Crude Oil Reserves

The amount of technically and economically recoverable crude oil is known as crude oil reserves. This may be reserve for an oil well, an oil reservoir, an oil field, a nation or for the world. Hence we refer to a particular oil well reserve, a field’s crude oil reserve, a country’s crude oil reserve and world crude oil reserve. The actual recoverable oil from crude oil reserves are more or less related to their degree of certainty. I mean… considering limitations in extraction techniques, only a fraction of crude oil reserves can be brought to surface.

Now, let’s look at different terms associated with crude oil reserves:

Crude oil proven reserve

Proven crude oil reserves are those crude reserves with reasonable certainty… at least 90% confidence of being recoverable under available economical, political and existing technological conditions. Proven reserves can further be classified as proven developed and proven undeveloped reserves. Proven developed reserves… as the name suggest are reserves that can be produced with existing wells and perforations, or from additional reservoirs where minimal operating expense is required while proven undeveloped reserves require additional capital investment to bring the oil to the surface.

Crude oil unproven reserves

Crude oil unproven reserves are those crude reserves with technical, contractual or regulatory uncertainties. Usually, unproven reserves are used internally by oil companies and government agencies for future planning purposes but are not routinely complied. Like proven reserves, unproven reserves also can be classified as probable and possible. Probable reserves are attributed to known accumulations and claim a 50% confidence level of recovery while possible reserves are attributed to known accumulations that have a less likely chance of being recovered than probable reserves. This term is often used for reserves which are claimed to have at least a 10% certainty of being produced.

Strategic Crude Oil Reserve

Strategic crude oil reserves are generally not counted when computing a nation’s crude oil reserve. It refers to government-controlled crude oil reserves for both economic and national security reasons. For example, United States Energy Information Administration states that approximately 4.1 billion barrels (650,000,000 m³) of oil are held in strategic reserves, of which 1.4 billion is government-controlled (m³=cubic meters).

Crude Oil Reserve Recovery Factor

Crude oil reserve recovery factor is the ratio of producible oil reserves to total oil in place for a given field. It varies greatly among oil fields and the recovery factor of any particular field may change over time based on operating history and in response to changes in technology and economics. The recovery factor may also rise over time if additional investment is made in enhanced oil recovery techniques such as gas injection, surfactants injection, water-flooding, or microbial enhanced oil recovery.

Crude Oil Reserve Growth

Crude oil reserve growth is a situation where most early estimates of the reserves of an oil field are conservative and tend to grow with time. This is because the geology of the subsurface cannot be examined directly and as such indirect techniques must be used to estimate the size and recoverability of the resource and while new technologies have increased the accuracy of these techniques, significant uncertainties still remain.

Estimated world crude oil reserves

Summary of Top 17 Proven World Crude Oil Reserve Data 2012

Country	Reserves 109 bbl
Venezuela	296.5
Saudi Arabia	265.4
Canada	175
Iran	151.2
Iraq	143.1
Kuwait	101.5
United Arab Emirates	136.7
Russia	74.2
Kazakhstan	49
Libya	47
Nigeria	37
Qatar	25.41
China	20.35
United States	20.35
Angola	13.5
Algeria	13.42
Brazil	13.2

Types of Crude Oil

Types of crude oil are commonly determined by location of its origin e.g., "West Texas Intermediate, WTI" or "Brent") and by its relative weight (API gravity) or viscosity ("light", "intermediate" or "heavy"); refiners may also refer to it as "sweet", which means it contains relatively little sulfur, or as "sour", which means it contains substantial amounts of sulfur and requires more refining in order to meet current product specifications.

The fact is; each crude oil has unique molecular characteristics which are understood by the use of crude oil assay analysis in petroleum laboratories. Actually, classification of crude oil types by geographical source generally offer little information about general toxicity, physical state and other changes that occur with weather and time.

