The dominant molecule in most natural gas accumulations is methane, but in many cases there are also minor amounts of other hydrocarbons such as propane, ethane, and butane. Thermally derived natural gas comes from sedimentary rocks that contain elevated levels of organic molecules rich in hydrogen and carbon atoms referred to as source rocks. The presence of the right kind of source rock in a sedimentary basin that has been subjected to the right conditions of burial and increased temperature may generate a natural gas accumulation, however many complimentary conditions must be met. To understand the requirements for an accumulation of natural gas (and oil), geologists use the petroleum system concept where a functioning petroleum system must include the following five elements:
- source rock,
- migration pathway,
- reservoir rock,
- seal rock, and
When source rocks are slowly heated to the right temperature (between approximately 150°F and 250°F) organic molecules react to form the mix of chainlike hydrocarbons we call crude oil. Source rocks heated to temperatures within this range, the ‘oil window,’ are said to be thermally mature for liquid hydrocarbons, but they commonly also begin generating natural gas in addition to oil. Source rocks capable of generating oil are referred to as oilprone and are typically derived from marine algae and other microorganisms. When source rocks are heated above 250°F they are described as overmature for oil, but can still generate significant quantities of natural gas. Source rocks that start out rich in carbon but leaner in hydrogen (coal, some shale, and limestone) can generate natural gas, but not the more hydrogen-rich liquid hydrocarbons found in crude oil. These types of source rocks are referred to as gas-prone and typically consist of organic material derived from land vegetation. These transformations occur due the rise in temperature with increasing depth below ground surface; geothermal heat. The rate at which the temperature increases with depth is described by the geothermal gradient which, on average in drilled sedimentary basins of the world, is about 50°F per 1,000 feet of depth. This means that the deeper we go beneath the earth’s surface, the warmer the rocks become. The part of a sedimentary basin where source rocks are buried deep enough for temperatures to be high enough to cause these thermal conversions is informally referred to as the “petroleum kitchen”.
When hydrocarbons are generated in the kitchen, their buoyancy quickly drives them to migrate out of the source rock following the path of least resistance through the most permeable strata they encounter. This migration out of the source rock creates the possibility of trapping and accumulation in a reservoir rock.
Reservoir rocks are porous and permeable formations that can store oil and gas in pore spaces between grains and later allow them to flow out of the rocks to wellbores, where they can be extracted. Sandstones, limestones, and dolomites, under the right conditions, can possess enough interconnected pores to form good reservoir rocks. Some low permeability rocks can still function as reservoirs for natural gas, due to the lower density and greater buoyancy of gas. In order for the pores in a reservoir rock to become filled with gas (or oil, if present), it must be located along a hydrocarbon migration pathway. If a pathway does not lead to a reservoir rock, the hydrocarbons may be lost to the surface environment.
Only where porous and permeable rocks are enclosed in trapping geometries does gas (and oil, if present) stop migrating and accumulate in the reservoir rock to form fields. Effective traps consist of reservoir rocks overlain and/or laterally bounded by impermeable seal rock, and are of two basic types. Structural traps occur where rock layers are deformed by folding or faulting to form concave-downward shapes capable of containing buoyant fluids such as gas. Stratigraphic traps occur where porous, permeable reservoir rocks are encased in impermeable seal rocks as a result of non-uniform deposition of sediments.
For example, clean sands on a wave-worked beach may grade laterally into a muddy offshore setting, and with time, the muddy offshore zone may migrate over the older beach sand, setting up a possible future stratigraphic trap, consisting of a wedge of porous reservoir sands between the impermeable muds above and below. Structural traps are usually much easier to identify and generally host the initial oil and gas discoveries in a basin. Stratigraphic traps are much harder to target, and their successful prediction normally requires more detailed mapping of the subsurface geology. This is best achieved by interpreting high-quality, closely spaced seismic data along with information gained from previously drilled surrounding wells. In any case, in order for traps to host gas fields (or oil), they must be created prior to hydrocarbon generation, expulsion, and migration from the kitchen. Moreover, they must then remain intact, uncompromised by later folding, faulting, or excessive burial.
Coal constitutes a special type of gas-prone source rock that can generate gas either as a result of the thermal maturation described above, or through microbial degradation at shallower depths in the absence of oxygen. Gas generated through the latter process is referred to as biogenic gas. In buried coal seams, biogenic gas molecules are typically dissolved in the surrounding pore waters and stored in the coal matrix, where methane molecules attach to coal particles. As long as the coal remains buried at this same depth, it is subjected to the pressure of the overlying rock and groundwater (hydrostatic pressure) and the methane molecules cannot form bubbles that can migrate out of the coal. If the coal seam is subsequently uplifted to shallower depths in the basin, the hydrostatic pressure is reduced, allowing the methane to bubble out of solution and migrate out of the coal seam. Once this migration starts, the gas follows the path of least resistance, as noted above, and will either migrate to a reservoir in a trapping configuration, get stranded in small quantities in the subsurface, or will eventually migrate to the surface and be lost to the atmosphere.