9.2 Energy resources (petroleum systems, coal)

The Petroleum System

A petroleum system encompasses all the geologic elements and processes required to generate, move, and trap hydrocarbons. Five essential elements must coincide in space and time: a source rock, sufficient maturation, a migration pathway, a reservoir rock, and a trap sealed by an impermeable cap rock. If any element is missing or out of sequence—for example, if a trap forms after the oil has already migrated away—no accumulation results.

Source Rock and Maturation

Hydrocarbons originate in organic-rich source rocks, typically dark, fine-grained shales or carbonate muds deposited in anoxic (oxygen-poor) settings where organic matter is preserved rather than oxidized. During burial the organic material is converted to kerogen, the solid precursor of petroleum. As burial deepens, temperature rises and thermal maturation slowly cooks the kerogen into oil and gas.

The oil window is the temperature range—roughly 60 to 120 degrees C, about 2 to 4 km depth—in which liquid oil is generated. Shallower and cooler, organic matter is thermally immature; hotter than about 120 to 150 degrees C the gas window dominates, cracking oil into natural gas, and at still higher temperatures only dry methane and inert carbon residue remain. A source rock's potential is judged by its total organic carbon (TOC), the type of kerogen it contains, and its thermal maturity, commonly measured by vitrinite reflectance.

Migration

Because petroleum is less dense than the formation water saturating the rock, buoyancy drives it upward and laterally out of the compacting source rock. Primary migration is the expulsion of hydrocarbons from the tight source rock; secondary migration is their movement through permeable carrier beds and along faults until they are trapped or leak to the surface as a seep.

Reservoir Rock

A reservoir rock stores and transmits hydrocarbons, so it must have both porosity—void space to hold fluids—and permeability—connected pores that allow fluids to flow. Sandstones and porous or fractured carbonates (limestone and dolomite) are the most common reservoirs. Porosity may be primary (depositional pore space between grains) or secondary (fractures and dissolution vugs created after deposition); compaction and cementation during burial reduce it, while dissolution and fracturing can restore it. High porosity with low permeability—as in a chalk or a shale—can hold abundant hydrocarbons that will not flow to a well, which is precisely the challenge of unconventional reservoirs.

Traps and Seals

A trap is a geometry that halts migration and concentrates hydrocarbons, and a seal or cap rock—an impermeable layer such as shale, evaporite, or tight carbonate—prevents further escape upward. Because gas floats on oil and oil floats on water, a trapped accumulation naturally segregates into gas, then oil, then water from top to bottom.

• Structural traps form by deformation: anticlines (arched folds), fault traps (a fault juxtaposes reservoir against an impermeable unit), and salt domes (rising salt pierces and arches the overlying strata).
• Stratigraphic traps form by depositional or diagenetic changes: pinch-outs, buried reefs, unconformities, and lateral facies changes where a reservoir grades into impermeable rock.
• Combination traps involve both structural and stratigraphic elements.

Conventional vs. Unconventional Resources

In a conventional reservoir, hydrocarbons have migrated into porous, permeable rock within a trap and flow readily to a well under natural pressure. Unconventional resources remain locked in low-permeability rock and require stimulation to produce:

• Shale (source-rock) and tight reservoirs hold oil and gas in rock of very low permeability—often the source rock itself, which never expelled its hydrocarbons.
• Hydraulic fracturing (fracking) combined with horizontal drilling injects high-pressure fluid, sand (proppant), and chemicals to create and prop open fractures, letting oil and gas flow to the well.
• Other unconventional resources include coalbed methane, tar (oil) sands, and gas hydrates.

Coal

Coal is a combustible sedimentary rock formed from plant matter that accumulated in swamps and was buried before it could fully decay. Burial and heating progressively drive off moisture and volatile compounds and increase the proportion of fixed carbon, a process called coalification. Rising rank records increasing heat and pressure over time.

Coal rank measures the degree of coalification (thermal maturity), whereas coal grade refers to impurity content such as ash and sulfur. Higher-rank coals carry more energy per unit mass and burn more cleanly. Sulfur content matters environmentally, because burning high-sulfur coal releases sulfur dioxide that contributes to acid rain. Anthracite, the highest rank, is hard, shiny, and rich in carbon; peat, at the other end, is the uncompacted precursor and is not technically coal. Most of the world's minable coal accumulated during two great intervals—the Carboniferous and Permian, when vast tropical swamps flourished, and the Cretaceous through early Cenozoic. Coal thus doubles as a key energy resource and as a source rock and reservoir for coalbed methane, tying the coal story directly back to the petroleum system that opens this section.