Petroleum reservoirs

A petroleum reservoir is a body of rock that contains crude oil, natural gas, or both, that can be extracted by a well. Two conditions must be met in tapping such resources. First, to contain and permit extraction of its fluids, the reservoir rock must be porous and permeable. Second, there must be a reservoir trap, a set of conditions that concentrates the petroleum and prevents its migration to the earth’s surface.

Sedimentary Reservoir Rock

Petroleum (from the Latin) literally means “rock oil.” Petroleum reservoirs are volumes of rock that contain or have the potential to contain hydrocarbons (crude oil and natural gas) that can be extracted by wells. A reservoir may contain one or more pools—that is, continuous bodies of oil or gas. One or more related pools form a field. The definition of a pool depends on economic as well as geologic considerations—as the price of oil goes up, the size of a pool goes down. There are four components to a petroleum reservoir: the rock, pores, trap, and fluid.

In more than 99 percent of cases, the reservoir rock is a sedimentary rock. Sedimentary rocks form when unconsolidated sediment, deposited on the earth’s surface by water or wind, becomes a solid rock. Two types of sedimentary rocks contain 95 percent of the world’s petroleum: sandstones and carbonates. Sandstone reservoirs contain approximately half of the United States’ petroleum. Freshly deposited sand is made up of mineral grains, mostly quartz, deposited from water in rivers, along shorelines, and in the ocean in the shallow water adjacent to the continent; less often, it is deposited by wind in sand dunes. Sand-sized grains range from 0.06 to 2.00 millimeters, or smaller than a sesame seed.

Carbonate rocks include limestones and dolostones. Limestones are made of the mineral calcite (calcium carbonate, CaCO₃). Sand-sized grains of calcite form when organisms, such as clams and corals, extract calcite (and aragonite, a related calcium carbonate mineral) from water to build their skeletons. When the animal dies, the skeletons are broken into fragments, which are then sorted by waves and currents and deposited along beaches or on the sea floor. Very fine-grained, mud-sized calcite and aragonite may be deposited with the sand-sized grains. This fine carbonate comes from calcareous algae (marine plants). In some cases, corals and algae may combine their activity to construct a wave-resistant mass, a coral reef. Reefs form the best potential reservoir rock. Dolostones, although they account for a relatively small portion of the world’s total carbonate mass, contain almost 80 percent of the petroleum in carbonate reservoirs in the United States. Dolostones are dominated by the mineral dolomite. Dolomite forms by the reaction of preexisting calcium carbonate minerals with magnesium-rich solutions. How this replacement takes place is not well understood.

Porosity and Permeability

A petroleum reservoir has the same general attributes as a water aquifer; it is a porous and permeable body of rock that yields fluids when penetrated by a borehole. Porosity is a measure of the pore space in a reservoir, the space available for storage of petroleum. It is usually expressed as a percentage. A freshly deposited unconsolidated sand contains about 25 percent intergranular pore space, normally occupied by water. As the sand is buried deeper and deeper, the grains interpenetrate, and the pores are reduced in size and percentage. At the same time, cements will precipitate in the pores. Thus, the pore space decreases, and the sand becomes a solid—a sandstone. To be a petroleum reservoir, a rock needs 10 percent porosity or more. A good reservoir will have 15 to 20 percent porosity. Sandstone reservoirs seldom exceed 25 percent porosity, but carbonate reservoirs can have up to 50 percent porosity. This greater porosity is a result of very high porosity in the open structure of reef rocks and cave systems of some limestones.

Porosity can be divided into primary and secondary types. Primary porosity is the porosity present at the time the rock was deposited. It depends on several factors, including the roundness and size range of the grains (sorting). Secondary porosity forms after the rock is solidified, when grains and cements are dissolved. Secondary porosity is most common in limestones but is also important in sandstones.

Permeability is a measure of the rate of flow of fluids through a porous medium. In general, the more porous the rock, the more permeable it is likely to be; however, the relation is not simple. For example, Styrofoam (a type of polystyrene plastic) has very high porosity but almost no permeability, which is why Styrofoam cups can hold liquids.

