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Approximately 104 x 106 m3 of oil (39 fields), 6.93 x 106 m3 of condensate (11 fields), and 64.92 x 109 m3 of gas (52 fields), were recovered in the Croatian part of the Pannonian Basin System during 64 years of exploitation (1941–2005). The production peak was attained between 1980–1989, when exploitation began in 12 new fields. Based on their cumulative production, the Croatian oil and gas fields can be divided into four groups, and the condensate fields into three groups. Such a division has been supported by analysis of recovery, number of reservoirs, porosity and permeability, age and lithology of reservoir rocks. The longest production period is assumed for the first group of fields; for oil it is approximately 55 years, for condensate 46 and gas 36 years. In the favourable first group the average number of reservoirs is 16 for oil and 11 for gas. Lithological composition is highly favourable, because reservoirs are represented mostly by sandstones of Pannonian and Pontian age with high porosities and permeabilities. A relatively homogeneous sandstone lithology, including good regional seals like marls, enables an increase in recovery through the use of secondary and tertiary recovery methods. Also, water-flooding will remain the dominant secondary-recovery method for increased production in the future.
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