We describe a process of phase fractionation, which is an mechanism by which a gas flux can alter and remobilize oil. Moving gas can dissolve and transport large fractions of oil. The composition of the oil dissolved in the gas is dependent on the pressure-temperature conditions of the oil, and the fluid flow history of the basin. The composition of the remaining oil can be interpreted to yield both the depth at which the oil fractionated and the volume of gas that fractionated that oil. The composition of the oil transported by gas can also be interpreted to yield information about processes deeper in the basin.

Several case studies of fractionation are examined. South Eugene Island Block 330 in the US Gulf Coast is a hydrocarbon province that is experiencing large gas fluxes at present, and probably in the past as well. The oils in the region show signs of progressive fractionation and remobilization due to gas transport. These signs include increased aromaticity and decreased paraffinicity in both the oils and the condensates of the region. The oils also are depleted of light n-alkanes. The Handil field in the Mahakam Delta, Indonesia, also contains oils that appear to be fractionated. The location of samples above an overpressured regime in the basin can yield insights into fluid leakage from that regime.

We compare the compositions of fluids in the study areas to results from a computer-based model of oil alteration that can simulation phase fractionation. The model provides a quantitative estimate of the amount of gas that has fractionated each oil in the region, and the depth at which that oil was fractionated.