The oil system

If one observes the distribution of hydrocarbons in the world, it is quite clear that it is not uniform all over the planet, but there are areas in which hydrocarbons are much more abundant and others that are totally devoid: the imbalance between the quantity of hydrocarbons contained in the reserves of the different world oil-yielding provinces is very clear. What conditions and determines the distribution of natural gas and oil in the subsoil? The factors that determine the quantity of hydrocarbons present in a region are manifold and all of a geological nature: to understand why one region is richer than another and to evaluate the potential of an oil-yielding province it is necessary to know its geology very well, both in terms of the different types of rock you can find and in terms of its geological history. This knowledge, that is not always easy to obtain, is very important to be able to implement a preliminary evaluation on areas that have not yet been explored as far as hydrocarbon research is concerned and to determine their productive potential. It is a fundamental step when an exploration is being carried out.

Oil system

The set of all the characteristics that lead to the formation of an oil field make up the so-called ‘oil system’. This system is made up of the following fundamental elements that will be dealt with in detail in the subsequent paragraphs:

• the presence of a mother rock (source rock);
• the presence of a reservoir rock;
• the presence of a cap rock (seal);
• the formation of traps with a suitable structure.

The mother rock

Hydrocarbons are formed due to the transformation of organic material scattered in rocks. Organic substances provide the two elements essential to the constitution of hydrocarbons: carbon and hydrogen. For the formation of a significant quantity of hydrocarbons, the source rock must contain more than 0.5% of its weight in organic carbon. Hence, the first prerequisite condition for the formation of an important oil field is the presence of rocks that are rich in organic substances.

Transformation of organic material

The sediments that are progressively deposited bury the ones below, that are therefore covered by growing layers of material that accumulate in time. As they are pushed deeper and deeper in the Earth’s crust, the sediments slowly loose the water that they contained originally, become denser and more compact and are subjected to growing temperature and pressure.

Hydrocarbons, being light and not very dense, tend to rise while they migrate. If they do not find obstacles on their ideal pathway, they scatter in the overlying rocks until they reach the surface and give rise to spontaneous evidence: the so-called oil seepage that remains on the surface. In short, for an important oil field to form it is necessary that the rock formations that surround the mother rocks should be able to trap and accumulate hydrocarbons within, and require three indispensable conditions:

• the presence of a rock that can contain the hydrocarbons, the so-called reservoir;
• the reservoir rock must be bounded by an impermeable rock, called ‘cap rock’, capable of stopping the migration of fluids and of confining them within the reservoir;
• the disposition and configuration of the reservoir and cap rocks must be such that they form a rather capacious container with a shape suited to hold the maximum amount of hydrocarbons and constitute the so-called ‘trap’.

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.

Capacious reservoirs

Reservoir rocks must have an elevated porosity and permeability: the higher these values are, the greater the quantity of hydrocarbons that the reservoir rock can contain and the easier it will be to extract oil and gas. Naturally, the greater the volume of the reservoir rock, the greater the volume of the oil field.

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.