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Fluid Evolution: Migration and Precipitation of Hydrocarbons and Metals |
Department of Geology, Queen’s University of Belfast, Belfast BT7 1NN, UK
Residues of hydrocarbons (bitumens sensu lato) are found in a wide range of settings in addition to reservoir porosity, including microfractures, megafractures, and several types of vein porosity. Paragenetic relationships between hydrocarbons and inorganic minerals provide information on the relative timing of hydrocarbon migration and the migration of other fluids. Hydrocarbons in reservoir rocks and mineralized fractures are paragenetically late as they are in pre-existing fluid pathways. Some other types of hydrocarbon occurrence are paragenetically earlier, especially proximal to the source rock. Different types of ore mineralization (notably base metal and uranium) in which hydrocarbons occur exhibit varying parageneses which represent distinct burial/thermal histories responsible for the ore types. Co-migration of hydrocarbons and silicon-bearing fluids is evinced by the occurrence of large quantities of authigenic silicate minerals within hydrocarbon residues.
Interactions between metals and hydrocarbons may be exploited in the exploration for both metals and hydrocarbons. Metal-rich fossil fuels can preserve a signature of anomalous metal concentrations in the vicinity of ore deposits. Hydrocarbons precipitated by irradiation-solidification processes about radioelement-bearing phases help to identify hydrocarbon migration pathways and in some circumstances allow isotopic dating of migration.