Now let’s look at different types of crude oil according to relative weight or viscosity and according to location of its origin:

Crude Oil Types according to relative weight or viscosity

Light crude oil

Type of crude oil known as Light Crude oil is liquid petroleum that has low density and that flows freely at room temperature. It has low viscosity, low specific gravity and high API gravity of about 37 and 42 degrees due to the presence of a high proportion of light hydrocarbon fractions. It generally has low wax content as well.

Medium Crude Oil

The type of crude oil known as Medium Crude is also a liquid petroleum that has low density, low viscosity, low specific gravity but with an API gravity between 22 and 38 degrees.

Heavy Crude Oil

Heavy crude oil type is any type of crude oil which does not flow easily. It is referred to as “heavy” because its density or specific gravity is higher than that of light crude oil. Heavy crude oil has been defined as any liquid petroleum with API gravity less than 20°.

Extra Heavy Crude Oil
Type of crude oil known as Extra heavy oil is defined with API gravity below 10.0 °API (API gravity, is a measure of how heavy or light a petroleum liquid is compared to water. If its API gravity is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks. )

Crude Oil types according to sulfur content

Type of crude oil with less than 0.42% sulfur is designate as sweet crude oil while the types containing higher levels of sulfur is called sour crude oil.

Light, sweet crude is more expensive than heavier, sourer crude because it requires less processing and produces a slate of products with a greater percentage of value-added products, such as gasoline, diesel, and aviation fuel.

Heavier, sourer crude typically sells at a discount to lighter sweeter grades because it produces a greater percentage of lower value-added products with simple distillation and requires additional processing to produce lighter products.

Crude Oil Types according to location of its origin

Brent Blend

Brent Blend is light, sweet North Sea crude with an API gravity of approximately 38 and a sulfur content of approximately 0.4%. Brent Blend is used for pricing around two-thirds of the crude traded internationally. Rolling price assessments are based on physical Brent-Forties-Oseberg crude oil cargoes loading not less than 10 days ahead and loaded free on board at the named port of shipment (‘Brent Dated).

Russian Export Blend

Russian Export Blend is a medium, sour crude oil with an API gravity of approximately 32 and a sulfur content of approximately 1.2%. Its spot price is reported at Augusta, Italy, and Rotterdam, the Netherlands, which act as the two primary delivery points.

West Texas Intermediate

West Texas Intermediate, the US benchmark crude oil, is a light, sweet crude oil with an API gravity of approximately 40 and a sulfur content of approximately 0.3%. The spot price of West Texas Intermediate is reported at Cushing, Oklahoma.

Other crude oil type according to location of its origin includes:

Saharan Blend (Algeria), Minas (Indonesia), Iran Heavy (Islamic Republic of Iran), Basra Light (Iraq), Kuwait Export (Kuwait), Es Sider (Libya), Bonny Light (Nigeria), Qatar Marine (Qatar), Arab Light (Saudi Arabia), Murban (UAE) and BCF 17 (Venezuela).

Origin of Crude Oil

Origin of crude oil has been a topic of debate by chemists and geologists for a long time. The most common theory is that the bodies of prehistoric plants and sea animals were trapped in sediments, which after many years due to heat and pressure were changed into crude oil and natural gas. 

Now let’s look at different theory that explains origin of crude oil in details:

Biogenic theory origin of crude oil

Most geologists view crude oil, like coal and natural gas, as the product of compression and heating of ancient organic materials over geological time. According to this theory, oil is formed from the preserved remains of prehistoric zooplankton and algae which have been settled to the sea bottom in large quantities under anoxic conditions. (Terrestrial plants tend to form coal, and very few dinosaurs have been converted into oil.) Over geological time this organic matter, mixed with mud, is buried under heavy layers of sediment. The resulting high levels of heat and pressure cause the remains to metamorphose, first into a waxy material known as kerogen which is found in various oil shale around the world, and then with more heat into liquid and gaseous hydrocarbons in a process known as catagenesis. 

