Phase fractionation at South Eugene Island Block 330

Persistent gas flux can dissolve, remobilize and alter reservoired or migra ting oil through a process of phase fractionation. Moving gas, when flowing through an oil, can dissolve large fractions of that oil. The composition of the oil dissolved in the gas is dependent on the pressure-temperature co nditions of the oil and the fluid flow history of the basin. The compositio n of the residual oil can be interpreted to yield both the depth at which t he oil fractionated and the volume of gas required to fractionate the oil. South Eugene Island Block 330 in the U.S. Gulf Coast is a hydrocarbon provi nce that has recently experienced large gas fluxes. Some of the oils in the region show signs of progressive fractionation and remobilization by gas t ransport. For example, the oils are more aromatic and less paraffinitic tha n unfractionated oils of similar maturity from the same area. The altered o ils are also depleted of light n-alkanes. We have developed a computer-base d model of oil alteration based on a fluid phase equilibria algorithm to si mulate progressive fractionation of oil by gas. Application of the model to the South Eugene Island Block 330 area shows that several of the oils in t he area have compositions that are compatible with alteration caused by equ ilibrating with approximately 12 to 14 mol of gas per mol of oil (2 to 2.7 g of gas per g of EI oil). The oils appear to have fractionated at approxim ately the depths of their present reservoirs. The model has great potential to examine hydrocarbon fluids for evidence of past migration and mixing. ( C) 1998 Elsevier Science Ltd, All rights reserved.