This paper is intended to introduce the student or nongeologist to some of the concepts of the origin and accumulation of natural gas. Natural gases consist of methane, carbon dioxide, and nitrogen, and small quantities of hydrogen sulfide, heavier hydrocarbons, helium, and, very rarely, hydrogen. It appears most probable from geochemical considerations that the major gases result from bacterial action on organic and inorganic matter within the sedimentary rocks of the earth's crust. Thermal processes may account for some of the gases, notably methane and carbon dioxide, in special cases. The inert gas, helium, is unique in that it is the product of radioactive decay of uranium and thorium which are present in trace quantities in most rocks and sediments.

Migration of gases from source beds to reservoir rocks is a result of both solution in ground waters, with subsequent release as pressure decreases, and migration as gas through the saturated waters within porous rocks. Entrapment and accumulation occur where the rising gases meet strata impermeable to gas.