SEEPAGE-INDUCED ANOMALIES, 'FALSE' ANOMALIES, AND IMPLICATIONS FOR ELECTRICAL PROSPECTING.

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 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: some hydrocarbons from the reservoir can seep to the near surface, and the near-surface section contains porous, iron-bearing host rocks. Study conclusions are combined with those drawn from 40 other electrical surveys to develop a strategy for using shallow-investigation electrical techniques in the search for petroleum which is discussed in the paper. Refs.