Hydrocarbon migration stands for the movement of hydrocarbon from source rock to
reservoir rocks. In a petroleum system, there are mainly two types of migration
In primary migration process, hydrocarbons are expelled from fine grained source rock into an adjacent permeable carrier bed. .
Oil Phase Migration:
Most of the hydrocarbons are expelled in liquid state from the source rock . The dynamic process of expulsion of the oil out of the source rock is driven itself by oil generation. The total organic carbon or TOC of Good source rocks range from 3 to 10%. When the TOC is high, then the kerogen is main load-bearing part. With the conversion of organic matter into oil, the load-bearing kerogen will convert into liquid. Most source rocks are carboniferous shales(also called black shales)There will be high fluid pressure of the oil within these carboniferous shales which leads to the formation of micro fractures in the rock. After the formation of micro fractures, the oil is squeezed out followed by collapse of source rock. Such kind of Micro fractures are found in most productive source rocks which are full of remnants of oil.
Secondary migration is a process of migration which involves the movement of hydrocarbons through fracture network in the source rock to reach reservoir rock. Migration mostly happens for only one or more separate hydrocarbons phases (gas or liquid depending on pressure and temperature conditions). Buoyancy is the key driving force which functions vertically and is proportional to the density difference between water and the hydrocarbon. Hence, migration will be quite stronger for gas as compare to heavier oil.