The migration of hydrocarbons

The hydrocarbons that form within the mother rock are generally scattered in the sediments and must have the possibility of migrating and concentrating to build up economically significant deposits. It has been calculated that only 5% of the hydrocarbons that form accumulate in oil fields of a certain importance. Migration takes place in two phases. Primary migration allows the expulsion of hydrocarbons from the mother rock with a mechanism similar to the one that provokes the alienation of the water originally present in the sediments due to the growing pressure to which they are subjected. Oil and gas are lighter and less dense than water so during  secondary migration  they tend to rise through pores and fractures in the rock.
For migration to take place, it is necessary that the surrounding rocks should be permeable, i.e. that they should have pores and fractures wide enough to allow the passage of oil drops and gas bubbles. The higher the permeability of the surrounding rocks, the higher the possibility that a  great quantity of hydrocarbons can accumulate. Permeability depends on the geological history of the region and is conditioned by two important factors:

  • initial permeability depends on the kind of rock and on the conditions of its formation; it is very high in gravel and sand, lower in sandstone and very low in limestone and in igneous and metamorphic rock;
  • secondary permeability is created by fractures and faults that form in the rock as a result of tectonic deformations or hollows that develop as a consequence of karstification processes.

The more productive oil-yielding provinces must therefore be sought for in places where the geological history has increased the probability of finding rocks with the characteristics of mother rocks surrounded by permeable rocks. Less permeable rocks, such as clay, act as an impermeable barrier that stops hydrocarbon migration and forms the ‘cap rock’

 

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