f Palaeothermicity of some European sedimentary basins derived from fluid inclusion and apatite fission track analyses

Oil-bearing fluids may be trapped in intracrystalline microcavities (fluid inclusions) created during crystal growth and microfracturing. Various minerals can trap hydrocarbons: silicates (quartz, feldspar), sulfates (anhydrite), carbonates (calcite, dolomite), halides (halite, fluorite) are the most common. These oil-bearing minerals are generally located in sedimentary environments (sandstone, limestone, salt), although occurences in metamorphic, hydrothermal and igneous environments are known. Oil inclusions are ubiquitous in petroleum reservoirs, and may be one-phase (liquid or vapor), two-phase (vapor bubble in a liquid phase) or three-phase (vapor, liquid and solid phases). Their composition and phase equilibrium are related to the PVTX conditions at the time of trapping. Fluid inclusions are used to determine an accurate basin paleothermicity. Comparisons can be made between the oil in the reservoir and the oil in the inclusions in order to determine the evolution of the oil between the time of trapping and the present. The bio-and/or thermal degradation can be estimated and the different generations of inclusions, observed in one sample, can be distinguished.