Rock Types

Where can petroleum be found?

Petroleum and gas are hydrocarbons found in nature, in sedimentary basins which are places where there is accumulation of sedimentary rocks. They are formed by the decomposition and deposit processes of other substances, such as other rocks, and chemical and organic elements, over depressions that occur on the Earth’s crust, known as “sedimentary basins”.   

Other conditions are also necessary for the accumulation of petroleum and/or gas in a sedimentary basin. The presence of source, reservoir, and cap rocks is necessary for the formation, accumulation, and prevention respectively of emission to the surface, of petroleum/gas. The synchronization of the maturation conditions of the source rocks (adequate temperature and pressure) and the migration of petroleum and/or gas to the reservoir rocks are also necessary.

Source rocks are the ones in which the accumulation and preservation of plant and animal organic matter takes place. When these are submitted to certain combinations of temperature and pressure, they enable the creation and posterior expulsion of hydrocarbons. Generally, they are fine textured rocks, such as shale, siltstone, calcilutite, and marlstone. Depending on the composition of the organic matter present in them and in the temperature and pressure conditions to which they are submitted, favorable conditions for the generation of petroleum and gas can be achieved. 

Reservoir rocks are those that manifest favorable porosity and permeability conditions for the accumulations and production of petroleum and/or gas, given that oil is found in between the rock grain. The image of an oil deposit is, therefore, closer to that of an soaked sponge rather than a cave containing the liquid. The bigger the porosity and permeability, the better the reservoir rocks’ productivity conditions. The main reservoir rocks are sandstone and conglomerate (detrital). Some examples of detrital rock are the post-salt turbidite reservoirs, found in Marlim, Albacora, Roncador, and Atlanta, and the carbonates (chemical and biogenic) present in important reservoirs of the pre-salt section of the Santos and Campos basins, such as Tupi, Libra, and Carcará. There are non-conventional reservoirs as well, composed of rocks with low original permeability, which, through a large quantity of fractures, allow for the flow of petroleum and/or gas, which can be extracted in cost effective ways. Reservoir rocks commonly have salted water at the bottom, above it oil, and on top of the oil, natural gas.

The oil will tend to migrate to the top finding its way through cracks or spaces in between the grain and will eventually reach the surface of the Earth if it is not stopped by a waterproof rock that traps it. Upon meeting one of these, the reservoir is formed. The rocks that act as “caps” or “traps” have very low permeability, which stops the petroleum and/or gas accumulated in the reservoir rocks from migrating to the surface. The main cap rocks are shale, siltstone, calcilutite, and marlstone, as well as evaporite rocks, such as halite, carnallite, and anhydrite. Sedimentary rocks can be divided into the following types: “detrital”, “chemical”, and “biogenic”. 

Stopping the oil from migrating to the surface is important otherwise it begins to lose its volatile components and turns into natural asphalt, as the one which is being used by humanity since three thousand years B.C..

  1. Detrital Sedimentary Rocks: rocks formed by the disintegration processes of pre-existing rocks (igneous, metamorphic, or sedimentary) and posterior lithification; ex: sandstone, siltstone, shale.
  2. Chemical Sedimentary Rocks: rocks formed by the precipitation of substances dissolved in the waters of rivers, lakes, and oceans; sedimentary rocks formed through chemical processes; ex: halite, carnallite, stalactite, travertine. 
  3. Biogenic rocks: rocks formed by the carapaces of organisms (ex: coquinas, which are carbonates composed of shells) or when there is the predominant action of living organisms, such as microbe, bacteria, and algae colonies, which are formed by carbonates of the pre-salt section of Brazil. 

Igneous rocks: rocks formed by the solidification of magma, which are, therefore, also known as “magmatic”, and which can be subdivided as “intrusive” and “extrusive”.

  • Intrusive, or “plutonic”, igneous rocks are those in which solidification occurs at great depths and with slow cooling speed, and, therefore, in good conditions for crystallization. Granite is a good example.  
  • Extrusive, or “volcanic”, igneous rocks are those in which solidification occurs in shallow depths or on the surface, with a fast cooling speed, and therefore, with little time for the formation of crystals. Basalt is a good example.

Metamorphic rocks: rocks formed by the deformation caused by the action of pressure and temperature on igneous, sedimentary and even pre-existing metamorphic rocks. Granite (igneous) can be transformed into gneiss (metamorphic), or limestone (sedimentary) can be transformed into marble (metamorphic), through metamorphic processes.

Last updated on 2015-06-08T12:02:20

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