Crude Oil Exploration

Crude oil exploration involves geological studies and technical examination to detect the presence of crude oil. To determine the location for crude oil exploration, various geology surveys such as magnetic survey, seismic reflection survey and gravity survey are carried out first. At a more complex stage, elaborate seismic surveys are conducted to measure the time taken by sound waves to travel through matter. After confirming the presence of crude oil, exploration activities will now commence.

Let’s look at different method of crude oil exploration:

Ancient methods of crude oil exploration

Digging or drilling near known oil and gas seeps constitutes earlier methods of crude oil exploration. There are cases of accidental finds while drilling for water. Also, because of abundant seeps, guesswork and good luck were sufficient for finding oil. Earlier geologist used geological hunches and knowledge of existing seeps and petroleum residue in soil samples collected from near a gas spring to promote drilling for oil. Generally speaking, oil companies held the use of geology in low regard till the advent of geophysical methods of crude oil exploration.

Geophysical methods used in crude oil exploration

Geophysical methods of oil exploration enhanced the oil prospector’s knowledge of subterranean strata and it demonstrates an advantage for finding crude oil. Tools used were fairly basic and depended on fundamental variables in the earth's physical condition: gravity change, magnetic field change, time change, and electrical resistance. 

Example is Eoetvoes's torsion balance. Instruments like Eoetvoes’s torsion balance made use of the earth's gravitational field and the way the field varied according to differences in mass distribution near the earth's surface. Looking at it, because the density of rocks varies, the gravitational force they exert necessarily varies. If very light rocks are found close to the surface, the gravitational force they exert will be less than those of very heavy rocks. With this in mind, geophysicists attempted to locate oil-bearing structure which will be associated with minimum gravity by using the torsion balance instrument.

Another variation of gravity tool used in geophysical method of crude oil exploration is the pendulum method. This method relied on the period of a pendulum's oscillation adjusted by variations in gravity due to changes in altitude and latitude. The pendulum method was superseded by the gravity meter. Advances in gravity instrument technology afforded geophysicists better equipment with which to make more accurate determinations. 

The most common gravitational instrument in use today is the gravity meter or gravimeter, which measures variations in the earth's gravitational field by the gravitational pull on a mass balanced against some form of elastic force.

Magnetic Methods used in crude oil exploration

Another method of crude oil exploration is the Magnetic method. Most oil occurs in sedimentary rocks that are nonmagnetic. Igneous and metamorphic rocks rarely contain oil and are highly magnetized. By conducting a magnetic survey over a given area, a prospector can determine where oil-bearing sedimentary rock is more likely to be found.

Two types of magnetic instruments are used to measure the slight difference in magnetism in rocks, the field balance and the airborne magnetometer. The field balance is used on the earth's surface to measure magnetism in specific locations. The airborne magnetometer is used to measure the magnitude of the earth's total magnetic field over a large area.

Seismic Method of crude oil exploration

Another method of crude oil exploration is the seismic method. The central physical property upon which seismic crude oil prospecting is established is the variation in speed of the transmission of elastic earth waves or sound waves through different geological structures measured by time. 

There are two principle seismic methods: refraction and reflection.

Refraction prospecting consists of elastic earth waves, initiated by some concussive force, traveling down to a dense or high velocity bed, then being carried along that bed until they are refracted up to seismic detector locations on the surface some distance from the shot point. What is recorded is the time required for the sound wave to reach each detector location from the shot point. The speed of transmission of the waves through different geological structures is proportional to the density or compactness of the formation. Unconsolidated formations such as sands and shale’s transmit waves with a low velocity, weak sandstones and limestone’s with higher speeds, and massive crystalline rocks such as limestone, rock salt, schist, and various igneous rocks with very high speeds.

The refraction method aided petroleum explorers in locating salt domes that transmitted elastic earth waves at high rates of speed.

Stratigraphic Method of crude oil exploration

A final method of crude oil exploration is the study of stratigraphy. Stratigraphic exploration consists of establishing correlations between wells, matching fossils, strata, rock hardness or softness, and electrical and radioactivity data to determine the origin, composition, distribution, and succession of rock strata. Sample logs, driller's logs, time logs, electrical logs, radioactivity logs, and acoustic logs help geologists predict where oil bearing strata occur. Sample logs, compiled from well cuttings and cores, are used to identify key beds and lithologic sequences.

A core is a narrow column of rock that is taken from the top to the bottom of a well and shows rock in sequential order as it appears in the ground. Core samples also provide information on porosity, permeability, and saturation of rock in the well. Cuttings are not a continuous record like core samples, but provide a means for identifying sections within larger thick layers through fossil and mineral deposits. 

The driller's log provides basic information to the stratigrapher concerning depth, type of rock, density, fluids, and other miscellaneous data. The driller's log keeps track of the time required to drill through various strata and the recognition of key beds he drills through. This data is correlated with other information to enhance the chance of finding oil.

Early electrical methods of exploration tested electrical resistivity and electro-magnetic potential. Occurrences of oil and gas can be located by this difference in resistance. The most useful application of electric testing has been in the development and impact of well logging. Schlumberger electric well logging is now standard in the industry. These logs record the conductivity of interstitial water in rock, the movement of drilling mud into porous strata, and the movement of formation water into the well bore.

Radioactivity Logs, which record both gamma-ray and neutron values, have been in use productively since 1941. Because radioactivity can be measured with precision it can be used to identify different layers within beds. Radioactivity logs give an indication of the type of rocks and fluids contained in those rocks.

Acoustic or sonic logs are used to measure the porosity of a formation. This tool measures the speed at which an acoustic or sonic impulse is carried through a specified length of rock. The speed of sound through the rock gives an indication of the porosity and can be helpful in locating reservoirs.

Maps, including contour, isopach, cross sections, and three dimensional computer images, also aid the petroleum explorer in locating oil and gas. Contour maps give details of subsurface structural features enabling geologists to visualize three dimensional structures. Contour maps include information about porosity, permeability, and structural arrangements such as faults, pinch-outs, salt domes, and old shorelines. Isopatch maps show variations in thickness of a given subsurface formation and are used in calculating the size of reservoirs and secondary recovery operations. A cross section map is a diagram of an imaginary vertical cut along a straight line that reveals subterranean features of a given area much like looking at a road cut. Three dimensional computer maps construct images of subterranean strata as deep as thirty miles.