Because most hydrocarbons are lighter than rock or water, these sometimes migrate upward through adjacent rock layers until they become trapped beneath impermeable rocks, within porous rocks called reservoirs. Concentration of hydrocarbons in a trap forms an oil field, from which the liquid can be extracted by drilling and pumping.

Geologists often refer to an "oil window" which is the temperature range that oil forms in—below the minimum temperature oil remains trapped in the form of kerogen, and above the maximum temperature the oil is converted to natural gas through the process of thermal cracking. Though this happens at different depths in different locations around the world, a 'typical' depth for the oil window might be 4–6 km. Note that even if oil is formed at extreme depths, it may be trapped at much shallower depths, even if it is not formed there. (In the case of the Athabasca Oil Sands, it is found right at the surface.)

Abiogenic theory origin of crude oil

The idea of abiogenic crude oil origin was championed in the Western world by astronomer Thomas Gold based on thoughts from Russia, mainly on studies of Nikolai Kudryavtsev. The idea proposes that large amounts of carbon exist naturally in the planet, some in the form of hydrocarbons. Hydrocarbons are less dense than aqueous pore fluids, and migrate upward through deep fracture networks. Thermophilic, rock-dwelling microbial life-forms are in part responsible for the biomarkers found in petroleum.

However, this theory is very much a minority opinion, especially amongst western geologists. It often pops up when scientists are not able to explain apparent oil inflows into certain oil reservoirs. However, most of these "abiotic" fields are explained as being the the result of geologic quirks. No western oil companies are currently known to explore for oil based on this theory.

Origin of crude oil reservoir

Three conditions must be present for crude oil reservoirs to form: first, a source rock rich in organic material buried deep enough for subterranean heat to cook it into oil; second, a porous and permeable reservoir rock for it to accumulate in; and last a cap rock (seal) that prevents it from escaping to the surface.

The vast majority of oil that has been produced by the earth has long ago escaped to the surface and been biodegraded by oil-eating bacteria. What oil companies are looking for is the small fraction that has been trapped by this rare combination of circumstances. Oil sands are reservoirs of partially biodegraded oil still in the process of escaping, but contain so much migrating oil that, although most of it has escaped, vast amounts are still present - more than can be found in conventional oil reservoirs. On the other hand, oil shale is source rocks that have never been buried deep enough to convert their trapped kerogen into oil.

Conclusion

Looking at origin of crude oil from most acceptable perspective, crude oil and gas form as a consequence of environmental conditions occurring in a sequence; the presence of organic material; organic remains are trapped and preserved in sediments, the material is buried deeply, then it is heated by increased temperature and pressure.

Importance of Crude Oil

Importance of crude oil as a vital source of energy and as the most important raw material for our industries can never be over stated. Let’s look at it this way crude oil is estimated to provide about 36% of the world’s energy needs and will likely remain so for many decades to come despite optimistic assumptions about growth in alternative energy sources. Yes… crude oil is still one of the cheapest sources of storable and transportable energy. Modern life is inconceivable without crude oil. The world consumes almost 14 billion liters of oil each day.

Now, let’s look at importance of crude oil in more details:

Importance of crude oil to our industrial needs

Considering the vast number of products that are derived from crude oil, its importance to our industrial growth is overwhelming. Tonnes of crude oil are processed into synthetic materials in our industries. For example: 40 percent of all textiles contain oil, for functional clothing this may be as much as 100 percent. 40 billion liters of oil a year are used to make CDs and DVDs. A single sofa contains 60 liters of oil. Common products like shampoo, detergent, solvents, paint, ink, tires, lubricants, candle wax, roofing material, carpet, synthetic clothing materials, shoes, and hundreds of thousands of other products are made from oil. Hence considering the vast number of industrial products that are derived from it, crude oil is a very versatile substance. Life as we know it today would be extremely difficult without crude oil and its by-products.

Importance of crude oil to our transportation needs

Crude oil can rightly be considered as the engine that drives our transportation needs. From building good road networks to fueling different means of transportation. Yes…about 90 percent of our transportation fuel needs are met by crude oil.

