Abstract The successful application of surface and nearsurface geochemical techniques in hydrocarbon exploration requires careful integration of many geological disciplines. Surface geochemistry can provide the explorationist with a means to screen large areas rapidly, economically, and qualitatively for overall petroleum source potential. Regions of favorable thermal environments for the formation of petroleum can be defined by identifying areas containing thermogenic hydrocarbons in soilgas. Soilgas techniques are also useful in determining areas of prospective accumulations prior to conducting additional and more costly exploration programs. Historically, the presence of an active oil or gas seep was sufficient to deem an area as prospective. Seep detection today rarely involves visible evidence of hydrocarbons; rather, it deals with seeps of extremely small magnitude (microseeps) that are not directly visible to the unaided eye. Typically, microseeps are detectable through the use of sensitive analytical instrumentation. In addition, it may be possible to detect active or inactive microseepage from its secondary manifestations such as bleaching of rocks in the area of the seep, the presence of stressed vegetation, or the formation of authigenic magnetite. Examples of some of these secondary indicators can be found in studies conducted by Dalziel and Donovan (1980), Donovan (1974), and Furguson (1975). Interpretation of these secondary indicators is often difficult because other natural processes can produce the same physicochemical features. Furthermore, the quantity and composition of hydrocarbon gases in the soil may be affected by the local geology. Consequently, the true meaning and character of these microseeps can only be realized after