,000 line km of reflection seismic have been acquired with analog recording of data during 1957-71.

Common depth point seismic acquisition started in 1986. About 23,000 line km of CDP data have been acquired off Latvia in 1975-91.

Some 200, onshore wells, penetrating the basement, have been drilled. Three off-shore wells have been drilled.

LATVIA SETTING

Latvia is on the northwest edge of the East European platform within the Baltic syneclise and the Latvian saddle (Fig. 1). The Latvian saddle has shallow basement and separates the Baltic and the Moscow syneclises.

The platform cover is subdivided into four structural complexes

The Baikalian complex occurs sporadically and is represented by terrigenous and volcanic rocks up to 300 m thick.
The Caledonian complex occurs all over the area (Fig. 2). In its base a terrigenous formation of lower-middle Cambrian age occurs, its thickness being 30-230 m. In its upper part Deimena sandstones occur. They are main oil reservoirs in the area.
Ordovician is represented by clayey and carbonate deposits 40-250 m thick. Lower Silurian consists of clay and marl, upper Silurian is represented by clay, marl, dolomite, and limestone layers. The thickness of Silurian deposits is up to 1,000 M.
The top part of Caledonian complex is occupied by the lower Devonian (Lochkovian) lagoon-continental terrigenous carbonate formation, its thickness being up to 200 m. The total thick-ness of the Caledonian complex reaches 1,500 m.
The Hercynian complex consists predominantly of terrigenous and carbonate-terrigenous Devonian formations up to 800 m thick. Carboniferous (Tournasian) terrigenous-carbonate deposits occur only in Southwest Latvia, their thickness being up to 100 m, The thickness of the Hercynian complex increases from north to south reaching 900 M.
The Alpine complex occurs in the southern part of the area. It incorporates Upper Permian carbonate rocks, Triassic and Jurassic terrigenous deposits. Their total thickness is up to 120 M.

The total thickness of the sedimentary cover of Latvia reaches 2,000 m.

The Caledonian complex has the most complex structure. It determines the tectonic subdivision of the area. In the Latvian part of the Baltic Syneclise the following structures are singled out (Fig. 3):

The Estonian-Latvian and Lithuanian monoclines are marginal structures of the Baltic syneclise; they are areas where the basement gently submerges from 500 to 1, 600 M.

The Liepaya depression is a distinctly asymmetrical depression with a gentle northern edge and a steep near-fault southern one. Its length is 200 km, width up to 70 km, trough amplitude 800 m. The thickness of the sedimentary cover reaches 2,000 m, that of the Caledonian complex 1,500 m.

The Liepaya-Saldus zone of highs crosses the Baltic syneclise, stretching from the Swedish offshore zone towards the northeast for about 400 km. The width of the zone is 25-80 km. From northeast towards southwest the basement submerges from 500-1,900 m. The zone is a complex system of disjunctive-plicative dislocations, the intensity of which exceeds that in other areas of the Baltic syneclise. Fault amplitudes reach 600 M.

The Gdansk-Kura depression is represented only by its northern peripheral part, thickness of the sedimentary cover there being 1,600-2,000 M.

The South Latvian step is a sublatitudinal tectonic block in southern Latvia. Its length is about 100 km. The amplitudes of boundary faults reach 400-500 m. The western, most deeply submerged, part of the South Latvian step comprises the Pape-Barta trough, with the sedimentary cover 1,6001,900 m thick.

Clayey Cambrian, Ordovician, and Silurian rocks are principal source rocks of the Baltic syneclise. The content of organic carbon in Cambrian clay sometimes reaches 2% while in middle and upper Cambrian and Tremadoc alum shale of the western part of the syneclise it reaches 10-20%.

In Ordovician deposits high organic carbon content is observed in black clay (10% or higher), its total thickness being 5-10 m. In the southern part of the syneclise the deposits of that type become prevailing in the Ordovician section. Similar high organic carbon content is observed in black graptolite clay of Lower Silurian age.

Oil generation in Paleozoic rocks started in Late Silurian in the southern perikraton part of the syneclise; the main oil generation stage took place in Late Devonian and Carboniferous when, in the southern and central parts of syneclise, Cambrian, Ordovician, and Silurian rocks entered the oil generation zone in the depth interval 2,000-3,000 m. The oil window reached Gdansk-Kura and Liepaya depressions.

