Two detailed case studies have been made of shallow electrical anomalies. In one study (Ashland field, Oklahoma), the anomalies are shown to be related to vertical seepage from a hydrocarbon reservoir; in the other (Salt Draw prospect, Texas), the anomalies are shown to be "false" anomalies and unrelated to any deeper hydrocarbons.

The Ashland study shows that methane seepage from the 3,000-ft (914-m) deep Hartshorne reservoir has resulted in near-surface accumulations of methane, pyrite and marcasite (FeS₂), and carbonate. The shallow pyrite and marcasite are the source of induced polarization (IP) anomalies, and the pore-plugging carbonate is the source of high-resistivity anomalies over this field.

The data from the Ashland study also illustrate the many geochemical features that are characteristic of seepage-induced alteration. These features provide criteria for determining whether shallow electrical anomalies are related to hydrocarbon seepage. The criteria were used for evaluation of IP anomalies over the Salt Draw prospect in west Texas. That evaluation suggests the IP anomalies at Salt Draw are "false"; that is, they are unrelated to the presence of any deeper hydrocarbon accumulations. The occurrence of these false anomalies stresses the importance of incorporating geochemical evaluations in the interpretation of electrical data used for hydrocarbon prospecting.

The results also suggest that seepage-induced pyrite and marcasite are likely to form only over hydrocarbon reservoirs where at least two geologic conditions are met: (1) some hydrocarbons from the reservoir can seep to the near surface, and (2) the near-surface section contains porous, iron-bearing host rocks. Because the combination of these conditions and other geologic features required for the formation of seepage-induced pyrite and marcasite is relatively rare, neither seepage-related FeS₂ nor the resultant shallow IP anomalies should be expected to occur over all hydrocarbon reservoirs.

These conclusions are combined with those drawn from 40 other electrical surveys to develop the following strategy for using shallow-investigation electrical techniques in the search for petroleum.

1. Avoid surveying areas with poor seepage potential or those which lack porous, iron-bearing host rocks.

2. Apply these surveys mainly to areas with relatively uniform near-surface geology.

3. For better interpretation of results over prospects, survey nearby production.

4. Determine which anomalies are "significant" and not simply variations in background response. This involves consideration of their magnitude, areal extent, and correlation of anomalies with known fields, prospects, and surface and subsurface geology.

5. Evaluate "significant" anomalies to determine if they are related to hydrocarbon seepage. This involves looking for independent evidence of seepage and determining the source of the IP response.

This strategy should significantly improve the effective use of this type of tool in hydrocarbon exploration.