Reservoir Traps

A petroleum trap is a geometric situation in which an impermeable layer of rock (a caprock) seals a permeable, petroleum-bearing reservoir rock from contact with the earth’s surface. A common type of caprock is shale, or consolidated, lithified mud. There are two common types of traps: structural and stratigraphic. Structural traps form where the originally horizontal sedimentary rock layers are disrupted by warping (folding) and breaking (faulting). A much sought-after type of fold trap is an anticline. Anticlinal folds have a cross-section that is concave down (like the letter A). Because oil and gas are lighter than water, they migrate to the high point on the anticline and are prevented from reaching the surface by an impermeable caprock. Fault traps occur where a break in a rock layer brings a reservoir rock into contact with an impermeable rock. Salt-dome traps form where bodies of salt flow upward and pierce overlying rock layers. Salt domes form both anticlinal and fault traps on their crest and margins. They are common on the Gulf Coast of the United States. Stratigraphic traps form where the permeability barrier is a result of lateral or vertical changes in the rock, changes that result from the conditions under which the original sediment was deposited. Stratigraphic traps include buried river channels, beaches, and coral reefs. Geologic methods dominate the search for stratigraphic traps.

Reservoir fluids consist of water, crude oil, and natural gas. When sediments are deposited, they are (or soon become) saturated with water, usually salt water. The oil forms from the alteration (called maturation) of organic material buried in sedimentary rocks. This process takes place in the absence of oxygen at temperatures of 60 to 150 degrees Celsius when the sediments are buried. Natural gas forms by further thermal alterations of the hydrocarbons (thermogenic gas) or by low temperature alteration of near-surface organic material (biogenic gas, the gas that can be seen bubbling up from the bottom in swamps). After formation, the oil and gas must be expelled from its source rock and migrate into a reservoir rock. As oil is lighter than water, it migrates up the water column to the highest location in the trap. If there is no trap, the oil will seep onto the earth’s surface and be destroyed by oxidation. Although oil seeps are common, in most cases the oil will be prevented from reaching the surface by a trap. The trapped reservoir fluids then stratify themselves on the basis of density, the lightest at the top (natural gas) and the densest on the bottom (water).

Areas of Concentration

Petroleum reservoirs are not uniformly distributed in time or space; certain areas of the world and certain intervals of geologic time have a disproportionate amount of the world’s petroleum reserves. One unifying characteristic of these major areas of concentration is the presence of a basin. A basin is an area where rocks thicken from the margin to basin center, rather like a mud-filled saucer. The geologic conditions in basins favor the formation of oil and gas in the basin center and its migration into traps along the basin margin.

According the CIA's World Fact Book's most recent estimates in April 2023, most of the world’s oil is in the Middle East, in such countries as Saudi Arabia (258.6 billion barrels), Kuwait (101.5 billion barrels), Iraq (145.02 billion barrels), and Iran (208.6 billion barrels). Major Latin American reserves are in Venezuela (303.81 billion barrels) and Mexico (5.9 billion barrels), while Africa’s largest reserves are in Nigeria (36.89 billion barrels) and Libya (48.36 billion barrels). The United States had 48.3 billion barrels in reserves in 2022, of which 85 percent was in four states: Texas, Alaska, New Mexico, and North Dakota. Oil reserve figures are, at best, moderately good indications of actual oil reserves. Members of OPEC may raise their reserve figures to obtain higher production quotas, and petroleum companies may understate or fail to estimate their reserves for tax and regulatory reasons. While the world’s major producing areas are widely separated, they were much closer together 100 million years ago (in the Cretaceous period). Many of them shared a common setting, a now vanished seaway between Eurasia and Gondwanaland that geologists call the Tethys Sea.

In terms of time, very little oil is found in rocks older than 500 million years, although there is record of oil up to a billion years old. Rocks of Jurassic and Cretaceous age (about 65 to 200 million years old) contain 54 percent of the world’s oil and those about 35 to 55 million years old, from the Eocene epoch, contain 32 percent of the world’s oil. It is not clear whether this young age (geologically speaking) is related to the origin of oil or results from the fact that deep wells cost more money so that older rocks are less thoroughly drilled. It may be that many older petroleum traps no longer exist because of erosion.

