Oil and natural gas exploration

Drilling for oil is the last step in oil exploration. Surface mapping, the use of seismic technology to study sedimentary rock sequences, and other geological and geophysical studies all precede drilling. Oil and gas exploration is also driven by market forces that lead companies to be more or less likely to look for new sources of petroleum depending on the price of oil and natural gas.

Background

Petroleum (crude oil and natural gas) is a mixture of hydrocarbons, which are organic compounds made largely of hydrogen and carbon. Exploration for accelerated during the early 1900s as demand for fluid fuels increased following the development of the internal combustion engine. By the early twenty-first century, oil companies were looking for traditional sources of oil and gas as well as new sources, such as oil shale and oil sands. Throughout the history of oil and gas exploration, the price of oil and gas has influenced oil companies’ decisions to explore for new resources. Technology has also influenced exploration both from the technical ability of companies to find new sources and the drilling capabilities of oil companies. As demand for oil and has increased, companies have explored new, often more challenging areas for supplies.

Origin of Petroleum

Oil and natural gas are formed over immense spans of time from microscopic floating marine organisms that live by the billions in the world's oceans. As these die and sink to the seafloor, they may be buried in areas where there is rapid influx of sediment from of adjacent land. When buried by mud, and thus removed from contact with oxygen dissolved in seawater, the organic matter cannot completely decompose to gases, as it otherwise would.

If sediment continues to accumulate, the high confining pressure and high temperature at depth can change the organic matter to liquid or gaseous hydrocarbons—crude oil or natural gas. As compaction changes the sediment to rock, the fluid are squeezed out of the sediment originally containing them and move upward through the compacting accumulation. The petroleum may migrate upward into a coarser sediment, such as sand. Sand can be very permeable to fluid migration because of the large interconnected pore spaces between adjacent sand grains. The petroleum continues to rise through such a sand layer because most of the pore space in marine sediments and sedimentary rocks is filled with water, and petroleum is less dense than water. During this upward migration, if the petroleum encounters a fine-grained sedimentary layer that will not permit fluids to move through it, then the petroleum can be restricted to a particular porous zone, such as a sandstone layer beneath the fine-grained barrier.

In some regions, the petroleum-bearing sedimentary rocks may be folded or otherwise deformed so that they are no longer a sequence of horizontal layers. In this way, the petroleum can be concentrated in commercial quantities. Commonly, petroleum is concentrated in the highest parts of the deformed or folded layer containing it, because all the pore space beneath is filled with seawater. Thus, exploration for oil and natural gas requires study of sedimentary sequences that extend hundreds of meters or even several kilometers into the subsurface. The settings in which these sequences accumulated and the geographic distributions and thickness variations of the sedimentary layers must be understood.

Some liquid petroleum is compressed into what are known asoil sands. Oil sands were probably formed when conventional oil was trapped in shallow reservoirs. Over time, the water and lighter hydrocarbons were washed away or consumed by bacteria, leaving dense compounds that are often contaminated by high levels of sulfur and some heavy metals. Because oil can be extracted from oil sands, these formations have become a potential source of oil.

Searching for Petroleum

Most sedimentary rocks were not deposited in settings in which large quantities of organic matter were preserved. Even if the sedimentary rocks are rich in petroleum-forming hydrocarbons, subsequent deformation of the rocks most commonly has not produced a potential to concentrate the petroleum in commercial quantities. Deep drilling is very expensive, and most wells do not find commercial concentrations of petroleum. In order to know where to locate exploratory wells, various kinds of geological studies must precede drilling.

The search for petroleum includes the study of sedimentary rocks at depth using geophysical techniques such as seismic studies. Seismic studies generate, commonly via explosions, sound waves that are reflected and refracted (bent) by the sedimentary layers in the subsurface. The sound returning to the surface is recorded by sensitive receivers, and reflections from layers of particular rock types can be recognized. The time required for the sound to travel to a particular sedimentary rock layer and be reflected from it back to the surface is used to determine the depth of the sedimentary layer. The velocity of sound through a given layer is a function of the rock density, which in turn is an indication of rock type. After surface mapping, seismic examination of the subsurface, and other geological and geophysical studies have revealed sufficient information, it is possible to place exploratory wells in locations where there is higher probability of petroleum discovery.

Initially, most oil exploration was done in the United States, especially in Texas, Louisiana, California, and Oklahoma. By the early twentieth century, geologists were finding large oil deposits in the Caspian Sea region and Mexico. The discovery of large oil deposits occurred subsequently in the Middle East, especially in Saudi Arabia. In the early twenty-first century, geologists continued to find oil and natural gas fields in these areas, but also in Indonesia, Libya, Nigeria, Sudan, China, Kazakhstan, and Russia. Offshore oil and gas deposits were first located in shallow water in the Gulf of Mexico in the early twentieth century. Offshore oil continues to be found in shallow water but also in water depths of more than 3,000 meters in the Gulf of Mexico as well as in the North Sea off the coast of Norway, off the Brazilian coast, along the east coast of Africa, and in Southeast Asia, near Indonesia. Greatly improved technology has made possible both the discovery of these fields and the ability to drill for oil and gas. The last two major unexplored regions of the world in terms of oil and natural gas deposits are the Arctic Ocean and Antarctica.

Petroleum Supply Problems

In spite of improving technology, the search for petroleum became less successful by the 1990s. However, late in that decade, exploration efforts increased. Petroleum exploration and large-scale petroleum production began in the United States earlier than in any other country, and for many decades prior to 1974, the United States was the world’s leading oil producer. Partly for these reasons, US petroleum resources are being depleted earlier than those of the rest of the world.

