First of all, it was necessary to find out under what geological conditions a deposit could be formed - an accumulation of oil and gas in rocks.
Geologists began to think: what combination of natural conditions can lead to the accumulation of oil? First of all, there should be so-called reservoirs in the region - rocks capable of absorbing and then giving away liquids and gases.
Among which rocks to look for collectors?
It should be said that by that time all rocks had already been divided into three large groups: sedimentary, magmatic and metamorphic.
Sedimentary rocks - as their name suggests - were formed from sediment that once sank to the bottom of the sea. Sometimes, however, such processes can also be observed on land: for example, the wind can carry fine sand and dust over considerable distances and precipitate it hundreds, if not thousands, of kilometers from the original formation. Such rocks include sand, sandstone, clay, limestone, dolomites and some others.
The second group is the magmatic rocks. And here the name suggests that this group includes rocks formed from magma. Cold magma thrown out by volcanoes turned into granites, basalts, porphyrites.
Finally, the third group is rocks that have undergone metamorphosis at birth. They could be formed from both sedimentary and magmatic rocks under the influence of underground high temperatures and pressures. Metamorphic rocks include shale, marble, jasper, etc.
As you now know, oil makes sense to look for among sedimentary rocks first of all. They are the ones with the best collector's properties.
However, the collector is not the same as the collector. Over time, they began to be distinguished by porosity, i.e. the total volume of all pores in a given rock, and by permeability - the ability to pass through the liquid and gas. In principle, porosity and permeability are mutually dependent values.
But there are also cracked collectors in nature: the rock itself has few pores - it is dense enough, but it is covered by a network of cracks, which are connected in a single network and can create channels that are tens of kilometers long. For example, some rock usually has poor reservoir properties, but if it is permeated by a network of macro- and micro-cracks, it may well become an oil storage facility.
However, if the entire sedimentary strata consisted only of reservoirs, it is unlikely that they could form any large deposit. After all, reservoirs not only accumulate but with the same ease and give away the accumulated. Oil and gas would go up to the surface and evaporate before they could concentrate in the field.
Therefore, it is necessary to have one more condition of formation of the deposit - it should be covered with some dense screen from above, i.e. a layer of rocks impermeable to oil and gas. Tires like this are usually clay, rock salt or limestone if they are not cracked.
Finally, for happiness to be complete, an anticline must be present in the area. That's what geologists call the bending of the layer, directed upward by the bulge. Underneath it, like in a trap, accumulate oil and gas reserves.
Anti-clinals are often formed by the plastic flow of rock salt. At some point, the overlying rocks have been overpressed. Underneath, the salt layer is also supported by hard rocks. And now, in search of a way out of the created vise, the salt layer begins to shift to the side. And in the place where the top pressure decreases, the salt immediately rushes up, forming an anticline fold.
Anti-clinical traps include the overwhelming majority of discovered oil and gas fields in the world - almost 90% in Russia and about 70% abroad. The size of the deposits can vary greatly: from small deposits of about 5 kilometers in length and 2-3 kilometers in width, with a height of 50-70 meters, to giant deposits of hundreds of kilometers in length, dozens in width and hundreds of meters in height. For example, one of the world's largest Ghawar fields in Saudi Arabia has a size of 225x25x0.4 kilometers!
There are also traps of other types. For example, tectonic screens appear at the rupture of layers during tectonic movements. Together with anticlinals, they are referred to as structural traps (formed during changes in the structure of the earth's interior).
In addition to them, traps of non-structural type are occasionally found. A typical example of such a trap is buried reefs. Once upon a time, in other geological epochs, these were actually reefs at the bottom of the primitive sea. Over time, however, they were covered by later impenetrable rocks and became traps for oil and gas in the depths of the Earth, as the coral reef is a chain of hills or even mountains made of porous limestone, in which there may even be cavern caves.
In addition to them, traps and non-structural types of traps are occasionally found. A typical example of such a trap is buried reefs. Once upon a time, in other geological epochs, it was actually reefs at the bottom of the primitive sea. Over time, however, they have been covered by later impenetrable rocks, found themselves in the depths of the Earth and have become traps for oil and gas, as the coral reef is a chain of hills or even mountains of porous limestone, which may even contain cavern caves.
The size of these mountain ranges can be very impressive. True, the largest deposits associated with underground reefs, which to date have been discovered in Mexico, are only about 200 kilometers long and 2-3 kilometers wide, but who knows what news will be presented to us by geologists tomorrow. After all, not all the secrets of underground storage rooms are revealed.
Well, for the sake of completeness, it is necessary to say a few words about the traps of non-structural type, which are formed as a result of lithological wedging. In the thickness of the Earth, rock layers can be placed at different angles. When two rock strata meet, a wedge trap filled with collector rocks can form when joining.
Sometimes, such deposits are formed along the channels of buried rivers. Such deposits were dubbed "shoelaces for shoes" by Americans because of their appearance.
It should be added that everything said about collectors, traps, buried reefs, anticlinals refers not only to the mainland deposits but also to the seabed, primarily the shelf of the World Ocean. It is here that sensational geological discoveries have been made in the last decade. They have become the basis of a new industry - offshore oil and gas production. But we will talk about it later.