Published 1996 as part of Memoir 66
Copyright © 1996 The American Association of Petroleum Geologists.   All Rights Reserved.

The surface expression of hydrocarbon-induced alteration of soils and sediments can take many forms, including (1) microbiological anomalies and the formation of "paraffin dirt"; (2) mineralogic changes such as formation of calcite, pyrite, uranium, elemental sulfur, and certain magnetic iron oxides and sulfides; (3) bleaching of red beds; (4) clay mineral alteration; (5) electrochemical changes; (6) radiation anomalies; and (7) biogeochemical and geobotanical anomalies.

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 surface manifestations of hydrocarbon 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 a variety of hydrocarbon-induced chemical and mineralogic changes. This hydrocarbon-induced alteration is highly complex, and its varied surface expressions have led to the development of an equally varied number of surface exploration techniques, including soil carbonate methods, magnetic and electrical methods, radioactivity-based methods, and remote sensing methods.

Exploration methods based on what are assumed to be hydrocarbon-induced soil or sediment alterations have long been popular. Many claims of success have been made for these methods. However, well-documented studies are rare, and the claims are seldom substantiated by a scientifically rigorous program of sampling and analysis. The cause of these altered soils and sediments may well be hydrocarbon-related, but hydrocarbons are an indirect cause at best and not the most probable cause. Although the occurrence of hydrocarbon-induced geochemical alteration is well established, considerable research is needed before we understand the many factors affecting the formation of these anomalies in the near surface. Only then will we realize their full value for hydrocarbon exploration.