Oil Floats
Anyone who has ever seen oil spilled in a water puddle will notice that it creates colourful patterns in the sunlight. This is because the oil, which is less dense than water, forms a separate layer which actually floats on the surface of the water. The different light refracting properties of the two layers create a prism effect, and hence, the colour patterns.
Droplets of oil in rocks buried deep underground will also float above the water that is also present within these rocks. Chapter 3 explained how the sedimentary rocks containing these oil droplets formed under water. Hence, these rocks must still contain some water. So, instead of running downhill as surface water does under the force of gravity, oil droplets in the subsurface tend to move upward, under the force of buoyancy, so as to float above the water that shares the same pore spaces. Driven by buoyancy, these oil droplets migrate upwards toward the surface through pores and cracks within the layers of rock. If these drops of oil encounter an impermeable surface through which they cannot flow, they will continue to flow upward along the underside of this impermeable sealing rock and collect in traps as shown in Figure 4.1. If no traps are encountered, the oil droplets will migrate all the way to the surface creating an oil seep. Oil seeps” are quite common in areas of petroleum potential and were used by the earliest oil prospectors to identify drilling locations.
Oil reservoirs in the subsurface in many ways resemble inverted ponds or lakes. Unlike ponds and lakes, however, accumulations of hydrocarbons are contained inside the pore spaces of solid rock.
(*) Oil seeps are common throughout the western portion of the island of Newfoundland especially in the Port au Port and Parsons Pond areas.