Formation and Geological Entrapment of Methane

Geological methane is methane generated within sediments or rocks and retained underground as free, dissolved or adsorbed gas. In sedimentary basins, it forms mainly through microbial activity at relatively low temperatures or through the thermal alteration of buried organic matter. It can migrate into porous rocks, become trapped beneath a low-permeability seal, and later escape through connected faults, joints, fissures or excavations.

Generation of methane

The origin of methane depends chiefly on organic matter, burial conditions, temperature and time.

• Biogenic methane is produced by methanogenic microorganisms during the anaerobic decomposition of organic matter, commonly in relatively shallow and low-temperature sediments.
• Thermogenic methane forms at greater burial depths when increasing heat and pressure transform organic matter into kerogen and subsequently crack kerogen or liquid hydrocarbons into gaseous hydrocarbons.
• Organic-rich mudstones, shales and coal-bearing strata commonly act as source rocks because they contain the material from which methane is generated.

Migration, storage and geological entrapment

A geological methane accumulation is generally not a large empty underground chamber. Gas is usually held under pressure within microscopic pores, fractures or adsorption sites in rock.

• After generation, methane moves from source rocks along permeable beds, pores and fractures under pressure gradients and buoyancy.
• Porosity determines how much fluid a rock can contain, while permeability determines how readily methane can move through its interconnected pores or fractures.
• Sandstone and some carbonate rocks can serve as conventional reservoir rocks when they possess adequate porosity and permeability.
• Methane may occur as free gas in pore spaces, dissolved in formation water or petroleum, or adsorbed on the internal surfaces of coal and organic-rich shale.
• An accumulation forms where a reservoir or fractured zone has geological closure and is overlain or bounded by a low-permeability seal such as shale, clay-rich rock or evaporite.
• Structural traps include folds and fault-bounded closures, while stratigraphic traps arise from changes such as reservoir-rock pinch-outs or unconformities.
• A fault can either transmit methane through an open, connected fracture network or seal it through clay-rich fault gouge, mineral filling or juxtaposition against low-permeability rock.
• The expression “geological pocket” therefore usually refers to a localized, sealed pore or fracture system rather than a cavern filled with gas.

Release through faults, fissures and excavations

Methane is released when a formerly sealed accumulation becomes hydraulically connected to a lower-pressure region such as the ground surface, a mine or a tunnel.

• Open or reactivated faults, joints and fissures can provide connected pathways from gas-bearing strata to the surface or an underground excavation.
• Tectonic movement, stress changes, weathering or excavation-induced fracturing may increase fracture connectivity and weaken an existing seal.
• Drilling, blasting or tunnelling may directly intersect a gas-bearing bed, sealed fracture or fault zone and create a sudden pressure outlet.
• A fall in pressure allows free gas to expand, dissolved methane to come out of solution and adsorbed methane, especially in coal, to desorb and enter fractures.
• The rate of discharge depends on gas pressure, gas content, fracture aperture and connectivity, and the permeability of the surrounding rocks.
• Because methane is lighter than air, it can accumulate near the roof or in poorly ventilated high points of an underground opening.
• Methane becomes a fire or explosion hazard when it mixes with air and encounters an ignition source; at high concentrations it can also create an oxygen-deficient atmosphere.
• Geological investigation, gas monitoring, adequate ventilation and control of ignition sources are therefore essential precautions in methane-prone underground works.

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