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The nature of petroleum fluid generation and migration often leaves reservoir gas and oil in equilibrium. The data suite of logs, fluid pressure gradients and fluid samples is often incomplete and cannot independently be used to define the fluid contacts and fluid properties of each phase. Using the constraints imposed by the equilibrium conditions can lead to valuable extra information to define fluid properties and contact depths. An understanding of these constraints can also lead to conclusions about the assumption of equilibrium between two separate phases.

Within reservoirs containing a significant hydrocarbon column height, there is the possibility of gravitational compositional gradients existing. This can be confirmed by detailed thermodynamic modelling of the fluids. The existence of a gravitational compositional gradient is an important indication of reservoir connectivity and can be used to differentiate vertical compositional variations due to the filling history of independent units and the oil diagenesis due to bacterial activity and solubility effects at the oil water contact.

Lateral compositional variations are also common and may be due to source maturity, biological activity, or secondary charging often linked to fault or fracture systems.

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