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Surface geochemical survey is a tool for the mapping and analyzing of the surface expression of subsurface buried petroleum hydrocarbons and minerals, whether deposited by natural earth processes or by manmade pollution, through the measuring of thermogenic soil gas along transects and/or in a grid. Thermogenic soil gas results from the vertical micro-migration of volatile compounds from hydrocarbons or minerals trapped in conventional and non-conventional deposits. Since 1930, more than 20 different surface geochemical techniques have been developed that have varying degrees of sensitivity and accuracy (direct soil analysis, active soil gas measurement, microbial and passive sorbent-based collector).

Surface geochemical surveys can be design for various objectives. The reconnaissance survey’s primary objective is to determine if unexplored basins (regions) are petroliferous and subsequently rank areas for additional geological and geophysical work. This can be followed by lead identification surveys that evaluate areas of hydrocarbon potential within the basin usually with geochemical line profiles tied to seismic and/or some other geophysical technique. Once the leads have been identified, a prospect ranking survey can be conducted to determine the prospect with the highest probability of trapping hydrocarbon of economic size. Frequently on large prospects, the reservoir parameters (porosity, permeability and hydrocarbon saturation) can vary therefore an anomaly evaluation survey can help to identify the final drilling location. Although many geological and geophysical techniques define the attributes of hydrocarbon exploration (source rock, reservoir rock, structure, seal, thermal history, migration, and entrapment), only surface geochemistry reveals important information about hydrocarbon charge, hydrocarbon type, and seepage of hydrocarbons to the surface.