Site Summaries

Florina, Greece

The Florina CO₂ field, located on the border with the Former Yugoslav Republic of Macedonia (FYROM), was discovered by chance in the 1960’s during lignite exploration. Production started in 1980 (current production 30 000 tonnes). The productive life of a well is one year because of problems caused by acid water and CO₂. The CO₂ occurs in sands below, above and between lignite seams at depths between 180 and 260 m. CO₂ may have formed by alteration of limestones and migration through faults in the overlying Tertiary sediments, although volcanic activity in the Almopia basin, 25 km to the east, may also be responsible. Clay horizons locally act as caprocks. Variations in CO₂ concentrations will be compared with seismicity to determine any correlation between seismic activity and leakage.

Location of the Florina CO₂ production field in northern Greece, operated by Air Liquide Hellas.

Simplified geological cross-section of the CO₂ reservoirs in the Florina Field, northern Greece.

Mihályi-Répcelak, Hungary The Mihályi-Répcelak area, discovered in 1933, is situated in the central part of the Little Hungarian Plain. The first borehole, drilled in 1935, produced a large amount of CO2 (94.6 v/v%) from an Early Palaeozoic phyllite (1602 m depth). Large CO2 reserves were discovered in the Répcelak basement (1460 m depth ) in 1945—46. CO2 production started in two sandstones in the 1950's, when a large number of wells were drilled (43 boreholes up to 1979). Two concessional blocks, the Mihályi (production in 1999: 43.6x106 m3) and the Répcelak fields (production: 19.0x106 m3) have been owned and operated by Linde Gas Ltd. since 1993. CO2 is produced from multi-layered reservoirs of sandy Mio-Pliocene turbidites and basin-fill sediments. The caprock is a Pannonian marly sequence.

Geological cross-section of the Mihályi-Répcelak area.

Mátraderecske, Hungary

This village is situated in the Matra Mountains. High CO₂ concentrations occur in homes and residents have installed cheap, effective mitigation equipment in basements. CO₂ is also used in medicinal 'spas' to aid circulation problems. This area is part of the Middle Miocene andesite volcanoes of north Hungary, close to the major Darnó Fault Zone. The Eocene andesite basement is overlain by clays and sands. The CO₂ is thought to be related to nearby copper-zinc mineralisation. The gases migrate upwards along faults and fractures from a karst water reservoir at ~1000 m depth. The gases migrate laterally against local seals and can escape along faults to the surface. The gas seepage occurs as bubbling in wells and streams, as well as strongly carbonated springs.

Top: Members of the NASCENT team enjoying a CO₂ bath at the Matraderescke mofetta. The CO₂ is invisible and comes to about waist height.
Middle and bottom left: The Mofetta in Matraderescke, northern Hungary. As CO₂ is absorbed through the skin, blood vessels expand and heart rate increases. This is used to improve patients' circulation problems.
Bottom right: Eszter Tòth of the RAD Lauder Institute, Budapest, Hungary, demonstrating the presence of CO₂—as Eszter lowers the candle, its flame is extinguished by the CO₂.

Latera geothermal field, Italy

This field is located within the Latera volcanic caldera. Although studied since the early 1970's for geothermal energy, the area was well known long before that for the carbonate-rich springs and CO₂—rich gas vents. The CO₂ is believed to be the result of decarbonation of carbonate minerals. These gas reservoirs are more than 0.1 Ma old. They are trapped by low permeability flysch rocks and laterally sealed fractures. Local inhabitants have lived for thousands of years above these reservoirs and associated gas vents.

LEFT: Simplified geological map of the Latera geothermal area.	RIGHT: Simplified geological cross-section of the Latera volcanic complex.

Vorderrhön, Germany

Natural CO₂ occurrences in the Werra potash district (Vorderrhön) have been known since the end of the 19th century. An East German company operated the CO₂ field until 1990. It was taken over by Air Liquide de France, but was shut down in 1994. Total production over ~100 years was ~528x10⁶ kg. CO₂ is hosted in fractured reservoirs in the Kupferschiefer, Zechstein Limestone, and Lower Werra Anhydrite. Evaporite seams form the caprock. Late Tertiary basalts are associated with fault zones. A direct relationship between these faults, volcanic rocks, and CO₂ is demonstrated in the Werra potash mines.

Location of the four CO₂ fields in the Vorderrhön area of Germany. Until reunification these fields produced CO₂.

Montmiral, France

Discovered in 1961, the CO₂ accumulation at Montmiral has been exploited since 1990 by Carboxyque, a subsidiary of Air Liquide France. Several CO₂ occurrences have been recognised below 2400m depth. The main CO₂ reservoir occurs in Triassic sandstone. Productivity is ensured by open fractures. The CO₂ saturation is estimated at 70%.

Hélène Pauwels, Isabelle Czernichowski-Lauriol and Phillipe Vigoroux of BRGM at the Carboxyque CO₂ production plant, Montmiral, France.

Offshore shallow gas accumulations

Shallow gas has frequently been observed in the Southern North Sea. Although such gas is almost exclusively composed of methane, these occurrences may provide useful information about physical processes associated with gas accumulations such as shallow trapping and migration processes. Some of these gas occurrences cause characteristic surface expressions, others are inferred by (very) high-resolution seismic methods. Gas accumulations in young unconsolidated fine-grained sediments may eventually (on a geological time scale) lead to over—pressured and under—compacted sediments.

The location of shallow gas fields in the Dutch sector of the North Sea. Although consisting of methane, these fields act as analogues for the migration of gas in the shallow sea bed.