Glen M. Gatenby¹

(1) Consultant, Missouri City, TX

ABSTRACT: Phase Separation - An Important Aspect Of Petroleum Migration

Major oil fields in the Deep Water GoM are found at subsurface depths coinciding with significant changes in the solubility of oil in gas - suggesting that oil and gas phase separation is a pervasive step in petroleum migration. Additionally, changes in the solubility of methane in water coincide with the depth of gas reservoirs - suggesting that gas exsolving from water is also an active process in hydrocarbon migration.

The solubility of oil in gas is calculated from Price's experimental data and plotted versus depth. With increasing temperature and pressure (depth) the solubility of oil in gas increases and eventually becomes asymptotic leading to the two becoming cosoluble. Viewing this process from the bottom up, vertically migrating hydrocarbons are most likely to phase separate at this depth. The importance of this "depth to phase separation" cannot be overstated as the largest fields are found proximally above this depth. Generally, as the magnitude of the change in solubility increases, the accumulation size also increases. Conversely, no commercial pays are found beneath the depth where oil and gas are cosoluble (an economic basement).

The solubility of methane in water is calculated using Haas' program and when plotted versus depth also reveals that major gas reservoirs are commonly found proximally above the depth vertically migrating water experience an abrupt decrease in methane solubility.

The solubility versus pay relationship is an improvement over the pressure versus pay relationship and can be done ahead of the bit.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado