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.
Efficient traps
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.
Trappole efficaci
Le caratteristiche delle rocce serbatoio e delle rocce di copertura appena descritte sono però requisiti necessari, ma non sufficienti per la formazione di giacimenti importanti. Un fattore decisivo è la forma della “trappola” che imprigiona gli idrocarburi, cosa che determina la forma e il volume del reservoir e l’entità delle riserve che questo può contenere. Le trappole possono essere di tipo strutturale o di tipo stratigrafico. Le trappole strutturali sono dovute a deformazioni tettoniche che hanno fratturato e piegato le rocce. La conformazione più favorevole è quella di rocce deformate in pieghe anticlinali con gli strati incurvati verso l’alto e che sono perciò le strutture più idonee a contenere fluidi che tendono a spostarsi verso l’alto perché meno densi.
Anche le rocce evaporitiche possono originare ottime trappole: i depositi salini, essendo più leggeri delle rocce circostanti tendono a migrare verso l’alto e incurvano gli strati soprastanti formando strutture dette “diapiri”, molto favorevoli all’accumulo di idrocarburi. Moltissimi bacini petroliferi in tutto il mondo sono associati alla presenza di diapiri salini (per esempio, in Europa Centrale).
Anche strutture tettoniche dove sistemi di faglie creano alternanze di bacini depressi e aree più rilevate (Horst e Graben) possono costituire efficaci sistemi di trappole come nel Bacino del Mare del Nord e nella Fossa Renana tra Francia e Germania.
Letrappole strutturali sono le strutture più facili da individuare con indagini geofisiche, motivo per cui la grande maggioranza dei giacimenti mondiali è contenuta in strutture di questo tipo.
Le trappole stratigrafiche sono invece dovute a cause sedimentarie, dove variano repentinamente le caratteristiche di permeabilità e porosità della roccia, per esempio in ambienti fluviali e ambienti marini poco profondi. Le trappole stratigrafiche, pur essendo molto numerose, contengono soltanto il 15% dei giacimenti mondiali, non tanto perchè siano meno efficaci delle trappole strutturali, quanto perchè la loro individuazione con metodi di indagine geofisica è molto più difficile.
All’interno di una trappola, a causa delle differenze di densità dei diversi componenti, troviamo: alla sommità il gas più leggero, al di sotto di questo gli idrocarburi e infine l’acqua. La superficie di separazione olio-acqua segna il limite inferiore del giacimento e la sua individuazione è fondamentale per calcolare il volume degli idrocarburi contenuti nel giacimento.

 

 

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