Petroleum reservoir

A petroleum reservoir or oil and gas reservoir is a special place deep underground where oil and gas are stored. These reservoirs form inside rocks that have tiny holes or cracks, letting the oil and gas move through. They are created when old plant material, called kerogen, is changed by heat and pressure deep inside the Earth's crust.

Reservoirs can be divided into two main types: conventional and unconventional. In conventional reservoirs, the oil and gas flow out naturally because they are trapped under rock layers that don’t let them pass through easily. In unconventional reservoirs, the rocks hold the oil and gas tightly, so special methods are needed to get them out. We find these reservoirs by using hydrocarbon exploration techniques to search for signs of oil and gas below the surface.

Oil field

An oil field is an area where liquid petroleum is stored underground in many connected reservoirs. This oil is trapped by rock layers that do not let it escape. Oil fields can be very large, stretching for hundreds of kilometers, and often need lots of equipment and workers to get the oil out.

Oil fields can be found anywhere the rocks allow, even far from towns or out at sea. Setting up an oil field is a big job, needing special equipment, roads, and places for workers to live. The oil industry has three main parts: getting the oil out of the ground, moving it through pipes or ships, and turning it into useful products like gasoline. There are over 65,000 oil fields around the world, with some of the biggest in Saudi Arabia and Kuwait.

Gas field

Natural gas comes from the same natural process that turns old plant material into oil. Because of this, oil and natural gas are often found close to each other. Places with lots of oil are called oil fields, while places with lots of natural gas are called natural gas fields.

Usually, oil forms in rocks buried about 1,000 to 6,000 meters deep, where it’s warm but not too hot. Natural gas forms in rocks buried even deeper and in hotter places. Both oil and natural gas are lighter than water, so they move upward until they get stuck under a layer of rock that doesn’t let them escape. We can reach this trapped gas and oil by drilling deep wells.

Some of the biggest natural gas fields are South Pars/Asalouyeh between Iran and Qatar, and Urengoy gas field and Yamburg gas field in Russia. Natural gas is also found under the ocean, like in the North Sea and near Sable Island. Getting gas from under the water uses big, special drilling equipment because working in the ocean is very difficult.

Formation

Crude oil comes from old plants and tiny sea creatures that lived millions of years ago. When these tiny plants and animals died, they sank to the ocean floor and got buried under layers of sand and mud. Over time, heat and pressure changed them into oil and natural gas.

For oil to form, there needs to be a special kind of rock underground called a sedimentary basin. The oil forms when the buried plants and animals are cooked by pressure and heat, then move into rocks that can trap the oil. Without these traps, the oil might escape. Some places, like the North Sea, had just the right conditions to trap lots of oil, creating many oil fields. Oil can be found very close to the surface or deep underground, and scientists are now also looking for oil in different kinds of rocks like trachyte and basalt.

Geology

Traps

A trap is formed when natural forces pushing oil and gas upward through rock layers are stopped by other forces in a special kind of rock. This stopping point is very important for oil and gas to collect and form a reservoir.

Petroleum geologists sort traps into three main types based on their shape and position: structural traps, stratigraphic traps, and the less common hydrodynamic traps. Structural traps happen when the shape of the underground rock changes due to folding and breaking, creating domes and folds. Stratigraphic traps happen when the rock layers change in thickness or type.

Structural traps

Structural traps are made when the underground rock bends and breaks, forming shapes like domes and folds. These traps are easier to find and hold most of the world's oil and gas.

Stratigraphic traps

Stratigraphic traps form when rock layers vary in thickness, texture, or type. Examples include traps under old surface layers and traps in ancient coral reefs.

Hydrodynamic traps

Hydrodynamic traps are rare and happen when differences in water pressure cause oil and gas to tilt and collect.

Seal / cap rock

A seal, also called a cap rock, is a special layer that stops oil and gas from moving upward. It works by having strong forces that keep the oil and gas in place until something breaks the seal.

Unconventional reservoirs

Main article: Unconventional (oil and gas) reservoir

Unconventional oil and gas reservoirs are different because the oil and gas are tightly held within the rock itself, not collected under a special layer like in regular reservoirs. Oil sands are an example of this type of reservoir. Getting oil from these reservoirs needs special methods, often more like mining than regular drilling. This can be more expensive and needs extra cleanup, but these reservoirs are important because regular oil reservoirs are becoming scarce.

Estimating reserves

Main article: Oil & gas reserves and resource quantification

After finding oil underground, experts try to learn more about it. They use special tools to map the area and drill test holes to see where the oil is. This helps them guess how much oil might be there.

They also study the rock to see how much space it has for oil and how easily the oil can flow. By doing lab tests and computer modeling, they can better estimate how much oil can actually be collected. This process is important because it helps companies decide how valuable the oil field might be.

Production

To get oil from an underground reservoir, we usually need to drill into the Earth. However, in some places like the La Brea Tar Pits in California and in Trinidad, oil can sometimes be seen on the surface.

Many things affect how much oil we can get from a reservoir. These include how the oil is spread out, how much oil is there to start with, the pressure in the reservoir, the properties of the oil and rock, the shape of the reservoir, the type of wells used, how many wells there are, where they are placed, and the methods used to extract the oil.

Today, we use special methods like thermal and gas injection along with chemical extraction to get more oil out of the ground.

Drive mechanisms

A petroleum reservoir may have enough natural pressure to push oil and gas to the surface. When this pressure drops as oil and gas are taken out, production can slow down. The reservoir might find ways to keep the pressure up naturally, but sometimes people need to help.

One way this happens is through solution-gas drive. This works because oil can hold gas inside it when under pressure. When pressure drops, the gas comes out and forms a cap on top of the oil, helping to push the oil out. Over time, this gas can run out, and production slows.

Another way is gas cap drive, where a reservoir already has a cap of gas on top of the oil. As oil is taken out, the gas cap grows and pushes more oil out. Managing where wells are placed helps get the most oil.

Water can also help. If there is water below the oil, it can push up on the oil as it is taken out, keeping pressure steady. But if too much water comes into the well, it can become costly.

When natural pressure isn’t enough, people can add water or gas to keep the pressure up. Gravity can also help move oil, especially if the rock allows it. In places where oil exists as gas, letting the gas expand can bring the oil out effectively.