Fluid Generation

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The main source of petroleum fluids is from the maturation of organic Kerogen within sedimentary  rocks. However important contributions particularly of contaminants can be generated by biological activity and the thermal destruction of carbonates and sulphates.

Sedimentary source rocks contain only around 3-10% of organic material. The majority of this organic material is categorised as Kerogen, an insoluble mixture of high molecular weight compounds. Kerogens release oil or natural gas under the extremes of temperature and pressure encountered during sediment burial. Kerogens can be classified into several types, each with a tendency to release a characteristic fluid, liquid or gas. The characteristics of the released fluid are also influenced by the source rock maturity. In the reducing environment in which hydrocarbon fluids are released, the remaining Kerogen becomes progressively more carbon rich, whereas the extracted fractions become more paraffin rich. A typical source rock under increasing temperature will initially release CO2, then aromatic oils, and finally simple paraffins such as methane.

Biogenic gas is generated in shallow cool reservoirs and consists predominantly of methane but can contain other significant contaminants such as CO2 and H2S. Biological activity particularly leading to H2S generation is also stimulated by water injection. A further cause of contaminant production is by the thermal destruction of carbonates and sulphates. The source of contaminant generation can be deduced by isotope analysis which can lead to deductions about the location and concentration of contaminants.

An understanding of fluid generation can help determine the filling sequence of reservoirs and lead to models of fluid properties and contaminant distributions.