Oil, Gas and Energy Resources
How Oil and Gas Form
Petroleum (oil) and natural gas originate from the remains of marine microorganisms — mainly phytoplankton and zooplankton — that accumulated in fine-grained sediments on the sea floor over millions of years. Buried under thick piles of sediment, this organic material is subjected to increasing heat and pressure in a process called maturation. At roughly 60–120°C (the “oil window”), long-chain organic molecules crack into shorter-chain hydrocarbons: oil. At temperatures above about 150°C, lighter natural gas is produced. The rock in which the organic material matures is called the source rock.
The Three Rock Types of an Oilfield
Once generated, oil and gas migrate upward through the subsurface (being less dense than water) until they are either trapped or reach the surface and are lost. A commercial deposit requires three rock types to be present together:
The reservoir rock is where the oil or gas is stored. It must be both porous (to hold the hydrocarbons in pore spaces) and permeable (to allow the fluids to flow when drilled). Sandstones and limestones with good interconnected porosity are the most common reservoir rocks. The Brent Group sandstones of the North Sea and the Jurassic limestones of the Middle East are classic examples.
The cap rock (or seal) is a tight, impermeable rock that prevents the hydrocarbons from escaping upward. Evaporites (halite, anhydrite), unfractured shale, and mudstone are effective cap rocks. Without a cap rock, any oil that migrates into the reservoir will continue upward and be lost.
A commercial oilfield requires: a source rock (organic-rich, fine-grained — generates the oil); a reservoir rock (porous and permeable — stores and yields the oil); and a cap rock (impermeable — prevents upward escape). All three must be in the right spatial arrangement.
Trap Structures
A trap is any geometry that causes oil and gas to pool rather than escape. The main trap types are:
Anticline traps form where rocks have been folded into an arch (anticline). Oil and gas migrate into the crest of the fold, where they are sealed by cap rock draped over the top. The anticline is the most commonly illustrated trap in geology exams. Cross-sections in your textbook will show the dome-shaped geometry, with oil sitting above water in the reservoir rock and gas above oil at the crest.
Fault traps form where a fault brings an impermeable rock (e.g. shale) against a porous reservoir rock, sealing the migration pathway. The juxtaposition must seal tightly enough to prevent leakage. Unconformity traps occur where an impermeable layer was deposited across an eroded, tilted sequence, sealing the truncated ends of reservoir rock layers beneath. Salt dome traps form where a buoyant dome of halite (rock salt) pierces upward through overlying sediments, doming and faulting reservoir rocks around its flanks and creating a structural closure above and beside the dome.
Students often mix these two up. Porosity is the total pore space in a rock — how much empty space it contains as a proportion of its volume. Permeability is the ability of fluids to flow through those pores — controlled by whether pores are connected to each other. A rock can be highly porous but impermeable: pumice has many pores but they are not well-connected, so fluids flow through it poorly. Conversely, a rock with moderate porosity but well-connected pores (like a well-sorted sandstone) can be highly permeable. Both properties are essential for a good reservoir rock — and permeability alone is what matters for a cap rock (it must have very low permeability to act as a seal).
Extraction Methods and Fracking
Conventional oil and gas extraction involves drilling into the reservoir rock through the cap rock; pressure in the reservoir pushes hydrocarbons up the well. As pressure declines, pumps or water injection maintain flow. Offshore extraction uses platforms drilled into the sea bed — the North Sea has over 180 oil and gas fields operated from fixed and floating platforms.
Hydraulic fracturing (fracking) is a technique used to extract oil and gas trapped in the pores of tight (very low permeability) source rocks such as shale, which are too impermeable for conventional production. High-pressure fluid is pumped into the well, fracturing the shale and propping the fractures open with sand or ceramic beads, allowing hydrocarbons to flow. Fracking has transformed oil and gas production in the United States but is controversial in the UK and elsewhere due to environmental concerns including induced seismicity, groundwater contamination risks, and methane leakage.
An anticline trap works because oil and gas migrate upward into the arched crest of a fold. The cap rock, draped over the crest, prevents further upward migration — trapping the hydrocarbons in the porous reservoir rock beneath. This is the most commonly examined trap type.
Onshore extraction disturbs habitats, requires access roads and pipelines, and risks blowouts and spills if well integrity fails. Offshore extraction risks catastrophic spills: the 2010 Deepwater Horizon blowout in the Gulf of Mexico released approximately 4.9 million barrels of oil, causing severe ecological damage to marine and coastal ecosystems. Gas flaring (burning off excess gas at the wellhead) releases CO₂ and black carbon. Subsidence above depleted fields can affect surface infrastructure. Fracking specifically raises concerns about water use (millions of litres per frack), disposal of contaminated fracking fluid, and small induced earthquakes caused by fluid injection affecting nearby faults. In the UK, induced seismicity was the primary reason for the moratorium on fracking operations.