Importance of crude oil to world energy needs

The fact is crude oil provided about 36% of the world’s energy needs, and in the future, crude oil is expected to continue to provide a leading component of the world’s energy mix. In most of the rest of the world, oil is more commonly used for space heating and power generation than for transportation.

Importance of crude oil to world economies

Most countries are significantly affected by developments in the oil market, either as producers, consumers, or both. Looking at it this way, Oil enabled modern agriculture to supply cheap food. Oil supplies cheap energy making the manufacture and transport of many products inexpensive enough to change the entire world economy. Ships, trains, and trucks powered by oil move the majority of the world's products. Oil consumption is directly correlated to Gross Domestic Product. For example, the US uses more oil than any other country, but the US also produces a similar percentage of the world's domestic product. As GDP has been falling in the US, so has oil consumption. At present, there is a fairly direct relationship between oil consumption and economic activity.

Importance of crude oil to agricultural needs

The truth is crude oil is a key product for the world’s agriculture industry, which helps feed the world’s population of more than six billion. If not for oil, modern agriculture might never have reached the yields it has today. Steam powered tractors, that weighed many tons, used coal or wood to power themselves. They were replaced by tractors powered by internal combustion, as well as combines and other farming equipment. Natural gas is used as the feedstock to manufacture most of the fertilizer used by industrial farm operations today.

History of Crude Oil

History of crude oil can be traced to the first oil wells drilled in China in the 4^th century. They have depths of up to about 243 meters deep and were drilled using bits attached to bamboo poles. The oil was mainly used in salt production by burning it to evaporate salt-water to produce salt. However, crude oil current status as integral component of politics, society, and technology has its roots in the early 20^th century with the invention of internal combustion engine.

Now, let’s look at early and modern history of crude oil in detail:

Early history of crude oil

Early history of crude oil can be traced to more than four thousand years ago. Looking at it, it was believed that asphalt (product of crude oil) was employed in the construction of the walls and towers of ancient city of Babylon. Also, ancient Persian tablets indicate the medicinal and lighting uses crude oil in the upper levels of their society.

The ancient records of Asian countries are said to contain many reference to the use of natural gas for lighting and heating purposes. Around 7^th century, crude oil was known as burning water in Japan. It is also on record that streets of ancient Baghdad were paved with tar (product of crude oil) gotten from natural fields in the region. 9^th century Azerbaijan was not left out as it was also on record that oil fields were exploited in the area with production estimated as hundreds of shiploads.

Crude oil distillation became available in Western Europe by the 12^th century through Islamic Spain. Also, crude oil recorded as pacura has been present in Romania since the 13^th century.

The earlier history of crude oil in Americas was in 1858 when James Miller Williams in Oil Springs, Ontario, Canada dug the first commercial oil well. This followed with the discovery of crude oil in 1859, near Titusville, Pennsylvania. The industry grew slowly in the 1800s, driven by the demand for kerosene and oil lamps.

Modern history of crude oil

The modern history of crude oil can be traced to the 19^th century growth in demand for petroleum as a fuel for lighting in North America and around the world. It actually began with the process to distill kerosene from crude oil. Looking at it, it all started in 1847 when James Young noticed a natural seepage in Ridding colliery at Alfreton, Derbyshire from which he distilled light thin oil suitable for use as lamp oil, and at the same time obtaining thicker oil suitable for lubricating machinery. He subsequently set up a small business refining the crude oil in 1848.

The subsequent increase in demand for refined petroleum prompted considerable search for crude oil. Looking at it, an early commercial well was hand dug in Poland in 1853 and another in nearby Romania in 1857. At around the same time the world's first, small, oil refinery was opened at Jasło in Poland, with a larger one opened at Ploiesti in Romania shortly after. In 1857, Romania became the first country in the world to have had crude oil output officially recorded at 275 tonnes.

