Near-Surface Evidence of Migration of Natural Gas from Deep Reservoirs and Source Rocks

ABSTRACT Gaseous hydrocarbons migrate to the surface in small quantities from deep source rocks and natural gas or crude oil accumulations. This process leads to anomalous concentrations of hydrocarbons in near-surface sediments. The hydrocarbon anomalies (amount and composition) are useful in exploration, because they may point to oil and gas reservoirs. The origin of hydrocarbon anomalies in shallow sediments is not a simple one. Methane in near-surface sediments may be caused by a combination of bacterial production of methane, leakage from reservoirs, and leakage of methane from thermal maturation of organic matter in source rocks. Organic geochemistry, especially carbon isotope techniques, have been used to recognize secondary fractionation processes caused by degassing or bacterial oxidation. Examples of geochemical surveys in onshore and offshore areas indicate secondary fractionations so small that gases from near-surface sediments and from reservoirs can be correlated by the geochemical data. The application of stable carbon isotopes is a significant step toward improving geochemical hydrocarbon surface exploration methods.