Brief background

 

The word "petroleum" is used in geology to designate any natural mixture consisting primarily of hydrocarbons, whether in solid, liquid or gaseous form at room temperature and pressure.

 

From a legal point of view, according to Decree-Law No. 109/94, of April 26 ("DL 109/94"), petroleum is: "all natural concentrations or mixtures hydrocarbons in the liquid or gaseous state, including all substances of any other nature that are found in combination, suspension or mixture with the hydrocarbons, with the exclusion of natural, solid hydrocarbons the exploitation of which can only be made by extraction of the reservoir rocks themselves”, being therefore included in the definition of petroleum, the natural gas.

 

Source GPEP: Oil impregnated cores

 

Petroleum is generated in sedimentary basins from accumulated organic matter, together with inorganic sediments, in oxygen-deficient environments. This accumulation takes place, in general, on the bottom of lakes, lagoons or seas with poor movement and currents close to the bottom. Thus, organic matter, although preserved from oxidation, undergoes modifications resulting from inorganic chemical reactions and the action of bacteria, resulting in the generation of some biogenic gas and the transformation of the remaining organic matter into kerogen, a material rich in very solid heavy hydrocarbons. Kerogen-rich rocks, in general fine detrital rocks (bituminous shales) or carbonate rocks (limestones and bituminous marls), are called source rocks, because it is in them that the generation of petroleum occurs.

 

With the continuation of the subsidence of the sedimentary basin in which the accumulation of organic matter took place, this matter is gradually subjected to higher temperatures and pressures and the kerogen is transformed, by decomposition and partition of its complex molecules, into simpler hydrocarbons, from oil to natural gas.

 

 

Other variables such as, time and the nature of kerogen, are fundamental to the type of hydrocarbon that is generated. There are 3 types of kerogen: 1) normally attributed to algae and bacteria – oil prone; 2) resulting mainly from the accumulation of residues of superior vegetables – gas prone; and 3) mixed, which is an intermediate type, which can generate both oil and gas.

 

The transformation of kerogen into petroleum is accompanied by an increase in volume that tends to expel the newly created hydrocarbons from the source rock, giving rise to the expulsion or primary migration of the petroleum which, eventually, may lead it to a suitable place for its accumulation or, as is often the case, to migrate to the land surface and seabed where it will naturally exude (“seeps”).

 

For petroleum to be exploited, i.e., extracted under economically viable conditions, it is necessary to accumulate in rocks with high porosity and permeability. Source-rocks generally do not have these characteristics and, therefore, petroleum can rarely be directly extracted from them. For this, it is necessary that the petroleum, after its expulsion from the parent rock, finds favorable conditions to migrate through the rocks, in a generally very slow process that can take tens of millions of years, to a reservoir rock where it accumulates. It should be noted that, in order to prevent seepage, another set of rocks are essential in this system: the sealing rocks of very low permeability, which, together with the arrangement and disposition of the geological formations and structures, traps, will constitute barriers to petroleum migration.

 

The set source or source rock, reservoir rock, seal rock and “trap” constitute a natural petroleum system.

 

The figure below illustrates the process of formation of oil and natural gas.

 

 

Source: Secondary Energy Infobook, NEED – National Energy Education Development, 2016

 

Due to the length of the process described above (millions of years), petroleum is considered a non-renewable natural resource, because at the human time scale, once consumed, it has an almost nil replacement rate in nature.

 

Before exploiting/extracting petroleum, it is necessary to carry out a long preliminary work of finding and researching the places where the resource may be accumulated in reservoirs with a size and location that allow for the economic viability of its exploitation.

 

In this context, petroleum exploration is based on determining the locations of sedimentary basins where the geological conditions necessary for the generation and accumulation of petroleum may be met, and proving through direct data the veracity of its existence.

 

The study of the subsurface geology of sedimentary basins is essential to determine the presence of potential source rocks, reservoir rocks and seal rocks, to understand the respective depositional environments and subsequently estimate their possible presence in depth to: i) analyze the tectonic deformation which could lead to the formation of “traps”; and, ii) outline the general features of the geological and structural evolution of the basins.

 

As a complement to the subsurface geological studies, geophysical prospecting techniques are used and, at a later stage of the exploration, the geological study of the subsurface through exploration wells, to identify, as best as possible, the geological conditions and properties in depth. In fact, it is not possible to guarantee the presence of petroleum in a reservoir without carrying out at least one exploration well.

 

Despite the subsurface geological and geophysical studies that are previously carried out, the probability of success of a exploratory well in new areas (i.e. without previous exploratory drilling) remains low (i.e. between about 10% to 20% [1]). If the analysis of the data obtained indicates good reservoir characteristics, a production test is carried out to determine the conditions under which petroleum can be produced.

 

Note that the economic value of a discovery can rarely be determined with a single exploration well. Although one has an idea of the volume of accumulation from the seismic and petrophysical interpretation (shape and dimensions of the trap and the porosity of the reservoir), it is generally necessary to drill more than one well to confirm the interpretation and safely determine the volume of the Recoverable reserves. If recoverable reserves are discovered in quantities that make the project viable in economic and financial terms, a new phase of the petroleum activity will evolve, related to the development and production of the petroleum field which has been delimited in the meantime.

 

It should be noted that, even with new technologies, petroleum exploration involves very high investments and is often unsuccessful, i.e., no discovery is made or a discovery is made without economic feasibility for its exploitation.

 

For more information, see “Livro Verde sobre a prospeção, pesquisa, desenvolvimento e produção de hidrocarbonetos em território nacional””, prepared by the former National Entity for the Fuel Market, E.P.E. (ENMC), current National Entity for the Energy Sector, E.P.E. (ENSE) (See Documentation/Disclosure - Publications)

 

[1] http://www.ifp-school.com/upload/docs/application/pdf/2015-02/5_exploration_well.pdf