The first most modern crude oil well was drilled near Titusville, Pennsylvania by Edwin Drake in 1859. However, it is on record that the first commercial oil well in Canada became operational in 1858 at Oil Spring, Ontario. Actually, the discovery at Oil Springs started an oil boom which brought hundreds of investors and workers to the area.

Advances in drilling continued into 1862 when local driller Shaw reached a depth of 62 metres using the spring-pole drilling method. After an explosion of natural gas in Canada on January 16, 1862 its first oil gusher came into production shooting into the air at a recorded rate of 3,000 barrels per day. By the end of the 19th century the Russian Empire, particularly the Branobel company in Azerbaijan, had taken the lead in production. Oil exploration in North America during the early 20th century later led to the U.S. becoming the leading producer by mid-century. As petroleum production in the U.S. peaked during the 1960s, however, the United States was surpassed by Saudi Arabia and the Soviet Union.

In essence, history of crude oil can first be traced to 7000 years of trying to find its uses, the period it was known to burn well than other source of oil to the 19^th century discovery of modern drilling technology which led to commercial production and other large scale uses of refined crude oil products.

Crude Oil Production Process

Crude oil production process involves operations that bring crude oil to the surface for further processing. Here we will be looking at exploration, extraction and recovering processes involved in crude oil production.  First, let’s look at the exploration process:

Exploration Processes

The first step involved in crude oil production process is exploration where geologists use seismic surveys and other methods to search for geological structures that may form oil reservoirs. As of 2010, no method has proven 100 percent effective in helping geologists locate crude oil deposits. The best, and most profitable, technique oil explorers’ use is seismic exploration.

“Classic” and “passive” methods are used in seismic surveys. The “classic” method includes making an underground explosion nearby and observing the seismic response that provides information about the geological structures under the ground while “passive” method involves extracting information from naturally-occurring seismic waves.

However, in most recent seismic exploration, a satellite or mobile seismic device shoots radio waves at prospective crude oil sites and then sends that data to a supercomputer which creates a digital model of the environment based on that data

Other instruments such as magnetometers and gravimeters are also sometimes used in crude oil exploration.

Extraction Processes

The second step involved in crude oil production process is extraction which normally starts with drilling wells into the underground reservoir. However, while the first drilling technique stuck a pipe straight into the ground, modern drillers now have slant drilling techniques. In general, slant drilling proves much more beneficial than vertical drilling. Slanted pumps are curved, allowing them to rotate 360 degrees and reach several different deposits from the same location.

Oil extraction also occurs at sea using the same methods as land-based oil extraction, but requires special types of platforms because the ocean floor may not support the weight of a normal rig or the size of a solid platform proves to costly to reach ocean depths. Most offshore operations tether a floating platform connected to cables attached to the ocean floor. Those drilling in shallow water or those who have found a large deposit might want to install a permanent structure. Movable rigs, such a ship or a barge, are good for finding oil deposits and gathering oil from several smaller sites.

Generally, when an oil well has been tapped, a geologist will note its presence. The mixtures of oil, gas and water from the well is separated on the surface. The water is disposed of and the oil and gas are treated, measured, and tested.

Recovering Processes

The third and final step involved in crude oil production process is recovering which include bringing the crude oil and associated gas to the surface, maintaining production, and purifying, measuring, and testing. Three stages are involved in crude oil recovering: Primary recovery stage, secondary recovery stage and tertiary recovery stage.

Now, let’s look at each of the recovery stages:

Primary recovery method of crude oil production process

Reservoir drive comes from a number of natural mechanisms during the primary recovery stage. These include: natural water displacing oil downward into the well, expansion of the natural gas at the top of the reservoir, expansion of gas initially dissolved in the crude oil, and gravity drainage resulting from the movement of oil within the reservoir from the upper to the lower parts where the wells are located. Typically, 5-15% recovery factor are achieved during the primary recovery stage.

