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Abstract

Fluid inclusion data provide pressure-temperature-time-composition (P-T-t-X) constraints for an episode of petroleum infiltration of the crystalline basement in South Norway. Petroleum inclusions associated with pyrobitumen occur in postmetamorphic quartz veins in the Modum Complex. Three groups of fluid compositions have been shown, ranging from CH4 +/- CO2 to condensates with alkanes up to C-15. The range in fluid composition is a result of petroleum decomposition at high temperature. Globular and massive pyrobitumen occurs in the quartz veins or in associated vein systems. Reflectance (%Rm) measurements of 3.20-3.35 correspond to a maximum temperature of 207-214degreesC for the pyrobitumen associated with group II and III inclusions. Geothermometry of chlorites included in the quartz show results of 226-231degreesC. Pressure conditions of trapping for all three groups of inclusion fluids have been estimated to 520-985 bar at 220degreesC. The pressure range is probably a result of fluctuations caused by repeated fracture opening and sealing due to seismic activity coupled with mineral growth. A lack of systematic textural relationships between the three groups of inclusions and similar pressure-temperature estimates for all fluid types indicate trapping at similar times and a process of rapid change. Fluid migration in fractures from an overlying, overpressured sedimentary basin into a dry, crystalline basement best explains the observed P-T-t-X constraints.