In the southernmost part of the syneclise Lower Paleozoic deposits entered the gas generation zone.

In Latvia, Cambrian, Ordovician, and Lower Silurian rocks are at the early maturation stage, the depth of the basement being 1,300-2,000 m. Thus the main oil generation area is the Gdansk-Kura depression. The Liepaya depression, the Pape-Barta trough, and adjacent submerged parts of the Liepaya-Saldus zone of highs may be considered to be the local oil kitchen.

KULDIGA OIL FIELD

One oil field, Kuldiga, was discovered in mid-Cambrian, and nine noncommercial accumulations in Ordovician have been found in Latvia. Some oil prospects can exist in conjunction with Silurian carbonates.

The middle Cambrian Daimena oil bearing horizon, as thick as 80 m, consists of five to seven sandstone layers separated by siltstone and clay. The sandstone has variable grain size, it is quartzose and well sorted. The porosity there reaches 25%, the median value is about 17%. Permeability varies from 100-1,000 md, sometimes reaching 1,500 md.

Kuldiga oil field is situated on the northern flank of the Liepaya-Saldus zone of highs and is associated with a near-fault anticline. The area of the structure is 6 sq km, the amplitude is 30 m, the depth of top basement is about 1,100 m.

The oil accumulation is contained in middle Cambrian sandstone 55 m thick, the oil bearing zone is 7 m thick. Mean porosity exceeds 20%, permeability is 300-1,000 md. The density of oil is 0.87-0.91 g/cc, sulfur content is 0.4-0.58%, tar 16.S-27.4%, asphaltenes 2.1-3.5% and wax 1-1.6%.

OTHER POTENTIAL

Traces of oil saturation in middle Cambrian sandstone have also been found on Liepaya-more offshore structure, which is one of the highest folds of the Liepaya-Saldus zone. The structure is ruptured by numerous faults.

The depth of top basement is about 1 km. In the tectonic block, where a well was drilled, the oil accumulation had probably been destroyed.

The Ordovician deposits are associated with three horizons. The lower one is represented by quartzose siltstone 0.2-1.5 m thick, porosity is up to 15%, permeability 80-85 md. Oil flows were up to 0.8 cu m/day. The middle horizon consists of Organogenies-detrital limestone 1.5-40 m thick, porosity is up to 10%, permeability is very low. The upper Ordovician (Porkuni) horizon is represented by calcarenites.

Small oil accumulations have been found on nine structures. The thickness of the oil-saturated section on the Liepaya-more structure is 9 m, porosity is 8-25%, permeability up to 25 md, The oil flow there was 0.7 cu m/6 hr. Specific weight of oil is 0.81 g/cc, tar content 3%, sulfur content less than 0.3%.

Oil prospects in Latvian Silurian deposits have not been confirmed yet. According to forecasts they may exist in reef limestone of Upper Silurian age. The caprock for the Middle Cambrian reservoir is represented by Lower Ordovician clay and marl, 20-40 m thick. Oil accumulations in Ordovician are sealed by adjacent clay and carbonates both vertically and horizontally. As regards the upper (Porkuni) horizon, the cap there is Lower Silurian clay and marl, their thickness; being more than 100 m.

In the Latvian part of the Baltic syneclise about 80 structures have been discovered, 23 of them offshore. Most of them are parts of structural groups and ramparts controlled by faults.

The size of local structures is a few tens, sometimes up to 100 sq km, the amplitudes are 25-60 m, sometimes reaching 200 m. The majority of structures is situated within the Liepaya-Saldus zone of highs.

Offshore seismic has determined about 40 Upper Ordovician reefs on Latvian-Estonian monocline, their height is up to 50 m, the area is 1-3 sq km. On Gotland similar Upper Ordovician reefs contain small oil accumulations. In about the same area seismic investigations have led to the discover-y of Silurian reefs being the northern chain of organic barrier structures of the marginal part of the Baltic syneclise. In Lithuania oil flows occurred from similar reefs.

CONCLUSIONS

Latvia is interested in cooperation with foreign companies as regards oil exploration and production.

The mining law and drafts of licensing blocks are being developed, and a regional report, "Geology and Hydrocarbon Potential of Western Latvia and Adjacent Baltic Sea," has been prepared for sale.