Secondary recovery method of crude oil production process

With diminishes in natural reservoir drive, secondary recovery methods are applied. They rely on replacing or increasing the natural reservoir drive with an artificial drive by the supply of external energy into the reservoir in the form of form of injecting fluid to increase reservoir pressure. This is achieved by water injection, natural gas reinjection and gas lift, which injects air, carbon dioxide or some other gas into the bottom of an active well, reducing the overall density of fluid in the wellbore. Typically, 35-45 recovery factors are achieved after primary and secondary recovery operations.

Tertiary Recovery method of crude oil production process

When secondary oil recovery isn’t enough for adequate crude oil recovery, tertiary recovery methods are applied, though when cost effective. This involves increase in the mobility of the crude oil in other to increase extraction. Different techniques are used in tertiary recovery methods which include:

·         First is the thermally enhanced oil recovery technique (TEOR). This is a tertiary recovery technique that involves heating the oil, thus reducing its viscosity and making it easier to extract. Steam injection is the most common form of TEOR, and is often done when a gas turbine is used to generate electricity and the waste heat is used to produce steam, which is then injected into the reservoir. In-situ burning is another form of TEOR, but instead of steam, some of the oil is burned to heat the surrounding oil.

·         Another method used to reduce viscosity is through carbon dioxide flooding.

·         Microbial treatments are another tertiary recovery method. Special blends of the microbes are used to treat and break down the hydrocarbon chain in oil thus making the oil easy to recover as well as being more economic versus other conventional methods.

Typically, tertiary recovery allows another 35%-45% of the reservoir’s oil to be recovered.

In essence, crude oil production process involves exploration, extraction and recovering of crude oil.

Crude Oil Production

Crude oil production involves the processes of exploration or location of oil filed, drilling and extraction of the crude. Now, let’s briefly look at each of the processes involve in crude oil production:

Exploration of crude oil

Geologist use seismic surveys to explore crude oil.  Seismic survey is used to search for geological structures that may form oil reservoirs. This can be done by two methods known as ‘classic’ and ‘passive’ method. The classic method include making an underground explosion nearby and observing the seismic response that provides information about the geological structures under the ground while passive methods involves extracting information from naturally-occurring seismic waves.

Other instruments such as gravimeters and magnetometers are also used to explore crude oil.

Drilling Crude oil wells

After exploration and discovery of crude oil, the next step in crude oil production is drilling wells into the underground reservoir. Drilling crew sets up and start the drilling operations and this is done by first, from the starter hole, they drill a surface hole down to a pre-set depth, somewhere the oil trap is located.

Once the pre-set depth is reached, they must place casing-pipe sections into the hole to prevent it from collapsing in on itself. The casing pipe has spacers around the outside to keep it centered in the hole. The crew completes the well to allow oil to flow into the casing in a controlled manner once the have reached the final depth. Finally, a collection of valves called a “Christmas Tree” is fitted to the top to regulate pressure and control flows.

Extraction of crude oil

Once the crude oil is flowing, the next step in crude oil production is to remove the oil rig from site and set up production equipment to extract the oil from the well. This is done by placing a pump at the well head. The pump system comprises a gear box driven by an electric motor to move a lever. The lever pushes and pulls a polishing rod up and down. The polishing rod is attached to a sucker rod, which is attached to a pump. This system forces the pump up and down, creating a suction that draws oil up through the well.

In cases where the oil is too heavy to flow, a second hole is drilled into the reservoir and steam is injected under pressure. The heat from the steam thins the oil in the reservoir, and the pressure helps push it up the well. This process is called enhanced oil recovery.

The amount of oil that is recoverable is determined by a number of factors including the permeability of the rocks, the strength of natural drives (the gas present, pressure from adjacent water or gravity), and the viscosity of the oil. When the reservoir rocks are "tight" such as shale, oil generally cannot flow through but when they are permeable such as in sandstone, oil flows freely. The flow of oil is often helped by natural pressures surrounding the reservoir rocks including natural gas that may be dissolved in the oil, natural gas present above the oil, water below the oil and the strength of gravity. 

