Seeps play an important role in the exploration of new basins or areas (Hunt, 1996). Only one basin in three is petroliferous enough to contain producible oil or gas, and only one in six contains even one large oil field (North, 1985). The importance of seeps has been minimized in this era of increased use of sophisticated instrumentation and decreased use of ground surveys. Nevertheless, nearly all the important oil-producing regions of the world were first discovered by surface oil and gas seeps (Hunt, 1981).
Less than 15% of the hydrocarbons generated by source rocks becomes recoverable oil and gas in reservoirs. Seeps occur wherever a permeable pathway leads to the surface from mature source strata or leaking petroleum reservoirs. These pathways include the outcrops of petroleum carrier beds, source rocks and unconformities, breached reservoirs and the surface expression of intrusions such as mud volcanoes and salt domes (Link, 1952). As oil migrates towards the surface, a number of weathering mechanisms begin to alter its composition. These include the evaporation of the more volatile hydrocarbons and consumption by microbes (biodegradation). The viscous liquids and asphalt found in surface seeps bear little chemical resemblance to their source, and a seep-reservoir oil correlation study requires very sophisticated analytical techniques. Except for indicating that an area has sourced liquid hydrocarbons, the presence of observable surface seeps reveals little about the source type or thermal maturity within a basin.
Hydrocarbons generated in petroliferous basins are composed of a large range of components, from the simplest, lightest methane molecule to very large and complex molecular structures. The light hydrocarbons (C1-C4), methane, ethane, propane and butane, migrate in the subsurface in gaseous form. Heavier hydrocarbons (C5+) migrate in the liquid phase. This difference is important with respect to seep detection. Oil seeps can only form where there is an uninterrupted path with adequate permeability to transport liquids to the surface. Consequently, observable macroseeps are rare and have often followed tortuous pathways to the surface. The light gaseous hydrocarbons, however, are more mobile in the subsurface and require a much less open pathway to be focused at the surface. They are also more likely to be located closer to their subsurface sources. These microseeps are much more widespread and, although generally invisible to the naked eye, are present in small concentrations that can now be sampled in soils, measured and mapped. The pattern and intensity of this microseepage can be combined with geologic information to predict areas having the greatest probability of containing subsurface reservoirs.
The relative concentrations of these light hydrocarbons (methane, ethane, propane, and butane) are directly related to production type and thermal maturity of hydrocarbons in the basin (Jones and Drozd, 1983). In other words, oil productive basins will contain greater proportions of ethane, propane and butane relative to methane, and gas prone areas will contain greater proportions of methane. Thus, light hydrocarbon surveys can be considered as a source rock tool applied at the surface. Although the presence of an oil seep proves the existence of oil generating sources, it does not provide adequate information on the level of thermal maturity and the potential gas (GOR) available to charge the subsurface reservoirs. This type of information is not generally present in liquid seeps because of weathering effects. The thermal maturity of the subsurface hydrocarbons that source the surface seeps can be determined by sampling the light hydrocarbons in the vicinity of observable seeps. Once established, this signature can be compared with compositions of light hydrocarbon seeps measured throughout the basin, with the goal of highlighting those areas that could be sourced from subsurface reservoirs.§

Macroseep Association with Production.- Link, W.K., 1952, Significance of Oil and gas Seeps in World Oil Exploration, AAPG Bulletin, V.36, pp. 1505-1541

Distribution of Oil and Gas Seeps in the United States.- Link, W.K., 1952, Significance of Oil and gas Seeps in World Oil Exploration, AAPG Bulletin, V.36, pp. 1505-1541