The oil fields closest to the Earth’s surface were the easiest to find and therefore were the first to be discovered. As exploration and exploitation of petroleum resources continued and grew, oil fields became increasingly difficult to find. In the United States, where exploration for petroleum began, oil companies have nearly run out of places to look for large, new oil fields. Even for the world as a whole, petroleum has become progressively more difficult or, at least, more expensive to find, so that the search for it has extended into hostile climatic environments and unstable political situations. In some cases, oil companies are also reevaluating old oil fields because new technologies allow extraction of oil that was previously unobtainable.

In the history of any oil-producing region, the oil discovery rate reaches its maximum several years before the maximum in oil production rate. For example, in the United States (exclusive of Alaska), the oil discovery rate reached its peak in the late 1950s and diminished greatly afterward. However, the US oil production rate reached its maximum in 1970 and declined afterward. The global oil discovery rate was at its maximum in the 1960s and diminished afterward in spite of record-high rates of exploration in the early 1980s. The global oil-production rate, however, as the end of the twentieth century, had not yet reached its maximum. Some geologists indicate that the world oil peak could occur early in the twenty-first century. Other scholars, particularly economists, indicate that as prices rise there will be an increased incentive to discover more oil and natural gas. There continues to be extensive debate concerning when oil and gas production will peak.

Global Oil Reserves

The reserves of a are that amount which has been discovered but not yet consumed. There is an ongoing debate about how to estimate reserves, and some countries and oil companies either over- or underestimate for their own ends. By the late twentieth century, rates were such that global reserves of conventional oil were expected to last for forty years. The consumption rate, however, does not remain constant; rather, it has grown historically. In 1996, the world oil consumption rate was growing more than 2 percent annually. A forty-year supply at the 1996 consumption rate becomes a thirty-year supply with a 2 percent growth in consumption per year. After some debate and the discovery of new oil deposits, published oil reserves in 2007 were more than 1.2 trillion barrels for conventional oil, extending the consumption time horizon. Although additional reserves were discovered in the early twenty-first century, consumption rates have increased, due in part to the rapid industrialization occurring in countries such as China and India. Global natural gas reserves plus estimates of undiscovered producible gas resources are a few decades larger than those for oil as measured in terms of remaining consumption time. By 2024, global proven oil reserves were estimated at 1,765.2 billion barrels.

Discovery of oil from 1985 to 1995 averaged less than 9 billion barrels annually, while consumption rates averaged more than 23 billion barrels annually. In other words, most of the oil burned during this interval had been discovered during earlier decades. By the early twenty-first century, the discovery rate had increased to about 11 billion barrels per year. Using these data, Saudi Arabia has the world’s largest oil reserves, followed by Iran. These reserve figures are based on conventional oil and gas supplies. If unconventional sources, such as oil sands, are taken into account, Canada’s potential oil reserves rival those of Saudi Arabia. If global oil consumption continues to grow at the 2 percent rate, and if oil discovery rates continue to decline, global reserves of oil will dwindle. Scientists agree that oil and natural gas are in limited supply, but increased exploration and drilling and recovery technologies implemented in the early twenty-first century are pushing back the date at which demand for oil and gas will exceed supply. Some commentators indicate that market forces are once again driving oil companies to explore all opportunities for oil and gas extraction.

Exploration for and Production of New Sources

As world demand for oil and natural gas has increased, oil companies have turned to alternative areas and techniques in the search for petroleum resources. High prices for oil and natural gas have made feasible drilling for oil in increasingly harsh environments or turning to unconventional oil such as oil sands.

In addition to seeking out new sources for liquid petroleum, oil companies have also turned to two other strategies. One is to return to old oil fields and re-explore them in order to evaluate how much oil remains in the ground. Earlier drilling did not extract all of the oil in a reservoir; often nearly one-half of the remained. New technologies such as saltwater infusion and side drilling make exploiting old oil fields possible. Another approach that some of the oil companies have used is to mine oil sands, especially in Alberta, Canada. Once the has been stripped away from the oil sands, these oil sands are heated and compressed so as to extract the oil. Because this process is expensive and requires a good deal of energy to remove the oil from the other material, oil companies have only recently begun to engage in full-scale exploration for oil sands as the price of conventional oil increased to more than one hundred dollars per barrel. Exploration of potential oil sands is ongoing in several areas of the world in addition to Canada, including Venezuela.

Oil companies are also exploring regions that have traditionally been ignored, such as East Africa. Most of the exploration for new sources of oil and natural gas has been concentrated on the ocean floor, especially off the west coast of Africa, off the coast of Brazil, and in areas bordering Indonesia. Some scientists also contend that oil and natural gas is available in the South China Sea, but the disputes between the bordering countries have made exploration unfeasible. Another area under consideration is the seabed of the Arctic Ocean. By 2022, drilling technology existed to drill through 3,660 meters of water and another 3,000 meters of rock. Thus, companies were engaged in looking for oil and natural gas in deepwater sites that would have been impossible to exploit in the past.

Oil and gas exploration continue to be driven by what seems to be an insatiable world demand for petroleum for energy and other uses, such as for the chemical industry. Improvements in technology have increased the ability of geologists and geophysicists to explore for oil and gas in new places. Improved technology also means that wells can be drilled in places that were impossible a few years ago. Therefore, companies are willing to explore for oil in challenging places.

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