Print this pageA Model Predicting the Geochemical Effects of Gas Solution as Means for Secondary Oil Migration
MEULBROEK, PETER T., and LAWRENCE CATHLES
We present the results from a one-dimensional computer model of gas-solution migration on a hydrocarbon mixture. In low TOC/lean source rock environments such as the US Gulf Coast, total oil saturation might be too low along migration pathways for single phase oil flow. Gas solution transport may, in these situations, provide a mechanism for oil migration.
The model is prescribed as follows: natural gas, produced either from the primary or secondary breakdown of kerogen, migrates to shallower depths. This gas, upon contacting oil, selectively dissolves some of that oil forming a single supercritical phase. Upon further migration, some of that dissolved oil exsolves as a liquid phase, and can be trapped. The gas may continue migrating, exsolving lighter and lighter oils.
The model calculates oil solubility in gas using an equation of state/fluid-phase equilibrium method. The model calculates the composition of the gas-oil mixture as it migrates upward, taking into account compositional changes due to additions (e.g., maturation), subtractions (e.g., trapping), and phase separation effects.
We have used this model to predict hydrocarbon zoning similar to that in the stacked reservoirs of Eugene Island Block 330. The deeper EI 330 oils are enriched in heavy alkanes, with the amount of lighter alkanes decreasing with depth. The model suggests that both of these effects (which are the inverse of the effects predicted by in situ thermal maturation) result from gas solution migration.