Crude oil is extracted, not mined. The distinction between these two terms is determined by the physical state of the resource being obtained from the earth. Crude oil is a liquid hydrocarbon, recovered using fluid dynamics and pressure manipulation, a process known as extraction. Mining, conversely, is the method used for retrieving solid materials, such as coal, gold ore, or other minerals.
Why Crude Oil Is Extracted, Not Mined
The geological conditions that hold crude oil dictate the method of recovery. Oil exists deep underground, soaked within the microscopic pore spaces of permeable rock formations, much like a sponge holds water. This formation, known as the reservoir rock, is typically a sedimentary layer such as sandstone or carbonate rock.
Above the reservoir, an impermeable layer called the caprock acts as a seal, preventing the buoyant oil from migrating upward. The oil and gas are trapped together in a structural or stratigraphic trap under immense natural pressure. Because the resource is a fluid, it is accessed by penetrating the reservoir with a narrow borehole.
This pressure-driven, fluid-based retrieval defines extraction, contrasting with the physical removal of a solid resource required in mining.
The Standard Method of Extraction: Drilling and Pumping
The first step in conventional oil extraction is exploration, relying on geophysical tools like three-dimensional (3D) seismic surveys. These surveys send acoustic waves into the earth and record the reflections that bounce back from subsurface rock layers. Geoscientists process this data to create detailed images of the geology, helping to identify potential hydrocarbon traps.
Once a location is selected, the process begins with rotary drilling to bore a hole thousands of feet into the earth. Steel pipe sections, called casing, are inserted to line the hole and prevent the surrounding rock from collapsing. A cement slurry is then pumped into the annulus—the space between the casing and the borehole wall—where it hardens to bond the pipe to the formation.
This cementing process creates an impermeable barrier, isolating geological zones and protecting groundwater formations. Once the wellbore is stable, the final stages of extraction, known as recovery, begin. Oil recovery is categorized into three distinct phases designed to maximize the volume of crude oil brought to the surface.
Primary Recovery
The initial phase, primary recovery, relies on natural forces within the reservoir, such as pressure from dissolved gas or an underlying water drive. This natural pressure forces the oil up the wellbore, often yielding only about ten percent of the reservoir’s original oil in place.
Secondary Recovery
When the natural pressure declines, secondary recovery methods are employed to extend the productive life of the field. The most common secondary technique is waterflooding, where water is injected into nearby wells to physically push the remaining oil toward the production well. Secondary recovery typically boosts the total recovery rate by an additional 20 to 40 percent.
Tertiary Recovery (Enhanced Oil Recovery)
The final phase is tertiary recovery, or Enhanced Oil Recovery (EOR), which uses advanced techniques to access oil trapped after the first two stages. EOR methods work by altering the properties of the oil itself to make it flow more easily. Thermal recovery involves injecting steam into the reservoir to heat the heavy, viscous crude, lowering its resistance to flow. Other EOR techniques include injecting gases like carbon dioxide or nitrogen to reduce viscosity or increase reservoir pressure.
Extraction Methods for Solid Hydrocarbons
While conventional crude oil is a liquid extracted through wells, other hydrocarbon resources require mining due to their solid or semi-solid state. Oil sands, for example, consist of sand, clay, water, and a highly viscous substance called bitumen. Since bitumen is too thick to flow naturally, the upper deposits are physically excavated using massive shovels and trucks in strip mining. The mined material is then transported to an extraction plant where hot water and solvents separate the bitumen from the sand.
For deeper oil sand deposits, an in-situ (in place) extraction method is used, such as Steam Assisted Gravity Drainage (SAGD). This process involves drilling two parallel horizontal wells into the deposit. Steam is continuously injected into the upper well, heating the bitumen until it becomes fluid enough to drain by gravity into the lower production well.
Another resource that involves mining is oil shale, a sedimentary rock containing a solid organic material called kerogen. To obtain a usable liquid fuel, the oil shale rock must be physically mined, often through underground or open-pit mining, and then crushed. The crushed rock is subsequently heated in a process called retorting, which converts the solid kerogen into a synthetic crude oil that can be refined.