So, How Does Oil Accumulation In Reservoirs Happen?
For a commercial sized oil accumulation to exist, an ample volume of oil has to be generated from a source rock; it has to migrate into a porous rock formation with fair permeability; and, it has to be trapped in that formation by some relatively impervious layer, or cap rock, above it.
Hydrocarbons are thought to be generated from organic-rich rocks known as shales. Shales typically have poor porosity and permeability, and are consequently considered poor reservoir rocks. Shales do, however, contain high organic content necessary for oil generation, and are considered prime source rocks.
As pressure and temperature work on the organic-rich source rock, oil is created and squeezed out of the formation and into an oil sand or limestone reservoir. Due to the relative densities of oil, water and gas, gas and oil tend to rise in relation to water present in the pores of the reservoir rock. The hydrocarbon continues to rise until it either reaches the surface, the top of the water column or the top of a trap. Hydrocarbon that is trapped forms today’s producing reservoirs.
So, what are traps again?
Traps are physical features, concave impediments to the upward migration of reservoir fluids. Traps can be structural (anticlines), stratigraphic (unconformities), and combinations of the two.
The result of these independent occurrences is the main objective of all oil exploration, the location of a column of reservoir fluids, usually the lightest on top (gas, oil, then water) with no place for the hydrocarbon to escape - a prime situation for exploitation.
Go to Oil Exploration from Oil Accumulation