In general, migration is from areas of relatively high pressure to low and from areas of relatively high temperature to low. The porosity of rock formations may be of two general types, described by petroleum geologists as primary and secondary. Primary porosity consists of the open spaces in the rock that existed at its formation. This kind of porosity is the open space that exists between the grains of the rock. Secondary porosity consists of open spaces, such as fractures caused by deformation or voids caused by solution of soluble rock, which were induced in the rock subsequent to lithification.
Permeability is the capability of a rock to transmit fluids.
Many factors are involved in the relative permeabilities of strata. In general, coarser grained sedimentary rocks are more permeable because they contain larger, more interconnected pores than the finer-grained Aones. Porosity and permeability are important, not only for permitting and enhancing fluid migration from source beds to reservoirs, but also for efficient draining of the reservoirs once they are drilled. Unless hydrocarbons are trapped somewhere within the earth, they will migrate to the surface and escape into the natural environment. Traps suitable for containing hydrocarbons may be formed in many different ways.