Oil Pools and Migration

The dispersed droplets of oil ultimately go to form oil pools under the following necessary conditions :

1. Migration and accumulation
2. Suitable resevoir and cap rocks
3. Suitable traps/structures

The migration of dipersed droplets of oil generated within the source rock is caused due to :

1. Compaction
2. Capillary action
3. Buoyancy
4. It’s lower sp. gravity than water
5. Currents of sub-surface water

Best rocks for Accumulation

The migration of oil leads to it’s accumulation in suitable reservoir rocks which must be both porous and permeable. The clay having high porosity but small permeability is not suited to the accumulation of oil.

• The most suitable reservoir rocks are sands and sandstones. Porous and cavernous limestone and dolomite and other fissured and jointed rocks may also form suitable reservoir rocks.
• The greater the porosity, the greater the amount of oil that the reservoir rock can contain, and the larger the pore size, the greater the amount of oil it will yield, since rocks with small grains and small pores create more frictional resistance to flow.
• Furthermore, a good impervious cap rock is necessary to retain oil in the reservoir rock, Clay and Shale are the most common cap rocks.

Some migration of petroleum could have occurred in a gaseous form along the highly impermeable micro-fractures and microscopes towards the low pressure areas.