About petroleum geology?

Question

why is the process of migration necessary for the production of oil. wont we get oil in the reservoir rock if it gets matured and also the trap is available there itself..

Answer 1

Oil is produced in "source rocks". These rocks do not act as a trap and petroleum will migrate as more petroleum is produced in the source rock. Source rocks are also poor reservoirs, so migration to rocks like sands and sandy shales make recovery easier.

Answer 2

Lady Geologist is right, most oil is generated in kerogen-rich rocks that have very poor permeability like shales and tight carbonates. The petroleum is expelled as the pressure of generation exceeds the fracture gradient of the source rock and the freed oil migrates upwards and through permeable beds, fractures and faults in the overlying rock column.  Most porous and permeable beds that are suitable for the storage and production of petroleum are poor source rocks due to ease of which any kerogen in them is degraded and destroyed.  Migration is not always necessary for petroleum production.  There are a few examples of a source rock also acting as the reservoir - the big shale plays like the Bakken in North Dakota, and the Barnett in Texas.  They produce because engineers enhance the natural fractures in the rock.