Hydrocarbon migration is a process that petroleum migrates from source rocks into reservoir. In petroleum geology, hydrocarbon migration is divided into two parts, which are a primary migration and a secondary migration.
The primary migration is when petroleum comes out of source rocks. The mechanisms behind the expulsion of hydrocarbons from source rocks into reservoir rocks are not clearly understood. There are several questions which nobody can clearly answer as (AAPG Wiki, 2015);
- How does oil escape from the source rock?
- Does oil migrate out of the trap?
- Why are there marked differences in oil gravity, wax content, and sulfur content in lateral and stratigraphically successive sands?
- Why are there differences in water salinity for multiple sands in one structural trap?
- What is the role of faults in transporting and trapping hydrocarbons?
- Why are there barren sands within sequences of productive sands?
- How is the cross-formational flow of hydrocarbons accomplished?
- Does the form change during migration and, if so, which form is dominant under what conditions?
- How can we estimate the timing, volumes, and compositions of transported hydrocarbons?
Figure 1 shows the diagram of a primary and secondary migration.
Figure 1 – Primary and Secondary Diagram
(Ref Image: http://www.ngdir.ir/Data_SD/GeoLab/Pics/GeoLabPic_865_2.jpg)
Secondary migration is the movement of hydrocarbon through reservoir rocks such as limestones and sandstones which are permeable. Hydrocarbon can travel through these rock as distinct phases in the upwards direction where there is a decrease in hydrostatic pressure.
Three forces relating to how hydrocarbons move in porous rocks are a gravity force, a buoyancy force and a capillary force. In order for hydrocarbons to migrate, the buoyancy force must overcome gravity and capillary pressure. Hydrocarbons will naturally flow through reservoir rocks until they are stopped by impermeable structures. Then they begin to accumulate. If there is no trap, hydrocarbon will reach surface and this is called “oil seepage.”
Hydrocarbons accumulating in reservoirs will separate as layer sequences – gas on top, oil in the middle and water at the bottom (Figure 2).
Figure 2 – Hydrocarbons in Reservoirs
(Ref Image http://1.bp.blogspot.com/-_U6qLaOf49s/VIHgXlKG5SI/AAAAAAAACnk/zMaHaKYmsRc/s1600/natural_gas172.jpg)