Changes

|}

|}

If a reservoir contains both a gas cap and an oil zone, then both fluids are normally at saturated conditions initially. Initial pressure equals the dewpoint of the gas cap fluid, and it equals the bubblepoint of the underlying oil (Figure 2). The repeat formation tester (RFT) has made the determination of initial [[fluid contacts]] possible in reservoirs with reasonable [[permeability]], that is, >1 md. A saturated gas cap in equilibrium with an underlying saturated oil, for example, will be seen as a sharp discontinuity in RFT pressures at the gas-oil contact.

If a reservoir contains both a gas cap and an oil zone, then both fluids are normally at saturated conditions initially. Initial pressure equals the dewpoint of the gas cap fluid, and it equals the bubblepoint of the underlying oil (Figure 2). The repeat formation tester (RFT) has made the determination of initial [[fluid contacts]] possible in reservoirs with reasonable [[permeability]], that is, >1 md. A saturated gas cap in equilibrium with an underlying saturated oil, for example, will be seen as a sharp discontinuity in RFT pressures at the [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=gas-oil%20contact gas-oil contact].

In the past 20 years, deeper petroleum reservoirs have been discovered and the traditional interpretation of a reservoir containing both gas and oil has changed. An alternative interpretation in some gas-oil reservoirs is that composition varies continuously with depth. Here the fluids at the shallowest elevations are gas condensates, while the fluids at greater depths are oils. Sometimes the initial reservoir pressure may be greater than the saturation pressure of all mixtures in the reservoir, implying that the reservoir is entirely undersaturated even though a gas is at the top and an oil is at the bottom of the reservoir. Reservoirs of this type would not show a sharp contrast in RFT pressures at the depth where the fluid changes from a near-critical gas to a near-critical oil. Instead they would show a continuously increasing pressure gradient (for example, from 0.2 to 0.3 psi/ft).

In the past 20 years, deeper petroleum reservoirs have been discovered and the traditional interpretation of a reservoir containing both gas and oil has changed. An alternative interpretation in some gas-oil reservoirs is that composition varies continuously with depth. Here the fluids at the shallowest elevations are gas condensates, while the fluids at greater depths are oils. Sometimes the initial reservoir pressure may be greater than the saturation pressure of all mixtures in the reservoir, implying that the reservoir is entirely undersaturated even though a gas is at the top and an oil is at the bottom of the reservoir. Reservoirs of this type would not show a sharp contrast in RFT pressures at the depth where the fluid changes from a near-critical gas to a near-critical oil. Instead they would show a continuously increasing pressure gradient (for example, from 0.2 to 0.3 psi/ft).