The oil system
A suitable ‘cap’
For the hydrocarbons to remain confined within the reservoir rock it is necessary that it should be surrounded by rocks that prevent the hydrocarbons from moving away. Cap rocks must therefore have characteristics that are in contrast to those necessary for a rock to be a good reservoir: in fact, they have to be as impermeable as possible. Usually they are made up of fine-grained sedimentary rock (such as clay, marl, clayey limestone) or of evaporite rocks (such as gypsum and halite) and must not be very fractured. 95% of the cap rocks of the world’s main oil fields is made up of clays or evaporites.
The above-mentioned characteristics of reservoir and cap rocks are conditions that are necessary but not sufficient for the formation of noteworthy oil fields. A decisive factor is the shape of the ‘trap’ that imprisons the hydrocarbons because it determines the shape and volume of the reservoir and the magnitude of the reserves that the latter can contain. Traps can be either structural or stratigraphic. Structural traps are caused by tectonic deformations that have fractured and folded the rocks. The conformation that is most favourable is that of rocks deformed in anticline folds with the layers upwardly convex. These structures are therefore the most suitable to contain fluids that tend to flow upwards because they are less dense. Even evaporite rocks can originate excellent traps: salt deposits, being lighter than the surrounding rocks, tend to flow upwards and curve the layers above forming structures called ‘diapirs’ that are favourable for the accumulation of hydrocarbons. Very many oil fields in the world are associated to the presence of salt diapers. (for example, in Central Europe). Even tectonic structures, where fault systems create an alternation of low-lying basins and protruding areas (Horst and Graben), can constitute efficient trap systems like in the North Sea Basin and in the Rhine Trench between France and Germany. Structural traps are the easiest to identify with geophysical surveys which explains why the majority of the world oil fields are contained in structures of this type. Stratigraphic traps, instead, are formed due to sedimentary causes, when there are sudden changes in the permeability and porosity of the rock, such as in river and relatively shallow sea environments. Even though stratigraphic traps are very numerous, they contain only 15% of the world oil fields, not because they are less efficient than structural traps but because their identification with geophysical survey methods is much more problematic. Within a trap, due to the difference in density of the various components, we find: at the top the lightest gas, below this, hydrocarbons and lastly, water. The surface that separates oil from water marks the lower limit of the oil field and its identification is fundamental to calculate the volume of hydrocarbons contained in the field.
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