school energy&environment

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. Organic substances derive from animal and plant organisms that, when dead, accumulate in sediments and detritus that deposit at the bottom of sedimentation basins. Accumulation can take place in both a marine or a continental environment and in both cases the organic substances are generally decomposed rapidly and only a small part (about one per thousand) avoids the attack of bacteria and the oxidation processes.
Organic substances can therefore accumulate in great quantities only in sedimentary rocks that form in basins that have been stable for a long time. The accumulation of organic material also depends on the production rate of the same, that is conditioned by environmental factors such as the availability of food and nutritious substances, light intensity and temperature. The sites that are more favourable to a high accumulation of organic material (remains of marine and land organisms) are warm, relatively shallow seas and those off the coast. In deep sea environments organic material is less, due to a scarce supply, while on land it decomposes very rapidly.
The first places to look for hydrocarbons are therefore those areas in which shallow water marine sediments that are rich in organic material are present: the ‘mother rocks’ from which hydrocarbons originate. It is also necessary that the organic substances should be preserved and protected from the decomposition processes as much as possible. This can be accomplished in regions of quick sedimentation, where the material is rapidly buried or in environments deprived of oxygen. Anoxic conditions can be found in closed basins with limited or no water circulation, as in lakes or lagoons with scarce communication with the open sea, in relatively shallow seas within emersed lands or in very deep oceanic basins. Anoxic basins are so tightly connected to the presence of noteworthy hydrocarbon deposits that they are considered among the most important generators of mother rocks and are therefore the object of exhaustive studies by those working on gas and oil exploration. Mother rocks formed in closed anoxic basins have been identified in all the main oil yielding provinces: Venezuela, Colombia, Gulf of Mexico, Saudi Arabia and Alaska, while mother rocks that have formed due to rapid sedimentation have been found in the oil-yielding provinces of Argentina, West Africa, the North Sea, USA and Italy (the Po basin).