Petroleum reservoir

Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by the presence of high heat and pressure in the Earth's crust.

An oil field is an area of accumulated liquid petroleum underground in multiple (potentially linked) reservoirs, trapped as it rises to impermeable rock formations.

In industrial terms, an oil field implies that there is an economic benefit worthy of commercial attention.

[1][2] Oil fields may extend up to several hundred kilometers across the surface, meaning that extraction efforts can be large and spread out across the area.

In addition to extraction equipment, there may be exploratory wells probing the edges to find more reservoir area, pipelines to transport the oil elsewhere, and support facilities.

Creating an operation at an oil field can be a logistically complex undertaking, as it involves the equipment associated with extraction and transportation, as well as infrastructure such as roads and housing for workers.

[6] Natural gas originates by the same geological thermal cracking process that converts kerogen to petroleum.

It uses a few, very large offshore drilling rigs, due to the cost and logistical difficulties in working over water.

Rising gas prices in the early 21st century encouraged drillers to revisit fields that previously were not considered economically viable.

For example, in 2008 McMoran Exploration passed a drilling depth of over 32,000 feet (9754 m) (the deepest test well in the history of gas production) at the Blackbeard site in the Gulf of Mexico.

Evidence indicates that millions of years of heat and pressure changed the remains of microscopic plants and animals into oil and natural gas.

[8] In addition to the aquatic ecosystem, which is usually a sea but might also be a river, lake, coral reef, or algal mat, the formation of an oil or gas reservoir also requires a sedimentary basin that passes through four steps:[9] Timing is also an important consideration; it is suggested that the Ohio River Valley could have had as much oil as the Middle East at one time, but that it escaped due to a lack of traps.

[11] In recent years, igneous reservoirs have become an important new field of oil exploration, especially in trachyte and basalt formations.

The timing of trap formation relative to that of petroleum generation and migration is crucial to ensuring a reservoir can form.

They are more easily delineated and more prospective than their stratigraphic counterparts, with the majority of the world's petroleum reserves being found in structural traps.

Stratigraphic traps are formed as a result of lateral and vertical variations in the thickness, texture, porosity, or lithology of the reservoir rock.

The seal (also referred to as a cap rock) is a fundamental part of the trap that prevents hydrocarbons from further upward migration.

This has tradeoffs, with higher post-production costs associated with complete and clean extraction of oil being a factor of consideration for a company interested in pursuing a reservoir.

Despite these tradeoffs, unconventional oil is being pursued at a higher rate because of the scarcity of conventional reservoirs around the world.

In a simple textbook example of a uniform reservoir, the first stage is to conduct a seismic survey to determine the possible size of the trap.

The porosity of an oil field, or the percentage of the total volume that contains fluids rather than solid rock, is 20–35% or less.

They have variable porosities and permeabilities and may be compartmentalized, with fractures and faults breaking them up and complicating fluid flow.

Because the evaluation of reserves has a direct impact on the company or the asset value, it usually follows a strict set of rules or guidelines.

[24][25] Modern production includes thermal, gas injection, and chemical methods of extraction to enhance oil recovery.

The virgin reservoir may be entirely semi-liquid but will be expected to have gaseous hydrocarbons in solution due to the pressure.

The gas/oil ratio and the oil production rate are stable until the reservoir pressure drops below the bubble point when critical gas saturation is reached.

In this case, the oil rate will not decline as steeply but will depend also on the placement of the well with respect to the gas cap.

This water gradually replaces the volume of oil and gas that is produced out of the well, given that the production rate is equivalent to the aquifer activity.

Recovery from a closed reservoir (i.e., no water drive) is very good, especially if bottom hole pressure is reduced to a minimum (usually done with compressors at the wellhead).