Measurements of methane and other hydrocarbon gases in near-surface marine sediments have been made with increasing frequency over the last 10 years as part of various geochemical prospecting efforts. Presumably, the presence of light hydrocarbon anomalies in sediments is indicative of seepage of hydrocarbons from nearby reservoirs. However, gas concentrations and compositions can be altered by filtering effects during gas migration through sediments as well as by microbially induced interferences and alterations. Methane is apparently consumed and oxidized by aerobic and anaerobic bacteria in near-surface sediments. The bacteria can alter isotopic compositions of microbially produced methane to yield thermal-like compositions which can be misinterpreted as oil-related ga . Ethane and higher (C₂+) hydrocarbon anomalies are considered more positive indicators of commercially prospective oil and gas accumulations but these gases can be selectively filtered by sediment chromatographic effects yielding bacteria-like compositions which might be passed over as non-anomalous.

These concerns, coupled with methodologic problems such as (1) the difficulty of measuring isotope ratios on small amounts of sediment gas, (2) the fact that the hydrocarbons which initially outgas from a sediment sample are different in composition than subsequent outgassing, and (3) disputes over the optimum depth for sediment gas measurement and anomaly detection, demand that surface gas anomalies used for prospect evaluation should be interpreted with care.

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