Assumptions

The underlying assumption of all near-surface geochemical exploration techniques is that hydrocarbons are generated and/or trapped at depth and leak in varying but detectable quantities to the surface. This has long been an established fact, and the close association of surface geochemical anomalies with faults, productive fairways, and specific leads and prospects is well known. The surface expression of hydrocarbon seeps is best exemplified in areas with numerous well developed migration pathways and an active petroleum system. It is further assumed, or at least implied, that the anomaly at the surface can be reliably related to a petroleum accumulation at depth. The success with which this can be done is greatest in areas of relatively simple geology, but becomes increasingly difficult as the geology becomes more complex. The geochemical anomaly at the surface represents the end of a petroleum migration pathway, a pathway that can range from short or long distance vertical migration to long distance lateral migration. Understanding geology, and hence petroleum dynamics, is the key to using seepage data in exploration

The surface geochemical expression of petroleum seepage can take many forms:

Bacteria and other microbes play a profound role in the oxidation of migrating hydrocarbons, and their activities are directly or indirectly responsible for many of the diverse surface manifestations of petroleum seepage. These activities, coupled with long-term migration of hydrocarbons, lead to the development of near-surface oxidation-reduction zones that favor the formation of this variety of hydrocarbon-induced chemical and mineralogical changes. This seep-induced alteration is highly complex and its varied surface expressions have led to the development of an equally varied number of geochemical and geophysical exploration techniques. Some detect hydrocarbons directly in surface and seafloor samples, others detect seep-related microbial activity, and still others measure the secondary effects of hydrocarbon-induced alteration (Schumacher, 1996; Saunders et al.,2000).

Macroseeps and Microseeps: As indicated above, there is a seepage continuum from the lowest detectable levels at one extreme to visible oil and gas seeps at the other. Macroseepage refers to the visible oil and gas seeps. Microseepage is defined as elevated concentrations of analytically detectable volatile or semivolatile hydrocarbons, or hydrocarbon-induced changes, in soils and sediments. The existence of microseepage is supported by a large body of empirical evidence:

Microseepage rates and surface hydrocarbon concentrations can vary significantly with time. Surface hydrocarbon seeps and soil geochemical anomalies appear and disappear in relatively short times: weeks to months to years. Results from studies of natural seeps and underground storage reservoirs, as well as repeat surveys of fields, indicate that the rate of hydrocarbon migration and microseepage varies from less than one meter per day to tens of meters per day. Empirical observations and computer simulations suggest that the mechanism for microseepage is a buoyancy-driven, continuous-phase gas flow through water-wet pores and fractures.

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