Surface indications of petroleum

Surface indications of the presence of oil and gas seldom if ever should be considered to be of primary importance unless they are interpreted in conjunction with the age, character, and especially the structure of the associated rocks of the area under investigation. Oil seeps and springs, for example, rarely indicate that the associated subsurface strata are oil-bearing, and the hydrocarbons are migrating and possibly being dissipated from adjacent structural traps. Usually the associated structure and pool of oil will be found at a more or less remote distance from the places of surface emission.

Asphaltic outcrops and bituminous sandstones and porous limestones have been found in many areas that are adjacent to commercial oil pools. Such outcrops of proliferous substances are believed to be the result of natural distillation and subsequent migration to surface exposures where the more volatile hydrocarbons escape, leaving the residue to dry and oxidize in the porous rocks near the surface.

Oil seeps may be found associated with many large petroleum producing regions of the world. The oil may be carried to the surface by water, in which case a slightest iridescent film or “rainbow” of oil may be seen on the surface of the water. Such a film is known to resemble closely a similar film of iron oxide, and the latter has sometimes been mistaken for a film of oil. However, an iron oxide film to be seen at the surface of water is brittle and will break up if stirred with a stick, but an oil film will not. Also, petroleum compounds may enter fissures and rock cavities and migrate to the surface, in which case bituminous dikes are formed.

Gas seeps that emit chlorine, nitrogen and carbon dioxide constitute poor evidence as surface indications, but seeps that contain perhaps some methane, and other hydrocarbon gases usually are considered to be a favorable criterion. Seep gases that contain the hydrocarbons of higher molecular weight should indicate the gas to have its source in close proximity to crude oil, and a dry gas escaping under considerable pressure seems to be a good indicator. In some areas, of the gulf Coast oil fields, the escape of sulphurous gases has been proved to be coming from deposits of crude oil in that vicinity.

While outcropping oil shales are usually considered to constitute good indications of associated petroleum deposits, such evidences as salt water seeps, paraffin dirt, mud dikes and mad volcanoes that emit gas, and sulphur gases and waters are usually of only minor consequence.