Abstract

Vertical migration is the foundation of petroleum geochemical exploration. Early vertical migration models included a molecular process called diffusion. Diffusion remained the accepted mechanism during the early years of geochemical exploration even though the diffusion model could not explain many observations, including vertical migration.

About 20 years ago Gerry Arp (formerly ARCO) quantified a buoyancy driven migration mechanism. Below the water table gases migrate vertically as a gas phase with buoyancy providing a mechanism for predominately vertical migration. The buoyancy model explained the gradients and data contrasts observed in many surface geochemical features. The model even predicted vertical migration velocities that later were verified by field measurements. The buoyancy model explained most of what we observed from vertical migration measurements.

Petroleum liquids migrate similarly, but slower and through larger fractures and faults. Geo-chemical gas data map reservoirs and geo-chemical liquid data map the surface expression of faults and fractures. Different surface expressions of these two migrating phases form the basis of modern geochemical exploration.

The buoyancy model also made possible improvements in vertical migration detection and interpretation. Oil discoveries in the Midland and Maverick Basins illustrate the significance of this vertical migration model in geochemical exploration.