A GEOCHEMICAL PERSPECTIVE OF NATURAL-GAS AND ATMOSPHERIC METHANE

Natural gases are key components in the description of the global carbon cycle. Although the estimated 120 Gt C reserves of natural gas are minute compared to the total carbon reservoir (c. 1.4 x 10(8) Gt C), the variations in composition and distribution can provide information on the magnitude of carbon fluxes between specific geo-, hydro- and atmospheric reservoirs. Natural gases, and in particular methane, influence not only our economies, but our atmosphere and ultimately our climate as well. Volatile hydrocarbons also represent, in certain environments, a significant carbon nutrient source for specialized organisms. The magnitudes of natural gas reservoirs and fluxes are put into perspective. On a global basis, natural gas supplies c. 20% of man's primary, external energy needs; following the consumption of oil an coal (38 and 30%, respectively). Currently, worldwide production of natural gas amasses 1.8 x 10(12) m3 y-1 or c. 2% y-1 of the proven geogas reserves (estimated total geogas reserves > 200 x 10(12) m3). Geochemical exploration for hydrocarbons relies heavily on the characterization of natural gases. As the volatile hydrocarbon phase, natural gases have physical and chemical properties which assist in their detection and application. They migrate readily, and are widely distributed in the sediment column. In addition, the significant range in molecular and isotope composition of natural gases provides interpretative information on the gases (e.g. conventional bacterial/diagenetic, thermogenic, geothermal or unconventional "deep gas") and on the maturity/type of the precursors source material from which they are derived. Reliable interpretation of natural gas data requires that secondary effects (e.g. migration, mixing, oxidation) can be identified. Artifacts, such as hydrocarbons generated during drilling or analysis, and sampling contamination/alteration must also be considered. These various aspects are reviewed. Occasionally, anomalous natural gas data lead us to the recognition of new gas types or processes. Some recent geochemical enigmas are presented for both the adventurous and skeptics.