Natural gases are derived from biogenic and nonbiogenic sources through diverse processes, including bacterial formation, catagenesis, hydrothermal/geothermal activity, and to an unknown degree, primordial or mantle emissions. Exploration for natural gas is aimed principally at accumulations of fossil fuel hydrocarbons, but other gas accumulations, notably those of noble gases, can also be economic targets. Investigators use the chemical and physical properties of these natural gases to identify and characterize the petroleum system using geochemical tools. Three categories of geochemical tools are commonly used to correlate natural gases to their sources and to others gases: (1) gas concentration, (2) molecular composition, and (3) stable isotope ratios. Analytical routi es are frequently applied to samples from near-surface sediments/soils and seepages, drill cuttings and cores, and oil and gas accumulations, all of which can be used to demonstrate the stratigraphic and geographic extent of a petroleum system.

Light hydrocarbons are the predominant compounds used to classify natural gases. Associated and co-genetic nonhydrocarbons, such as CO₂, N₂, H₂S, He, and Rn, however, can also provide ancillary information that can help identify the appropriate source of active source rock. Interpretative schemes, combining concentration and composition, are founded extensively on empirical observations of natural gas occurrences. Significant, measurable, and predictable differences among the various gas types provide the basis for these geochemical tools to identify and characterize primary gases, which are the original, unaltered gases from a single source of active source rock. These tools can also differentiate them from secondary gases, which are mixtures of diffe ent gas types or are primary gases with altered signatures due to oxidation or migration. In addition, natural gas fingerprints can be used to estimate the type and thermal maturity of the organic matter or kerogen sourcing the hydrocarbons. For fingerprinting gases, stable isotope ratios of the light hydrocarbons are key diagnostic parameters.