How does petroleum migrate upward through porous rock beneath Earth’s surface?

Petroleum migrates upward through subsurface rock primarily due to the lower density of hydrocarbon fluids and gases compared to the water that saturates nearly all subsurface sedimentary formations.12

This process occurs in two main stages:

• Primary Migration: Hydrocarbons are expelled from fine-grained source rocks (such as shales or clays) into more permeable layers. This is often initiated by the pressure of overlying sediments during compaction and the dehydration of clay minerals.3
• Secondary Migration: The hydrocarbons move through the wider, interconnected pores of coarser-grained carrier beds, such as sandstones or carbonates. Because these rocks are water-saturated, the oil and gas float upward through the aqueous environment over geologic time until they are trapped by nonporous barriers or reach the surface as seeps.1

This movement can be further influenced by active water flow, diffusion, or structural changes in the Earth's crust, such as folding or plate tectonics.1