Three distinct types of petroleum systems can be defined based on the difference in the basic physics of hydrocarbon accumulation. Conventional petroleum systems (Type 1) have the traditional components of source, seal, reservoir, trap, and timing that must be evaluated and that must be favorable for a hydrocarbon accumulation to be present. Hydrocarbons migrate from the source rock to the reservoir and trap based on the density difference between oil, gas, and water. Continuous basin-centered accumulations (Type 2) trap migrating hydrocarbons in tight rocks by relative-permeability conditions that develop between the hydrocarbons and interstitial water. Reservoir conditions, therefore, also define the seal and the trap. Source rock reservoirs (SRRs, Type 3) have a much lower permeability and much smaller pore throats than even continuous accumulations. The unexpelled and unmigrated hydrocarbons that remain in the porosity of the SRR are available to be produced if sufficient fracture conductivity is induced by hydraulic fracturing. Coal-bed methane, oil sands, and oil SRRs are variations on, and composites of, the three basic petroleum system end members.

The physics of gas flow in SRRs are different from other petroleum systems at the small pore throat sizes that are present in the secondary oil or gas-wet pores within the kerogens and their associated microfractures, from in the water-wet portions of the SRR, or in the migration pathways. In the absence of water within the nanopores, gas is present in a number of diffuse systems. Adsorbed gas is present as a diffuse layer on the surface of the organic porosity. If there is a gradient along that surface, diffusion occurs in a linear fashion. The free gas in the pore space moves from high concentration to low concentration by slippage flow, as described by Knudsen diffusion. There is free interchange between the free and adsorbed gas molecules by “hopping” from one diffuse system to the other. Gas is absorbed within the kerogen matrix and diffuses out to become adsorbed. The relative contribution and rates of flux in these various systems are an active research topic, but the high deliverability of gas from SRRs is a result of the different physics of gas flow.