Oil-filled fluid inclusions occur in cements in petroleum reservoirs and are evidence for the generation and migration of hydrocarbons in a basin. Generally, oil-filled inclusions occur together with aqueous inclusions in the same cement crystal. Geothermetric studies of the aqueous inclusions provide thermal and compositional data pertinent to interpreting the time of cementation and hydrocarbon migration relative to source rock maturation.

Oil-filled inclusions occur both in random locations and in alignment with crystal cleavages or fractures. Random distributions of fluid inclusions suggest oil entrapment during growth of the cement crystal into primary porosity whereas the occurrence of fluid inclusions along sealed fractures suggests migration through secondary porosity. Generally, the oil-filled inclusions consist of liquid hydrocarbon and a gas phase, but inclusions containing oil, water, and gas also occur. Those different compositions suggest differences in the migration and mechanism of petroleum.

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Oil-filled inclusions are characterized by fluorescence spectra. In many cases, different episodes of hydrocarbon migration are indicated by the occurrence in the same crystal of oil-filled inclusions whose fluorescence spectra are different.

The organic chemical compositions of aqueous and oil-filled inclusions are determined by decrepitation-gas chromatography. Those compositions are compared to organic compositions of whole reservoir rock, reservoir oils, and source rock to decipher the history of oil emplacement and maturation. Oil alteration effects, possibly induced during the early stages of migration, are also detected.

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