Oils tend to span a large range of viscosity from liquids as light as gasoline to heavy as tar. The lightest forms tend to result in higher extraction rates.

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.

Formation of crude oil

Formation of crude oil can be credited to dead organisms. Over the years, the organisms decayed in the sedimentary layers. In these layers, there was little or no oxygen present. So microorganisms broke the remains into carbon-rich compounds that formed organic layers. The organic material mixed with the sediments, forming fine-grained shale, or source rock. As new sedimentary layers were deposited, they exerted intense pressure and heat on the source rock. The heat and pressure distilled the organic material into crude oil and natural gas.

The oil flowed from the source rock and accumulated in thicker, more porous limestone or sandstone, called reservoir rock. Movements in the Earth trapped the oil and natural gas in the reservoir rocks between layers of impermeable rock, or cap rock, such as granite or marble.

Let’s look closely at organic and chemical formation of crude oil:

Organic formation of crude oil

Crude oil is formed through the heating and compression of organic materials over a long period of time. It is widely believed that most of the oil we extract today comes from the remains of prehistoric plants and animal whose remains settled on the bottom of an Ocean or Lake. Over time this organic material combined with mud and was then heated to high temperatures from the pressure created by heavy layers of sediment.

For instance, there were certain warm nutrient-rich environments where the large amounts of organic material falling to the ocean floor exceeded the rate at which it could decompose. This resulted in large masses of organic material being buried under subsequent deposits such as shale formed from mud. This massive organic deposit later became heated and transformed under pressure into oil.

Geologists often refer to the temperature range in which oil forms as an "oil window" below the minimum temperature oil remains trapped in the form of kerogen, and above the maximum temperature the oil is converted to natural gas through the process of thermal cracking. Sometimes, oil formed at extreme depths may migrate and become trapped at a much shallower level.

Chemical formation of crude oil

In a more detail explanation of chemical formation of crude oil, it is widely believed that Formation of crude oil occurs from hydrocarbon pyrolysis in a variety of mainly endothermic reactions at high temperature and/or pressure. Looking at it, bacterial decomposition of the plants and animals removed most of the oxygen, nitrogen, phosphorus and sulfur from the matter, leaving behind a sludge made up mainly of carbon and hydrogen. As the oxygen was removed from the detritus, decomposition slowed. 

Over time the remains became covered by layers upon layers of sand and silt. As the depth of the sediment reached or exceeded 10,000 feet, pressure and heat changed the remaining compounds into the hydrocarbons and other organic compounds that form crude oil and natural gas.

The type of petroleum formed by the plankton layer depended largely on how much pressure and heat were applied. Low temperatures (caused by lower pressure) resulted in a thick material, such as asphalt. Higher temperatures produced lighter crude oil. Ongoing heat could produce gas, though if the temperature exceeded 500°F, the organic matter was destroyed and neither oil nor gas was produced.

Formation of crude oil reservoir

Conditions necessary for the formation of crude oil reservoirs are a source rock rich in hydrocarbon material buried deep enough for subterranean heat to cook it into oil; a porous and permeable reservoir rock for it to accumulate in; and a cap rock (seal) or other mechanism that prevents it from escaping to the surface.

Within these reservoirs, fluids will typically organize themselves like a three-layer cake with a layer of water below the oil layer and a layer of gas above it, although the different layers vary in size between reservoirs. Because most hydrocarbons are less dense than rock or water, they often migrate upward through adjacent rock layers until either reaching the surface or becoming trapped within porous rocks (known as reservoirs) by impermeable rocks above.

However, the process is influenced by underground water flows, causing oil to migrate hundreds of kilometers horizontally or even short distances downward before becoming trapped in a reservoir. When hydrocarbons are concentrated in a trap, an oil field forms, from which the liquid can be extracted by drilling and pumping.