Note that coal does generate methane and coal deposits have given rise to natural gas resources, for instance those beneath the southern North Sea. So coal can be a petroleum source rock, but usually for gas fields.
Table 2 highlights the major differences between the four kerogen types in terms of their chemical properties and biological origins. Type I kerogen is comparatively rare as it is derived mainly from algal sources in lake and/or lagoonal environments: the Scottish Midland Valley 'oil shales' used by 'Paraffin' Young contain kerogen of this kind. Type II kerogen, the most abundant, is typically derived from plant debris, phytoplankton and bacteria in marine sediments; it is the common source of crude oil but also yields some natural gas. Type III kerogen comes mainly from remains of land plants found in coals and it principally generates natural gas. Type IV kerogen includes oxidised plant remains and fragmentary charcoal derived from forest fires; it has virtually no petroleum potential being devoid of hydrogen.