Exploration for Reservoirs

The exploration for petroleum reservoirs has two intimately related aspects, one geophysical and the other geological. In geophysical exploration, the scientist, called an exploration geophysicist, uses the physics of the earth to locate petroleum reservoirs in structural traps. The principal technique is seismic reflection profiling. In this approach, the geophysicist sets off deliberate explosions and uses the energy reflected from subsurface rock layers to interpret the folds, faults, and salt domes that may be present. Seismic reflection is similar to sonar. Variations in the gravity, magnetic, and heat-flow characteristics of the earth may also point to the location of oil and gas pockets in the subsurface.

Geological exploration dominates in the search for stratigraphic traps. Petroleum geologists use facies models to predict the location and extent of petroleum reservoirs. Facies models are based on information about the size, internal characteristics, and large-scale association of modern sediment accumulations. Data based on well logs, obtained from previously drilled wells, are used to prepare structure maps, which show the “topography” of the reservoir surface, and thickness variation (isopachous) maps. Data from well logs, well cuttings, and cores are used to prepare lithofacies maps, which show the lateral variation in the rocks. These variations influence porosity and permeability trends in the reservoir. Such maps can then be compared to the facies models to make predictions about the size and location of the edges of stratigraphic traps in the subsurface. Commonly applied facies models for sandstones include those for rivers, beaches, deltas, and sand dunes. Those for carbonate rocks include beaches, reefs, and dolostones.

Seismic stratigraphy is a technique that combines geology and geophysics. In this approach, the data from artificial explosions are used to interpret the depositional system of the rocks in a way that is similar to radar and sonar. Echoes from the explosions are used to map the three-dimensional structure of the rocks. For example, a delta system has a structure that is distinct from that of sediments deposited in the shallow waters of the continental shelf. On the whole, the most favorable “prospect” is the portion of a trap closest to the earth’s surface. Since oil and gas rise to the top of a trap, this location is the most likely volume to contain petroleum.

Economic Considerations

Petroleum supplies a major portion of the world’s fossil energy and, consequently, is an important element in the complex international play of economic forces. In other words, what decides whether a body of rock is a petroleum reservoir is not simply geology but also the reservoir’s economic and political setting. The basic problem is that petroleum is a nonrenewable resource, present in finite amounts, and not randomly distributed in the Earth’s subsurface. Millions of oil and gas wells have been drilled in the continental United States, making it the most mature country in the world from the point of view of petroleum exploration. The “easy” oil has been found, and it is becoming harder to find the fewer and fewer undiscovered and economically exploitable petroleum pools. On a more positive note, however, as drilling density has increased, so has knowledge. Better information, better understanding of how and when petroleum enters a reservoir, and better techniques for finding traps have improved the success ratio of oil drilling.

Principal Terms

field: one or more pools; where multiple pools are present, they are united by some common factor

natural gas: a flammable vapor found in sedimentary rocks, commonly but not always associated with crude oil; it is also known simply as gas or methane

permeability: a measure of the rate of flow of fluids through a porous medium

petroleum: a dark green to black, flammable, organic liquid commonly found in sedimentary rocks; it looks like used crankcase oil and is also called crude oil or liquid hydrocarbon

pool: a continuous body of petroleum-saturated rock within a petroleum reservoir; a pool may be coextensive with a reservoir

porosity: the percentage of pore, or void, space in a reservoir

reservoir: a body of porous and permeable rock; petroleum reservoirs contain pools of oil or gas

trap: a seal to fluid migration caused by a permeability barrier; traps may be either stratigraphic or structural

well log: a strip chart with depth along a well borehole plotted on the long axis and a variety of responses plotted along the short axis; there are many varieties, including borehole logs, geophysical logs, electric logs, and wireline logs; information may be obtained about lithology, formation fluids, sedimentary structures, and geologic structures

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