Hydrogeochemical Prospecting for Natural Gas: The Geochemical Chimney, Shallow Groundwater Hydrogeology, and Interpretation of Near-Surface Data

Summary Near-surface geochemical prospecting methods are playing an increasingly important role in the search for new natural gas accumulations, especially in onshore frontier areas. The approach involves analysis of soil gas samples from shallow borings and gas/water samples from shallow water wells for hydrocarbon gas concentrations and composition, stable isotopes (carbon, hydrogen), and ancillary hydrochemical parameters. These data are then used to identify the location, origin, source, extent, and thermal maturity of gas accumulations at depth. Nearsurface exploration techniques commonly assume that fugitive hydrocarbon gas leaks from a deep gas reservoir and migrates vertically upward by buoyancy to the surface, so that the surface expression is located essentially vertically above the hydrocarbon accumulation. This vertical upward migration of hydrocarbon gas has been referred to as a “geochemical chimney,” and is traditionally used as a working model for interpreting near-surface geochemical prospecting data. However, the geochemical chimney model for gas migration implicitly assumes certain hydrogeologic conditions are met (i.e., very little or no lateral groundwater flow, negligible vertical gradients, and/or separate phase transport of hydrocarbon gases). If these conditions are not met, then the plume of upward-migrating fugitive gases can be deflected laterally within the shallow (< 1000 meters bgs) groundwater system, or even downwards, depending on the local hydrogeologic regime. In addition, groundwater transport would mix hydrocarbon gases, thereby affecting gas concentration and isotope signature. Hydrogeochemical and hydrocarbon gas data from the Columbia River Basin of Washington and Oregon are used to illustrate the effects of lateral transport and downwelling of fugitive hydrocarbon gases. A properly planned and executed groundwater investigation can result in a more accurate characterization of the underlying hydrocarbon gas accumulation.