There are no more blank spots on the modern geographical map of the world. The surface of the Earth has been studied as well as possible. At the same time, if someone undertook to make a detailed map of the Earth’s interior, with the exception of some individual areas, it would be a solid white spot. Even taking into account all the data known to date, the Earth’s bowels are a very poorly studied area. At present, their study is actively continuing, as well as the search for new oil and gas deposits.
From the “wildcat” to the rational scientific approach
The growing need for oil has led to an explosion of knowledge about the earth’s interior and the processes that have taken place in it over millions of years. The path that geology has taken over the past century is on a par with the space and nuclear industries in terms of scale and innovation. The development of geology has made it possible to successfully identify the most promising areas for oil prospecting and to determine geological structures in which oil fields can be formed. At the same time, oil prospecting is still an art in which the experience and skills of specific specialists often mean more than the techniques themselves and scientific research.
The science of oil exploration has come a long way from drilling “for luck” (the so-called “wildcat” method) to rigorous scientific approaches. In the past, the search for oil fields was concentrated in the areas where oil came to the surface of the earth. This was an obvious and, for the time, quite reasonable approach. Over time, oil prospecting methods became more and more sophisticated, while the targets themselves became more and more difficult to reach and often smaller in size.
In order to distribute resources more rationally and reduce costs, oil prospecting is usually carried out according to the principle from the general to the particular. That is at first a large oil prospective region is identified and gradually narrowing the area of searching identify in this area the most promising points for drilling exploration wells.
The main purpose of all the prospecting work is to identify in the oil prospective area of geological structures that can accumulate and retain oil. Such structures, called traps, may have different configurations, but they all have in common the presence of permeable rock bounded by impermeable rock strata.
So what are the methods that help find oil?
Methods of oil field prospecting are divided into:
- geological;
- geophysical;
- geochemical.
Geological methods focus on surface data. To do this, geologists study and describe the rocks that come to the surface of the earth. To do this, they locate rock outcrops or drill small pits to find out what rocks lie beneath the modern surface layer of sedimentary material. Photographs taken from high altitude (from an airplane or even from space) are also studied. These images often reveal surface signs of deep structures favorable for oil and gas accumulation. The data are used to create a geological map, which is a projection of rock outcrops on the surface.
Such surface data, of course, is not enough to identify oil deposits. To “see” what are the depths of the bowels of the earth, we use geophysical methods.
Geophysical surveys are methods of studying the earth’s interior using physical phenomena. These include electrical prospecting, gravity prospecting, magnetic prospecting and seismic prospecting.
Electrical exploration is based on studying the parameters of a constant or alternating electromagnetic field. Because different rocks and the fluids saturating them conduct electric current in different ways, certain conclusions can be drawn about the nature of the underlying rocks by studying changes in the electromagnetic field.
Gravity exploration is based on studying changes in the gravity field. Dense rocks can affect the gravity field. Even the smallest changes in the gravity field can indicate the types of rocks and their saturating fluids that lie deep within the Earth’s interior.
Magnetic exploration, as the name implies, studies changes in the magnetic field. Sedimentary rocks saturated with oil do not have magnetic properties, while magmatic and metamorphic rocks that do not contain oil do. Thus, magnetic prospecting can also suggest the types of rocks in the subsurface.
For fields at a late stage of development, it is especially important to increase the recovery of reserves. This task can be successfully solved by drilling sidetracks from the existing stock of wells.
Finally, seismic exploration is the most important way to explore the earth’s subsurface.
Seismic surveys
Seismic surveys are one of the most effective methods of searching for oil deposits. They are based on the study of the propagation of elastic vibrations in the rock strata. The general scheme of research is as follows. On the surface (or near it) a sound wave is generated which spreads deep into the subsurface in an expanding sphere. At the boundaries of rocks occur various effects of refraction, reflection of elastic waves, which are registered on the surface of the ground with special devices. The data obtained are recorded, processed, and brought to a single format. The result is a fairly accurate image of the geological structure in the study area.
Sound (elastic) waves, which are used to obtain data on the deep structure of the earth’s crust, can be generated in different ways. When conducting research on land, small charges are detonated or special vibration generators are used. At sea, in order not to cause harm to marine inhabitants, airguns are most often used.
Attempts to use seismic surveys to search for oil deposits have been made since the 1920s. Until the 1990s, the only method used was 2D seismic, which produces a flat image of a cross section of the earth’s crust. With the development of computer technology, it became possible to analyze huge amounts of data, and therefore three-dimensional (3D) seismics began to develop. Needless to say, the 3D image obtained as a result of 3D seismic is much more informative than the flat image obtained by 2D seismic. Three-dimensional seismic allows not only identifying a prospective geological structure and estimating its size, but also helps determine the most expedient points for drilling wells.
Search for oil deposits – a complex activity
With the help of these methods it is possible to identify with high accuracy the structure of deep layers of rocks, types of rocks and determine the presence of promising traps, in which oil deposits could be formed. To verify the presence of hydrocarbons in the identified traps, hydrogeochemical methods are used. For example, an increase in arenes in the groundwater may indicate the presence of hydrocarbons in deeper layers of the subsoil. Gas surveys can confirm the presence of a halo of hydrocarbon gases that form on the surface of the earth around any oil or gas deposit.
All of these methods of searching for oil deposits go a long way toward identifying the most favorable structures. But the final verdict on the presence of commercial oil reserves can only be made by drilling exploratory wells. Nothing can replace the need to drill wells and conduct a test operation of a promising geological structure. Wells not only confirm the presence of oil reserves in the identified structures. They are used to determine the commercial potential of discovered reserves.
Thus, oil companies use many different technologies to help identify oil reservoirs deep in the earth. In the last 150 years major oil companies and independent oil explorers have drilled more than two million wells in search of oil deposits. The development of scientific approaches and exploration techniques has greatly increased the chances of discovering new oil and gas reserves. And the development of seismic surveys has greatly reduced the cost of drilling unsuccessful prospecting and exploration wells.
But despite the long and successful development of methods and techniques for prospecting and exploration, finding oil fields is still an extremely complex, complex and fairly risky endeavor. And the success of the work carried out is never guaranteed.
