Crude oil, or petroleum, forms in regions of rich in biological activity which gave rise to a layer of mud containing an abundance of organic matter. This usually occurs in oceans, lakes or river basins. Intense heat and pressure build up inside this layer as it becomes buried deeper and deeper under newly-formed layers. The organic matter undergoes density-dependent chemical modifications. The heat drives the formation of shorter and lighter hydrocarbons while liquids expand with the rising temperatures and ascend to newer layers. In the absence of a trapping layer, the low density oil flows upwards and emerges from the surface, giving rise to petroleum seeps. Under different geological conditions an oil reservoir can from when the oil becomes trapped between an overlying non-permeable layer and a porous layer such as sandstone. The trapping layer can crack during earthquakes, in which case the oil may surface.
Curvature of the rock layers creates a fold trap, a region in which the presence of a trapping layer (green) allows for the collection of light oil (red) within porous rock (yellow). Adopted from Wikipedia.
Crude oil and its different derivatives have been in use since 4000 years ago. The writings of the ancient Greek historian Herodotus describe how asphalt was used for constructing the walls and towers of Babylon. Since the region is rich with tar pits and asphalt seeps, large volumes of viscous oil would collect on the banks of the river Issus, a tributary of the Euphrates. Persian tablets documented the use of oil in medicine and for lighting already in ancient times. So sometimes it is the oil that finds the people…
… While others have to go looking. The intensity of the exploration for oil increased in the middle of the 19th century, as distillation of crude oil into kerosene became an inexpensive alternative to whale oil. Initially, searches focused on petroleum seeps (above the ground) followed by drilling in the region, combined with a fair share of superstition and peculiar instruments such as the famous y-shaped dowsing rod.
Dowsing – the "spiritual" method for exploring for underground water, metals or oil. Adopted from Wikipedia.
The Paraffin Oil Company was the first to take ground samples and analyze their composition prior to drilling, but it took almost 20 years before oil companies began to employ geophysical methods. One of the first tools utilized by oil explorers was the torsion balance, an instrument that measures tiny variations in gravity. It was used for locating regions with low density rock formations in which collecting layers such as salt domes are likely to be found. Other versions of gravity-measuring instruments, such as pendulums, were employed throughout the 1930s, until the invention of the gravimeter.
While sedimentary rock has no magnetic signature, igneous rock and metamorphic rock both leave quite an intense signature. This fact is also used in oil exploration. Since collecting layers must be porous, only those made of sedimentary rock can contain oil. Two instruments are used by explorers to measure tiny variations in the magnetic fields between rocks or different regions. The airborne magnetometer measures the average magnetic signature in large regions and is used for mapping large geological formations, while the field balance measures variations in specific regions.
Another technique for finding potential oil reservoirs is the seismic method. During World War I, a German by the name of Ludger Mintrop developed a method for locating enemy artillery through the measurement of seismic waves by three detectors set at different locations. Having been familiar with the geological features of the terrain, Mintrop was able to pinpoint the exact location of the guns by triangulation. After the War Mintrop started using his method in a reverse fashion – collecting geological data from a region of interest by setting an explosion at a known distance and measuring the ensuing seismic waves. Basically, the more dense or crystalline a rock is, the faster that the waves travel. Explorers measure the velocities of refracted and reflected waves, which differ according to the densities of the rock layers underneath the ground.
Map showing seismic reflection of rock layers up to a depth of 250 meters (adopted from Wikipedia).
The final scientific method I will discuss is called stratigraphy, the study of rock layers and layering. Through high precision mapping of rocks, fossils and wells and the analysis of seismographic and radioactive data, it is possible to accurately map underground geological formations, and to evaluate the probability of locating oil reservoirs. Core samples are taken prior to drilling in specific spots, and a variety of measurements are recorded and integrated into three dimensional maps of the layers, their composition and their depth. Low electrical conductivity is an indicator for the presence of natural gas or oil, since their conductivity is lower than that of interstitial water (water within the rock pores). Radioactivity is measured in the form of gamma radiation and neutrons to accurately identify the different layers, and acoustic measurements, including the flow rate of the drilling fluid, are taken to assess the porosity of the rock.
So now that you know everything that you need to know (well, almost…), wouldn't you prefer to try dowsing?
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